CHANGELOG.md

In this changelog, we document "large-ish" changes to Iris that affect even the
way the logic is used on paper.  We also document changes in the Coq
development; every API-breaking change should be listed, but not every new
lemma.

## Iris master

Coq 8.11 is no longer supported in this version of Iris.

**Changes in `algebra`:**

* Generalize the authorative elements of the `view`, `auth` and `gset_bij`
  cameras to be parameterized by a [discardable fraction](iris/algebra/dfrac.v)
  (`dfrac`) instead of a fraction (`frac`). Normal fractions are now denoted
  `●{#q} a` and `●V{#q} a`. Lemmas affected by this have been renamed such that
  the "frac" in their name has been changed into "dfrac". (by Simon Friis Vindum)
* Generalize `namespace_map` to `reservation_map` which enhances `gmap positive
  A` with a notion of 'tokens' that enable allocating a particular name in the
  map. See [algebra.reservation_map](iris/algebra/reservation_map.v) for further
  information.
* Add `dyn_reservation_map` which further extends `reservation_map` with the
  ability to dynamically allocate an infinite set of tokens. This is useful to
  perform synchronized allocation of the same name in two maps/APIs without
  dedicated support from one of the involved maps/APIs. See
  [algebra.dyn_reservation_map](iris/algebra/dyn_reservation_map.v) for further
  information.
* Demote the Camera structure on `list` to `iris_staging` since its composition
  is not very well-behaved.
* Extend `gmap_view` with lemmas for "big" operations on maps.
* Typeclasses instances triggering a canonical structure search such as `Equiv`,
  `Dist`, `Op`, `Valid`, `ValidN`, `Unit`, `PCore` now use an `Hint Extern`
  based on `refine` instead of `apply`, in order to use Coq's newer unification
  algorithm.
* Set `Hint Mode` for the classes `OfeDiscrete`, `Dist`, `Unit`, `CmraMorphism`,
  `rFunctorContractive`, `urFunctorContractive`.
* Set `Hint Mode` for the stdpp class `Equiv`. This might require few spurious
  type annotations until
  [Coq bug #14441](https://github.com/coq/coq/issues/14441) is fixed.
* Add `max_prefix_list` RA on lists whose composition is only defined when one
  operand is a prefix of the other. The result is the longer list.
* Add `NonExpansive` instances for `curry` and friends.

**Changes in `bi`:**

* Add new lemmas `big_sepM2_delete_l` and `big_sepM2_delete_r`.
* Rename `big_sepM2_lookup_1` → `big_sepM2_lookup_l` and
  `big_sepM2_lookup_2` → `big_sepM2_lookup_r`.
* Add lemmas for swapping nested big-ops: `big_sep{L,M,S,MS}_sep{L,M,S,MS}`.
* Rename `big_sep{L,L2,M,M2,S}_intuitionistically_forall` →
  `big_sep{L,L2,M,M2,S}_intro`, and `big_orL_lookup` → `big_orL_intro`.
* Rename `bupd_forall` to `bupd_plain_forall`, and add
  `{bupd,fupd}_{and,or,forall,exist}`.
* Decouple `Wp` and `Twp` typeclasses from the `program_logic.language`
  interface. The typeclasses are now parameterized over an expression and a
  value type, instead of a language. This requires extra type annotations or
  explicit coercions in a few cases, in particular `WP v {{ Φ }}` must now be
  written `WP (of_val v) {{ Φ }}`.
* Improve `make_laterable`:
  - Adjust definition such that `Laterable P` iff `P ⊢ make_laterable P`.
    As a consequence, `make_laterable_elim` got weaker: elimination now requires
    an except-0 modality (`make_laterable P -∗ ◇ P`).
  - Add `iModIntro` support for `make_laterable`.
* Improvements to `BiMonoPred`:
  - Use `□`/`-∗` instead of `<pers>`/`→`.
  - Strengthen to ensure that functions for recursive calls are non-expansive.
* Add `big_andM` (big conjunction on finite maps) with lemmas similar to `big_andL`.
* Add transitive embedding that constructs an embedding of `PROP1` into `PROP3`
  by combining the embeddings of `PROP1` into `PROP2` and `PROP2` into `PROP3`.
  This construct is *not* declared as an instance to avoid TC search divergence.
  (by Hai Dang, BedRock Systems)
* Improve notation printing around magic wands, view shifts, `WP`, Texan
  triples, and logically atomic triples.
* Slight change to the `AACC` notation for atomic accessors (which is usually
  only printed, not parsed): added a `,` before `ABORT`, for consistency with `COMM`.
* Add the lemmas `big_sepM_impl_strong` and `big_sepM_impl_dom_subseteq` that
  generalize the existing `big_sepM_impl` lemma. (by Simon Friis Vindum)

**Changes in `proofmode`:**

* Add support for pure names `%H` in intro patterns. This is now natively
  supported whereas the previous experimental support required installing
  https://gitlab.mpi-sws.org/iris/string-ident. (by Tej Chajed)
* Add support for destructing existentials with the intro pattern `[%x ...]`.
  (by Tej Chajed)
* `iMod`/`iModIntro` show proper error messages when they fail due to mask
  mismatches. To support this, the proofmode typeclass `FromModal` now takes an
  additional pure precondition.
* Fix performance of `iFrame` in logics without `BiAffine`.
  To adjust your code if you use such logics and define `Frame` instances,
  ensure these instances to have priority at least 2: they should have either at
  least 2 (non-dependent) premises, or an explicit priority.
  References: docs for `frame_here_absorbing` in
  [iris/proofmode/frame_instances.v](iris/proofmode/frame_instances.v) and
  https://coq.inria.fr/refman/addendum/type-classes.html#coq:cmd.Instance. (by
  Paolo G. Giarrusso, BedRock Systems)
* Rename the main entry point module for the proofmode from
  `iris.proofmode.tactics` to `iris.proofmode.proofmode`. Under normal
  circumstances, this should be the only proofmode file you need to import.

**Changes in `bi`:**

* Add lemmas characterizing big-ops over pure predicates (`big_sep*_pure*`).
* Move `BiAffine`, `BiPositive`, `BiLöb`, and `BiPureForall` from
  `bi.derived_connectives` to `bi.extensions`.
* Strengthen `persistent_fractional` to support propositions that are persistent
  and either affine or absorbing. (by Paolo G. Giarrusso, BedRock Systems)

**Changes in `base_logic`:**

* Add `ghost_map`, a logic-level library for a `gmap K V` with an authoritative
  view and per-element points-to facts written `k ↪[γ] w`.
* Generalize the soundness lemma of the base logic `step_fupdN_soundness`.
  It applies even if invariants stay open accross an arbitrary number of laters.
  (by Jacques-Henri Jourdan)
* Rename those `*G` typeclasses that must be global singletons to `*GS`, and
  their corresponding `preG` class to `GpreS`. Affects `invG`, `irisG`,
  `gen_heapG`, `inv_heapG`, `proph_mapG`, `ownPG`, `heapG`.

**Changes in `program_logic`:**

* Change definition of weakest precondition to use a variable number of laters
  (i.e., logical steps) for each physical step of the operational semantics,
  depending on the number of physical steps executed since the begining of the
  execution of the program. See merge request [!595](https://gitlab.mpi-sws.org/iris/iris/-/merge_requests/595).
  This implies several API-breaking changes, which can be easily fixed in client
  formalizations in a backward compatible manner as follows:
   - Ignore the new parameter `ns` in the state interpretation, which
     corresponds to a step counter.
   - Use the constant function "0" for the new field `num_laters_per_step` of
     `irisG`.
   - Use `fupd_intro _ _` for the new field `state_interp_mono` of `irisG`.
   - Some proofs using lifting lemmas and adequacy theorems need to be adapted
     to ignore the new step counter.
  (by Jacques-Henri Jourdan)
* Remove `wp_frame_wand_l`; add `wp_frame_wand` as more symmetric replacement.
* Swap the polarity of the mask in logically atomic triples, so that it matches
  regular `WP` masks.
* Rename `iris_invG` to `iris_invGS`.

**Changes in `heap_lang`:**

* Rename `Build_loc` constructor for `loc` type to `Loc`.
* Add atomic `Xchg` ("exchange"/"swap") operation. (by Simon Hudon, Google LLC)

The following `sed` script helps adjust your code to the renaming (on macOS,
replace `sed` by `gsed`, installed via e.g. `brew install gnu-sed`).
Note that the script is not idempotent, do not run it twice.
```
sed -i -E -f- $(find theories -name "*.v") <<EOF
# auth and view renames from frac to dfrac
s/\b(auth|view)_(auth|both|update)_frac_(is_op|op_invN|op_inv|inv_L|validN|op_validN|valid|op_valid|valid_2|valid_discrete|includedN|included|alloc|validI|validI_2|validI_1|validI|)\b/\1_\2_dfrac_\3/g
s/\bgset_bij_auth_frac_(\w*)\b/gset_bij_auth_dfrac_\1/g
s/\bgset_bij_auth_empty_frac_valid\b/gset_bij_auth_empty_dfrac_valid/g
s/\bbij_both_frac_valid\b/bij_both_dfrac_valid/g
# big_sepM renames
s/\bbig_sepM2_lookup_1\b/big_sepM2_lookup_l/g
s/\bbig_sepM2_lookup_2\b/big_sepM2_lookup_r/g
# big_*_intro
s/\bbig_sep(L|L2|M|M2|S)_intuitionistically_forall\b/big_sep\1_intro/g
s/\bbig_orL_lookup\b/big_orL_intro/g
s/\bbupd_forall\b/bupd_plain_forall/g
# "global singleton" rename
s/\b(inv|iris|(gen|inv)_heap|(Gen|Inv)Heap|proph_map|ProphMap|[oO]wnP|[hH]eap)G\b/\1GS/g
s/\b([iI]nv|iris|(gen|inv)_heap|(Gen|Inv)Heap|proph_map|ProphMap|[oO]wnP|[hH]eap)PreG\b/\1GpreS/g
# iris.proofmode.tactics → iris.proofmode.proofmode
s/\bproofmode\.tactics\b/proofmode.proofmode/
s/(From +iris\.proofmode +Require +(Import|Export).*)\btactics\b/\1proofmode/
# iris_invG → iris_invGS
s/\biris_invG\b/iris_invGS/g
EOF
```

## Iris 3.4.0 (released 2021-02-16)

The highlights and most notable changes of this release are as follows:
* Coq 8.13 is now supported; the old Coq 8.9 and Coq 8.10 are not supported any
  more.
* The new `view` RA construction generalizes `auth` to user-defined abstraction
  relations. (thanks to Gregory Malecha for the inspiration)
* The new `dfrac` RA extends `frac` (fractions `0 < q ≤ 1`) with support for
  "discarding" some part of the fraction in exchange for a persistent witness
  that discarding has happened. This can be used to easily generalize fractional
  permissions with support for persistently owning "any part" of the resource.
  (by Simon Friis Vindum)
* The new `gmap_view` RA provides convenient lemmas for ghost ownership
  of heap-like structures with an "authoritative" view. Thanks to `dfrac`, it
  supports both exclusive (mutable) and persistent (immutable) ownership of
  individual map elements.
* With this release we are beginning to provide logic-level abstractions for
  ghost state, which have the advantage that the user does not have to directly
  interact with RAs to use them.
  - `ghost_var` provides a logic-level abstraction of ghost variables: a mutable
    "variable" with fractional ownership.
  - `mono_nat` provides a "monotone counter" with a persistent witnesses
    representing a lower bound of the current counter value. (by Tej Chajed)
  - `gset_bij` provides a monotonically growing partial bijection; this is
    useful in particular when building binary logical relations for languages
    with a heap.
* HeapLang provides a persistent read-only points-to assertion `l ↦□ v`.
  (by Simon Friis Vindum)
* We split Iris into multiple opam packages: `coq-iris` no longer contains
  HeapLang, which is now in a separate package `coq-iris-heap-lang`. The two
  packages `coq-iris-deprecated` (for old modules that we eventually plan to
  remove entirely) and `coq-iris-staging` (for new modules that are not yet
  ready for prime time) exist only as development versions, so they are not part
  of this release.
* The proofmode now does a better job at picking reasonable names when moving
  variables into the Coq context without a name being explicitly given by the
  user. However, the exact variable names remain unspecified. (by Tej Chajed)

Further details are given in the changelog below.

This release of Iris was managed by Ralf Jung and Robbert Krebbers, with
contributions by Arthur Azevedo de Amorim, Dan Frumin, Enrico Tassi, Hai Dang,
Michael Sammler, Paolo G. Giarrusso, Rodolphe Lepigre, Simon Friis Vindum, Tej
Chajed, and Yusuke Matsushita.  Thanks a lot to everyone involved!

**Changes in `algebra`:**

* Add constructions to define a camera through restriction of the validity predicate
  (`iso_cmra_mixin_restrict`) and through an isomorphism (`iso_cmra_mixin`).
* Add a `frac_agree` library which encapsulates `frac * agree A` for some OFE
  `A`, and provides some useful lemmas.
* Add the view camera `view`, which generalizes the authoritative camera
  `auth` by being parameterized by a relation that relates the authoritative
  element with the fragments.
* Add the camera of discardable fractions `dfrac`. This is a generalization of
  the normal fractional camera.
  See [algebra.dfrac](iris/algebra/dfrac.v) for further information.
* Add `gmap_view`, a camera providing a "view of a `gmap`". The authoritative
  element is any `gmap`; the fragment provides fractional ownership of a single
  key, including support for persistent read-only ownership through `dfrac`.
  See [algebra.lib.gmap_view](iris/algebra/lib/gmap_view.v) for further information.
* Add `mono_nat`, a wrapper for `auth max_nat`. The result is an authoritative
  `nat` where a fragment is a lower bound whose ownership is persistent.
  See [algebra.lib.mono_nat](iris/algebra/lib/mono_nat.v) for further information.
* Add the `gset_bij` resource algebra for monotone partial bijections.
  See [algebra.lib.gset_bij](iris/algebra/lib/gset_bij.v) for further information.
* Rename `agree_op_inv'` → `to_agree_op_inv`,
  `agree_op_invL'` → `to_agree_op_inv_L`, and add `to_agree_op_invN`.
* Rename `auth_auth_frac_op_invL` → `auth_auth_frac_op_inv_L`,
  `excl_auth_agreeL` → `excl_auth_agree_L`,
  `frac_auth_agreeL` → `frac_auth_agree_L`, and
  `ufrac_auth_agreeL` → `ufrac_auth_agree_L`.
* Fix direction of `auth_auth_validN` to make it consistent with similar lemmas,
  e.g., `auth_auth_valid`. The direction is now `✓{n} (● a) ↔ ✓{n} a`.
* Rename `auth_both_valid` to `auth_both_valid_discrete` and
  `auth_both_frac_valid` to `auth_both_frac_valid_discrete`. The old name is
  used for new, stronger lemmas that do not assume discreteness.
* Redefine the authoritative camera in terms of the view camera. As part of this
  change, we have removed lemmas that leaked implementation details. Hence, the
  only way to construct elements of `auth` is via the elements `●{q} a` and
  `◯ b`. The constructor `Auth`, and the projections `auth_auth_proj` and
  `auth_frag_proj` no longer exist. Lemmas that referred to these constructors
  have been removed, in particular: `auth_equivI`, `auth_validI`,
  `auth_included`, `auth_valid_discrete`, and `auth_both_op`.  For validity, use
  `auth_auth_valid*`, `auth_frag_valid*`, or `auth_both_valid*` instead.
* Rename `auth_update_core_id` into `auth_update_frac_alloc`.
* Rename `cmra_monotone_valid` into `cmra_morphism_valid` (this rename was
  forgotten in !56).
* Move the `*_validI` and `*_equivI` lemmas to a new module, `base_logic.algebra`.
  That module is exported by `base_logic.base_logic` so it should usually be
  available everywhere without further changes.
* The authoritative fragment `✓ (◯ b : auth A)` is no longer definitionally
  equal to `✓ b`.
* Change `*_valid` lemma statements involving fractions to use `Qp` addition and
  inequality instead of RA composition and validity (also in `base_logic` and
  the higher layers).
* Move `algebra.base` module to `prelude.prelude`.
* Strengthen `cmra_op_discrete` to assume only `✓{0} (x1 ⋅ x2)` instead of `✓
  (x1 ⋅ x2)`.
* Rename the types `ofeT`→`ofe`, `cmraT`→`cmra`, `ucmraT`→`ucmra`, and the
  constructors `OfeT`→`Ofe`, `CmraT`→`Cmra`, and `UcmraT`→`Ucmra` since the `T`
  suffix is not needed. This change makes these names consistent with `bi`,
  which also does not have a `T` suffix.
* Rename typeclass instances of CMRA operational typeclasses (`Op`, `Core`,
  `PCore`, `Valid`, `ValidN`, `Unit`) to have a `_instance` suffix, so that
  their original names are available to use as lemma names.
* Rename `frac_valid'`→`frac_valid`, `frac_op'`→`frac_op`,
  `ufrac_op'`→`ufrac_op`, `coPset_op_union` → `coPset_op`, `coPset_core_self` →
  `coPset_core`, `gset_op_union` → `gset_op`, `gset_core_self` → `gset_core`,
  `gmultiset_op_disj_union` → `gmultiset_op`, `gmultiset_core_empty` →
  `gmultiset_core`, `nat_op_plus` → `nat_op`, `max_nat_op_max` →
  `max_nat_op`. Those names were previously blocked by typeclass instances.

**Changes in `bi`:**

* Add big op lemmas `big_op{L,L2,M,M2,S}_intuitionistically_forall` and
  `big_sepL2_forall`, `big_sepMS_forall`, `big_sepMS_impl`, and `big_sepMS_dup`.
* Add lemmas to big-ops that provide ownership of a single element and permit
  changing the quantified-over predicate when re-assembling the big-op:
  `big_sepL_lookup_acc_impl`, `big_sepL2_lookup_acc_impl`,
  `big_sepM_lookup_acc_impl`, `big_sepM2_lookup_acc_impl`,
  `big_sepS_elem_of_acc_impl`, `big_sepMS_elem_of_acc_impl`.
* Add lemmas `big_sepM_filter'` and `big_sepM_filter` matching the corresponding
  `big_sepS` lemmas.
* Add lemmas for big-ops of magic wands: `big_sepL_wand`, `big_sepL2_wand`,
  `big_sepM_wand`, `big_sepM2_wand`, `big_sepS_wand`, `big_sepMS_wand`.
* Add notation `¬ P` for `P → False` to `bi_scope`.
* Add `fupd_mask_intro` which can be conveniently `iApply`ed to goals of the
  form `|={E1,E2}=>` to get rid of the `fupd` in the goal if `E2 ⊆ E1`. The
  lemma `fupd_mask_weaken Enew` can be `iApply`ed to shrink the first mask to
  `Enew` without getting rid of the modality; the same effect can also be
  obtained slightly more conveniently by using `iMod` with `fupd_mask_subseteq
  Enew`. To make the new names work, rename some existing lemmas:
  `fupd_intro_mask` → `fupd_mask_intro_subseteq`,
  `fupd_intro_mask'` → `fupd_mask_subseteq` (implicit arguments also changed
  here), `fupd_mask_weaken` → `fupd_mask_intro_discard`. Remove `fupd_mask_same`
  since it was unused and obscure.  In the `BiFUpd` axiomatization, rename
  `bi_fupd_mixin_fupd_intro_mask` to `bi_fupd_mixin_fupd_mask_subseteq` and
  weaken the lemma to be specifically about `emp` (the stronger version can be
  derived).
* Remove `bi.tactics` with tactics that predate the proofmode (and that have not
  been working properly for quite some time).
* Strengthen `persistent_sep_dup` to support propositions that are persistent
  and either affine or absorbing.
* Fix the statement of the lemma `fupd_plainly_laterN`; the old lemma was a
  duplicate of `fupd_plain_laterN`.
* Strengthen `big_sepL2_app_inv` by weakening a premise (it is sufficient for
  one of the two pairs of lists to have equal length).
* Rename `equiv_entails` → `equiv_entails_1_1`,
  `equiv_entails_sym` → `equiv_entails_1_2`, and `equiv_spec` → `equiv_entails`.
* Remove the laws `pure_forall_2 : (∀ a, ⌜ φ a ⌝) ⊢ ⌜ ∀ a, φ a ⌝` from the BI
  interface and factor it into a type class `BiPureForall`.

**Changes in `proofmode`:**

* The proofmode now preserves user-supplied names for existentials when using
  `iDestruct ... as (?) "..."`. This is backwards-incompatible if you were
  relying on the previous automatic names (which were always "H", possibly
  freshened). It also requires some changes if you were implementing `IntoExist`
  yourself, since the typeclass now forwards names. If your instance transforms
  one `IntoExist` into another, you can generally just forward the name from the
  premise.
* The proofmode also preserves user-supplied names in `iIntros`, for example
  with `iIntros (?)` and `iIntros "%"`, as described for destructing
  existentials above. As part of this change, it now uses a base name of `H` for
  pure facts rather than the previous default of `a`. This also requires some
  changes if you were implementing `FromForall`, in order to forward names.
* Make `iFrame` "less" smart w.r.t. clean up of modalities. It now consistently
  removes the modalities `<affine>`, `<absorbing>`, `<persistent>` and `□` only
  if the result after framing is `True` or `emp`. In particular, it no longer
  removes `<affine>` if the result after framing is affine, and it no longer
  removes `□` if the result after framing is intuitionistic.
* Allow framing below an `<affine>` modality if the hypothesis that is framed is
  affine. (Previously, framing below `<affine>` was only possible if the
  hypothesis that is framed resides in the intuitionistic context.)
* Add Coq side-condition `φ` to class `ElimAcc` (similar to what we already had
  for `ElimInv` and `ElimModal`).
* Add a tactic `iSelect pat tac` (similar to `select` in std++) which runs the
  tactic `tac H` with the name `H` of the last hypothesis of the intuitionistic
  or spatial context matching `pat`. The tactic `iSelect` is used to implement:
  + `iRename select (pat)%I into name` which renames the matching hypothesis,
  + `iDestruct select (pat)%I as ...` which destructs the matching hypothesis,
  + `iClear select (pat)%I` which clears the matching hypothesis,
  + `iRevert select (pat)%I` which reverts the matching hypothesis,
  + `iFrame select (pat)%I` which cancels the matching hypothesis.

**Changes in `base_logic`:**

* Add a `ghost_var` library that provides (fractional) ownership of a ghost
  variable of arbitrary `Type`.
* Define a ghost state library on top of the `mono_nat` resource algebra.
  See [base_logic.lib.mono_nat](iris/base_logic/lib/mono_nat.v) for further
  information.
* Define a ghost state library on top of the `gset_bij` resource algebra.
  See [base_logic.lib.gset_bij](iris/base_logic/lib/gset_bij.v) for further
  information.
* Extend the `gen_heap` library with read-only points-to assertions using
  [discardable fractions](iris/algebra/dfrac.v).
  + The `mapsto` connective now takes a `dfrac` rather than a `frac` (i.e.,
    positive rational number `Qp`).
  + The notation `l ↦{ dq } v` is generalized to discardable fractions
    `dq : dfrac`.
  + The new notation `l ↦{# q} v` is used for a concrete fraction `q : frac`
    (e.g., to enable writing `l ↦{# 1/2} v`).
  + The new notation `l ↦□ v` is used for the discarded fraction. This
    persistent proposition provides read-only access to `l`.
  + The lemma `mapsto_persist : l ↦{dq} v ==∗ l ↦□ v` is used for making the
    location `l` read-only.
  + See the [changes to HeapLang](https://gitlab.mpi-sws.org/iris/iris/-/merge_requests/554)
    for an indication on how to adapt your language.
  + See the [changes to iris-examples](https://gitlab.mpi-sws.org/iris/examples/-/commit/a8425b708ec51d918d5cf6eb3ab6fde88f4e2c2a)
    for an indication on how to adapt your development. In particular, instead
    of `∃ q, l ↦{q} v` you likely want to use `l ↦□ v`, which has the advantage
    of being persistent (rather than just duplicable).
* Change type of some ghost state lemmas (mostly about allocation) to use `∗`
  instead of `∧` (consistent with our usual style).  This affects the following
  lemmas: `own_alloc_strong_dep`, `own_alloc_cofinite_dep`, `own_alloc_strong`,
  `own_alloc_cofinite`, `own_updateP`, `saved_anything_alloc_strong`,
  `saved_anything_alloc_cofinite`, `saved_prop_alloc_strong`,
  `saved_prop_alloc_cofinite`, `saved_pred_alloc_strong`,
  `saved_pred_alloc_cofinite`, `auth_alloc_strong`, `auth_alloc_cofinite`,
  `auth_alloc`.
* Change `uPred_mono` to only require inclusion at the smaller step-index.
* Put `iProp`/`iPreProp`-isomorphism into the `own` construction. This affects
  clients that define higher-order ghost state constructions. Concretely, when
  defining an `inG`, the functor no longer needs to be applied to `iPreProp`,
  but should be applied to `iProp`. This avoids clients from having to push
  through the `iProp`/`iPreProp`-isomorphism themselves, which is now handled
  once and for all by the `own` construction.
* Rename `gen_heap_ctx` to `gen_heap_interp`, since it is meant to be used in
  the state interpretation of WP and since `_ctx` is elsewhere used as a suffix
  indicating "this is a persistent assumption that clients should always have in
  their context". Likewise, rename `proph_map_ctx` to `proph_map_interp`.
* Move `uPred.prod_validI`, `uPred.option_validI`, and
  `uPred.discrete_fun_validI` to the new `base_logic.algebra` module. That
  module is exported by `base_logic.base_logic` so these names are now usually
  available everywhere, and no longer inside the `uPred` module.
* Remove the `gen_heap` notations `l ↦ -` and `l ↦{q} -`. They were barely used
  and looked very confusing in context: `l ↦ - ∗ P` looks like a magic wand.
* Change `gen_inv_heap` notation `l ↦□ I` to `l ↦_I □`, so that `↦□` can be used
  by `gen_heap`.
* Strengthen `mapsto_valid_2` conclusion from `✓ (q1 + q2)%Qp` to
  `⌜✓ (q1 + q2)%Qp ∧ v1 = v2⌝`.
* Change `gen_heap_init` to also return ownership of the points-to facts for the
  initial heap.
* Rename `mapsto_mapsto_ne` to `mapsto_frac_ne`, and add a simpler
  `mapsto_ne` that does not require reasoning about fractions.
* Deprecate the `auth` and `sts` modules. These were logic-level wrappers around
  the underlying RAs; as far as we know, they are unused since they were not
  flexible enough for practical use.
* Deprecate the `viewshift` module, which defined a binary view-shift connective
  with an implicit persistence modality. It was unused and too easily confused
  with `={_}=∗`, the binary view-shift (fancy update) *without* a persistence
  modality.

**Changes in `program_logic`:**

* `wp_strong_adequacy` now applies to an initial state with multiple
  threads instead of only a single thread. The derived adequacy lemmas
  are unchanged.
* `pure_exec_fill` is no longer registered as an instance for `PureExec`, to
  avoid TC search attempting to apply this instance all the time.
* Merge `wp_value_inv`/`wp_value_inv'` into `wp_value_fupd`/`wp_value_fupd'` by
  making the lemmas bidirectional.
* Generalize HeapLang's `mapsto` (`↦`), `array` (`↦∗`), and atomic heap
  connectives to discardable fractions. See the CHANGELOG entry in the category
  `base_logic` for more information.
* Opening an invariant or eliminating a mask-changing update modality around a
  non-atomic weakest precondition creates a side-condition `Atomic ...`.
  Before, this would fail with the unspecific error "iMod: cannot eliminate
  modality (|={E1,E2}=> ...) in (WP ...)".
* In `Ectx_step` and `step_atomic`, mark the parameters that are determined by
  the goal as implicit.
* Deprecate the `hoare` module to prevent accidental usage; the recommended way
  to write Hoare-style specifications is to use Texan triples.

**Changes in `heap_lang`:**

* `wp_pures` now turns goals of the form `WP v {{ Φ }}` into `Φ v`.
* Fix `wp_bind` in case of a NOP (i.e., when the given expression pattern is
  already at the top level).
* The `wp_` tactics now preserve the possibility of doing a fancy update when
  the expression reduces to a value.
* Move `IntoVal`, `AsVal`, `Atomic`, `AsRecV`, and `PureExec` instances to their
  own file [heap_lang.class_instances](iris_heap_lang/class_instances.v).
* Move `inv_head_step` tactic and `head_step` auto hints (now part of new hint
  database `head_step`) to [heap_lang.tactics](iris_heap_lang/tactics.v).
* The tactic `wp_apply` no longer performs `wp_pures` before applying the given
  lemma. The new tactic `wp_smart_apply` repeatedly performs single `wp_pure`
  steps until the lemma matches the goal.

The following `sed` script helps adjust your code to the renaming (on macOS,
replace `sed` by `gsed`, installed via e.g. `brew install gnu-sed`).
Note that the script is not idempotent, do not run it twice.
```
sed -i -E -f- $(find theories -name "*.v") <<EOF
# agree and L suffix renames
s/\bagree_op_inv'/to_agree_op_inv/g
s/\bagree_op_invL'/to_agree_op_inv_L/g
s/\bauth_auth_frac_op_invL\b/auth_auth_frac_op_inv_L/g
s/\b(excl|frac|ufrac)_auth_agreeL/\1_auth_agree_L/g
# auth_both_valid
s/\bauth_both_valid\b/auth_both_valid_discrete/g
s/\bauth_both_frac_valid\b/auth_both_frac_valid_discrete/g
# gen_heap_ctx and proph_map_ctx
s/\bgen_heap_ctx\b/gen_heap_interp/g
s/\bproph_map_ctx\b/proph_map_interp/g
# other gen_heap changes
s/\bmapsto_mapsto_ne\b/mapsto_frac_ne/g
# remove Ts in algebra
s/\bofeT\b/ofe/g
s/\bOfeT\b/Ofe/g
s/\bcmraT\b/cmra/g
s/\bCmraT\b/Cmra/g
s/\bucmraT\b/ucmra/g
s/\bUcmraT\b/Ucmra/g
# _op/valid/core lemmas
s/\b(u?frac_(op|valid))'/\1/g
s/\b((coPset|gset)_op)_union\b/\1/g
s/\b((coPset|gset)_core)_self\b/\1/g
s/\b(gmultiset_op)_disj_union\b/\1/g
s/\b(gmultiset_core)_empty\b/\1/g
s/\b(nat_op)_plus\b/\1/g
s/\b(max_nat_op)_max\b/\1/g
# equiv_spec
s/\bequiv_entails\b/equiv_entails_1_1/g
s/\bequiv_entails_sym\b/equiv_entails_1_2/g
s/\bequiv_spec\b/equiv_entails/g
EOF
```

## Iris 3.3.0 (released 2020-07-15)

This release does not have any outstanding highlights, but contains a large
number of improvements all over the board.  For instance:

* `heap_lang` now supports deallocation as well as better reasoning about
  "invariant locations" (locations that perpetually satisfy some Coq-level
  invariant).
* Invariants (`inv N P`) are more flexible, now also supporting splitting
  and merging of invariants with respect to separating conjunction.
* Performance of the proofmode for BIs constructed on top of other BIs (e.g.,
  `monPred`) was greatly improved, leading to up to 70% speedup in individual
  files. As part of this refactoring, the proofmode can now also be instantiated
  with entirely "logical" notion of BIs that do not have a (non-trivial) metric
  structure, and still support reasoning about ▷.
* The proof mode now provides experimental support for naming pure facts in
  intro patterns.  See
  [iris/string-ident](https://gitlab.mpi-sws.org/iris/string-ident) for details.
* Iris now provides official ASCII notation. We still recommend using the
  Unicode notation for better consistency and interoperability with other Iris
  libraries, but provide ASCII notation for when Unicode is not an option.
* We removed several coercions, fixing "ambiguous coercion path" warnings and
  solving some readability issues.
* Coq 8.10, 8.11, and 8.12 are newly supported by this release, and Coq 8.7 and
  8.8 are no longer supported.

Further details are given in the changelog below. We always first list the
potentially breaking changes, then (some of) the additions.

This release of Iris received contributions by Abel Nieto, Amin Timany, Dan
Frumin, Derek Dreyer, Dmitry Khalanskiy, Gregory Malecha, Jacques-Henri Jourdan,
Jonas Kastberg, Jules Jacobs, Matthieu Sozeau, Maxime Dénès, Michael Sammler,
Paolo G. Giarrusso, Ralf Jung, Robbert Krebbers, Simon Friis Vindum, Simon
Spies, and Tej Chajed.  Thanks a lot to everyone involved!

**Changes in `heap_lang`:**

* Remove global `Open Scope Z_scope` from `heap_lang.lang`, and leave it up to
  reverse dependencies if they want to `Open Scope Z_scope` or not.
* Fix all binary operators performing pointer arithmetic (instead of just the
  dedicated `OffsetOp` operator doing that).
* Rename `heap_lang.lifting` to `heap_lang.primitive_laws`. There now also
  exists `heap_lang.derived_laws`.
* Make lemma names for `fill` more consistent
  - Use the `_inv` suffix for the the backwards directions:
   `reducible_fill` → `reducible_fill_inv`,
   `reducible_no_obs_fill` → `reducible_no_obs_fill_inv`,
   `not_stuck_fill` → `not_stuck_fill_inv`.
  - Use the non-`_inv` names (that freed up) for the forwards directions:
   `reducible_fill`, `reducible_no_obs_fill`, `irreducible_fill_inv`.
* Remove namespace `N` from `is_lock`.

* Add support for deallocation of locations via the `Free` operation.
* Add a fraction to the heap_lang `array` assertion.
* Add `lib.array` module for deallocating, copying and cloning arrays.
* Add TWP (total weakest-pre) lemmas for arrays.
* Add a library for "invariant locations": heap locations that will not be
  deallocated (i.e., they are GC-managed) and satisfy some pure, Coq-level
  invariant.  See `iris.base_logic.lib.gen_inv_heap` for details.
* Add the ghost state for "invariant locations" to `heapG`.  This affects the
  statement of `heap_adequacy`, which is now responsible for initializing the
  "invariant locations" invariant.
* Add lemma `mapsto_mapsto_ne : ¬ ✓(q1 + q2)%Qp → l1 ↦{q1} v1 -∗ l2 ↦{q2} v2 -∗ ⌜l1 ≠ l2⌝`.
* Add lemma `is_lock_iff` and show that `is_lock` is contractive.

**Changes in `program_logic`:**

* In the axiomatization of ectx languages, replace the axiom of positivity of
  context composition with an axiom that says if `fill K e` takes a head step,
  then either `K` is the empty evaluation context or `e` is a value.

**Changes in the logic (`base_logic`, `bi`):**

* Rename some accessor-style lemmas to consistently use the suffix `_acc`
  instead of `_open`:
  `inv_open` → `inv_acc`, `inv_open_strong` → `inv_acc_strong`,
  `inv_open_timeless` → `inv_acc_timeless`, `na_inv_open` → `na_inv_acc`,
  `cinv_open` → `cinv_acc`, `cinv_open_strong` → `cinv_acc_strong`,
  `auth_open` → `auth_acc`, `sts_open` → `sts_acc`.
  To make this work, also rename `inv_acc` → `inv_alter`.
  (Most developments should be unaffected as the typical way to invoke these
  lemmas is through `iInv`, and that does not change.)
* Change `inv_iff`, `cinv_iff` and `na_inv_iff` to make order of arguments
  consistent and more convenient for `iApply`. They are now of the form
  `inv N P -∗ ▷ □ (P ↔ Q) -∗ inv N Q` and (similar for `na_inv_iff` and
  `cinv_iff`), following e.g., `inv_alter` and `wp_wand`.
* Rename `inv_sep_1` → `inv_split_1`, `inv_sep_2` → `inv_split_2`, and
  `inv_sep` → `inv_split` to be consistent with the naming convention in boxes.
* Update the strong variant of the accessor lemma for cancellable invariants to
  match that of regular invariants, where you can pick the mask at a later time.
  (The other part that makes it strong is that you get back the token for the
  invariant immediately, not just when the invariant is closed again.)
* Rename `iProp`/`iPreProp` to `iPropO`/`iPrePropO` since they are `ofeT`s.
  Introduce `iProp` for the `Type` carrier of `iPropO`.
* Flatten the BI hierarchy by merging the `bi` and `sbi` canonical structures.
  This gives significant performance benefits on developments that construct BIs
  from BIs (e.g., use `monPred`). For, example it gives a performance gain of 37%
  overall on lambdarust-weak, with improvements for individual files up to 72%,
  see Iris issue #303. The concrete changes are as follows:
  + The `sbi` canonical structure has been removed.
  + The `bi` canonical structure contains the later modality. It does not
    require the later modality to be contractive or to satisfy the Löb rule, so
    we provide a smart constructor `bi_later_mixin_id` to get the later axioms
    "for free" if later is defined to be the identity function.
  + There is a separate class `BiLöb`, and a "free" instance of that class if
    the later modality is contractive. A `BiLöb` instance is required for the
    `iLöb` tactic, and for timeless instances of implication and wand.
  + There is a separate type class `BiInternalEq` for BIs with a notion of
    internal equality (internal equality was part of `sbi`). An instance of this
    class is needed for the `iRewrite` tactic, and the various lemmas about
    internal equality.
  + The class `SbiEmbed` has been removed and been replaced by classes
    `BiEmbedLater` and `BiEmbedInternalEq`.
  + The class `BiPlainly` has been generalized to BIs without internal equality.
    As a consequence, there is a separate class `BiPropExt` for BIs with
    propositional extensionality (i.e., `■ (P ∗-∗ Q) ⊢ P ≡ Q`).
  + The class `BiEmbedPlainly` is a bi-entailment (i.e., `⎡■ P⎤ ⊣⊢ ■ ⎡P⎤`
    instead of `■ ⎡P⎤ ⊢ ⎡■ P⎤`) as it has been generalized to BIs without a
    internal equality. In the past, the left-to-right direction was obtained for
    "free" using the rules of internal equality.
* Remove coercion from `iProp` (and other MoSeL propositions) to `Prop`.
  Instead, use the new unary notation `⊢ P`, or `⊢@{PROP} P` if the proposition
  type cannot be inferred. This also means that `%I` should not be necessary any
  more when stating lemmas, as `P` above is automatically parsed in scope `%I`.
* Some improvements to the `bi/lib/core` construction:
  + Rename `coreP_wand` into `coreP_entails` since it does not involve wands.
  + Generalize `coreP_entails` to non-affine BIs, and prove more convenient
    version `coreP_entails'` for `coreP P` with `P` affine.
  + Add instance `coreP_affine P : Affine P → Affine (coreP P)` and
    lemma `coreP_wand P Q : <affine> ■ (P -∗ Q) -∗ coreP P -∗ coreP Q`.
* Remove notation for 3-mask step-taking updates, and made 2-mask notation less
  confusing by distinguishing it better from mask-changing updates.
  Old: `|={Eo,Ei}▷=> P`. New: `|={Eo}[Ei]▷=> P`.
  Here, `Eo` is the "outer mask" (used at the beginning and end) and `Ei` the
  "inner mask" (used around the ▷ in the middle).
  As part of this, the lemmas about the 3-mask variant were changed to be about
  the 2-mask variant instead, and `step_fupd_mask_mono` now also has a more
  consistent argument order for its masks.

* Add a counterexample showing that sufficiently powerful cancellable invariants
  with a linear token subvert the linearity guarantee (see
  `bi.lib.counterexmples` for details).
* Redefine invariants as "semantic invariants" so that they support
  splitting and other forms of weakening.
* Add lemmas `inv_combine` and `inv_combine_dup_l` for combining invariants.
* Add the type `siProp` of "plain" step-indexed propositions, together with
  basic proofmode support.
* New ASCII versions of Iris notations. These are marked parsing only and
  can be made available using `Require Import iris.bi.ascii`. The new
  notations are (notations marked [†] are disambiguated using notation scopes):
  - entailment: `|-` for `⊢` and `-|-` for `⊣⊢`
  - logic[†]: `->` for `→`, `/\\` for `∧`, `\\/` for `∨`, and `<->` for `↔`
  - quantifiers[†]: `forall` for `∀` and `exists` for `∃`
  - separation logic: `**` for `∗`, `-*` for `-∗`, and `*-*` for `∗-∗`
  - step indexing: `|>` for `▷`
  - modalities: `<#>` for `□` and `<except_0>` for `◇`
  - most derived notations can be computed from previous notations using the
    substitutions above, e.g. replace `∗` with `*` and `▷` with `|>`. Examples
    include the following:
    - `|={E1,E2}=* P` for `|={E1,E2}=∗`
    - `P ={E}=* Q` for `P ={E}=∗ Q`
    - `P ={E1,E2}=* Q` for `P ={E1,E2}=∗ Q`
    - `|={E1}[E2]|>=> Q` for `|={E1}[E2]▷=> Q`
    The full list can be found in [theories/bi/ascii.v](theories/bi/ascii.v),
    where the ASCII notations are defined in terms of the unicode notations.
* Add affine, absorbing, persistent and timeless instances for telescopes.
* Add a construction `bi_rtc` to create reflexive transitive closures of
  PROP-level binary relations.
* Slightly strengthen the lemmas `big_sepL_nil'`, `big_sepL2_nil'`,
  `big_sepM_nil'` `big_sepM2_empty'`, `big_sepS_empty'`, and `big_sepMS_empty'`.
  They now only require that the argument `P` is affine instead of the whole BI
  being affine.
* Add `big_sepL_insert_acc`, a variant of `big_sepL_lookup_acc` which allows
  updating the value.
* Add many missing `Proper`/non-expansiveness lemmas for big-ops.
* Add `big_*_insert_delete` lemmas to split a `<[i:=x]> m` map into `i` and the rest.
* Seal the definitions of `big_opS`, `big_opMS`, `big_opM` and `big_sepM2`
  to prevent undesired simplification.
* Fix `big_sepM2_fmap*` only working for `nat` keys.

**Changes in `proofmode`:**

* Make use of `notypeclasses refine` in the implementation of `iPoseProof` and
  `iAssumption`, see <https://gitlab.mpi-sws.org/iris/iris/merge_requests/329>.
  This has two consequences:
  1. Coq's "new" unification algorithm (the one in `refine`, not the "old" one
     in `apply`) is used more often by the proof mode tactics.
  2. Due to the use of `notypeclasses refine`, TC constraints are solved less
     eagerly, see <https://github.com/coq/coq/issues/6583>.
  In order to port your development, it is often needed to instantiate evars
  explicitly (since TC search is performed less eagerly), and in few cases it is
  needed to unfold definitions explicitly (due to new unification algorithm
  behaving differently).
* Strengthen the tactics `iDestruct`, `iPoseProof`, and `iAssert`:
  - They succeed in certain cases where they used to fail.
  - They keep certain hypotheses in the intuitionistic context, where they were
    moved to the spatial context before.
  The latter can lead to stronger proof mode contexts, and therefore to
  backwards incompatibility. This can usually be fixed by manually clearing some
  hypotheses. A more detailed description of the changes can be found in
  <https://gitlab.mpi-sws.org/iris/iris/merge_requests/341>.
* Remove the long-deprecated `cofeT` alias (for `ofeT`) and `dec_agree` RA (use
  `agree` instead).

* Add `auto` hint for `∗-∗`.
* Add new tactic `iStopProof` to turn the proof mode entailment into an ordinary
  Coq goal `big star of context ⊢ proof mode goal`.
* Add new introduction pattern `-# pat` that moves a hypothesis from the
  intuitionistic context to the spatial context.
* The tactic `iAssumption` also recognizes assumptions `⊢ P` in the Coq context.
* Better support for telescopes in the proof mode, i.e., all tactics should
  recognize and distribute telescopes now.
* The proof mode now supports names for pure facts in intro patterns. Support
  requires implementing `string_to_ident`. Without this tactic such patterns
  will fail. We provide one implementation using Ltac2 which works with Coq 8.11
  and can be installed with opam; see
  [iris/string-ident](https://gitlab.mpi-sws.org/iris/string-ident) for details.

**Changes in `algebra`:**

* Remove `Core` type class for defining the total core; it is now always
  defined in terms of the partial core. The only user of this type class was the
  STS RA.
* The functions `{o,r,ur}Functor_diag` are no longer coercions, and renamed into
  `{o,r,ur}Functor_apply` to better match their intent. This fixes "ambiguous
  coercion path" warnings.
* Rename `{o,r,ur}Functor_{ne,id,compose,contractive}` into
  `{o,r,ur}Functor_map_{ne,id,compose,contractive}`.
* Move derived camera constructions (`frac_auth` and `ufrac_auth`) to the folder
  `algebra/lib`.
* Rename `mnat` to `max_nat` and "box" it by creating a separate type for it.
* Move the RAs for `nat` and `positive` and the `mnat` RA into a separate
  module. They must now be imported from `From iris.algebra Require Import
  numbers`.
* Make names of `f_op`/`f_core` rewrite lemmas more consistent by always making
  `_core`/`_op` the suffix:
  `op_singleton` → `singleton_op`, `core_singleton` → `singleton_core`,
  `discrete_fun_op_singleton` → `discrete_fun_singleton_op`,
  `discrete_fun_core_singleton` → `discrete_fun_singleton_core`,
  `list_core_singletonM` → `list_singleton_core`,
  `list_op_singletonM` → `list_singleton_op`,
  `sts_op_auth_frag` → `sts_auth_frag_op`,
  `sts_op_auth_frag_up` → `sts_auth_frag_up_op`,
  `sts_op_frag` → `sts_frag_op`,
  `list_op_length` → `list_length_op`,
  `list_core_singletonM` → `list_singletonM_core`,
  `list_op_singletonM` → `list_singletonM_op`.
* All list "map singleton" lemmas consistently use `singletonM` in their name:
  `list_singleton_valid` → `list_singletonM_valid`,
  `list_singleton_core_id` → `list_singletonM_core_id`,
  `list_singleton_snoc` → `list_singletonM_snoc`,
  `list_singleton_updateP` → `list_singletonM_updateP`,
  `list_singleton_updateP'` → `list_singletonM_updateP'`,
  `list_singleton_update` → `list_singletonM_update`,
  `list_alloc_singleton_local_update` → `list_alloc_singletonM_local_update`.
* Remove `auth_both_op` and rename `auth_both_frac_op` into `auth_both_op`.
* Add lemma `singleton_included : {[ i := x ]} ≼ ({[ i := y ]} ↔ x ≡ y ∨ x ≼ y`,
  and rename existing asymmetric lemmas (with a singleton on just the LHS):
  `singleton_includedN` → `singleton_includedN_l`,
  `singleton_included` → `singleton_included_l`,
  `singleton_included_exclusive` → `singleton_included_exclusive_l`.

* Add notion `ofe_iso A B` that states that OFEs `A` and `B` are
  isomorphic. This is used in the COFE solver.
* Add `{o,r,ur}Functor_oFunctor_compose` for composition of functors.
* Add `pair_op_1` and `pair_op_2` to split a pair where one component is the unit.
* Add derived camera construction `excl_auth A` for `auth (option (excl A))`.
* Make lemma `Excl_included` a bi-implication.
* Make `auth_update_core_id` work with any fraction of the authoritative
  element.
* Add `min_nat`, an RA for natural numbers with `min` as the operation.
* Add many missing `Proper`/non-expansiveness lemmas for maps and lists.
* Add `list_singletonM_included` and `list_lookup_singletonM_{lt,gt}` lemmas
  about singletons in the list RA.
* Add `list_core_id'`, a stronger version of `list_core_id` which only talks
  about elements that are actually in the list.

The following `sed` script helps adjust your code to the renaming (on macOS,
replace `sed` by `gsed`, installed via e.g. `brew install gnu-sed`).
Note that the script is not idempotent, do not run it twice.
```
sed -i -E '
# functor renames
s/\b(o|r|ur)Functor_(ne|id|compose|contractive)\b/\1Functor_map_\2/g
# singleton_included renames
s/\bsingleton_includedN\b/singleton_includedN_l/g
s/\bsingleton_included\b/singleton_included_l/g
s/\bsingleton_included_exclusive\b/singleton_included_exclusive_l/g
# f_op/f_core renames
s/\b(op|core)_singleton\b/singleton_\1/g
s/\bdiscrete_fun_(op|core)_singleton\b/discrete_fun_singleton_\1/g
s/\bsts_op_(auth_frag|auth_frag_up|frag)\b/sts_\1_op/g
s/\blist_(op|core)_singletonM\b/list_singletonM_\1/g
s/\blist_op_length\b/list_length_op/g
# list "singleton map" renames
s/\blist_singleton_valid\b/list_singletonM_valid/g
s/\blist_singleton_core_id\b/list_singletonM_core_id/g
s/\blist_singleton_snoc\b/list_singletonM_snoc/g
s/\blist_singleton_updateP\b/list_singletonM_updateP/g
s/\blist_singleton_update\b/list_singletonM_update/g
s/\blist_alloc_singleton_local_update\b/list_alloc_singletonM_local_update/g
# inv renames
s/\binv_sep(|_1|_2)\b/inv_split\1/g
s/\binv_acc\b/inv_alter/g
s/\binv_open(|_strong|_timeless)\b/inv_acc\1/g
s/\bcinv_open(|_strong)\b/cinv_acc\1/g
s/\b(na_inv|auth|sts)_open\b/\1_acc/g
# miscellaneous
s/\bauth_both_frac_op\b/auth_both_op/g
s/\bmnat\b/max_nat/g
s/\bcoreP_wand\b/coreP_entails/g
' $(find theories -name "*.v")
```

## Iris 3.2.0 (released 2019-08-29)

The highlight of this release is the completely re-engineered interactive proof
mode.  Not only did many tactics become more powerful; the entire proof mode can
now be used not just for Iris but also for other separation logics satisfying
the proof mode interface (e.g., [Iron] and [GPFSL]). Also see the
[accompanying paper][MoSeL].

[Iron]: https://iris-project.org/iron/
[GPFSL]: https://gitlab.mpi-sws.org/iris/gpfsl/
[MoSeL]: https://iris-project.org/mosel/

Beyond that, the Iris program logic gained the ability to reason about
potentially stuck programs, and a significantly strengthened adequacy theorem
that unifies the three previously separately presented theorems.  There are now
also Hoare triples for total program correctness (but with very limited support
for invariants) and logical atomicity.

And finally, our example language HeapLang was made more realistic
(Compare-and-set got replaced by compare-exchange and limited to only compare
values that can actually be compared atomically) and more powerful, with added
support for arrays and prophecy variables.

Further details are given in the changelog below.

This release of Iris received contributions by Aleš Bizjak, Amin Timany, Dan
Frumin, Glen Mével, Hai Dang, Hugo Herbelin, Jacques-Henri Jourdan, Jan Menz,
Jan-Oliver Kaiser, Jonas Kastberg Hinrichsen, Joseph Tassarotti, Mackie Loeffel,
Marianna Rapoport, Maxime Dénès, Michael Sammler, Paolo G. Giarrusso,
Pierre-Marie Pédrot, Ralf Jung, Robbert Krebbers, Rodolphe Lepigre, and Tej
Chajed. Thanks a lot to everyone involved!

**Changes in the theory of Iris itself:**

* Change in the definition of WP, so that there is a fancy update between
  the quantification over the next states and the later modality. This makes it
  possible to prove more powerful lifting lemmas: The new versions feature an
  "update that takes a step".
* Add weakest preconditions for total program correctness.
* "(Potentially) stuck" weakest preconditions and the "plainly modality" are no
  longer considered experimental.
* Add the notion of an "observation" to the language interface, so that
  every reduction step can optionally be marked with an event, and an execution
  trace has a matching list of events.  Change WP so that it is told the entire
  future trace of observations from the beginning.
* The Löb rule is now a derived rule; it follows from later-intro, later
  being contractive and the fact that we can take fixpoints of contractive
  functions.
* Add atomic updates and logically atomic triples, including tactic support.
  See `heap_lang/lib/increment.v` for an example.
* Extend the state interpretation with a natural number that keeps track of
  the number of forked-off threads, and have a global fixed proposition that
  describes the postcondition of each forked-off thread (instead of it being
  `True`).
* A stronger adequacy statement for weakest preconditions that involves
  the final state, the post-condition of forked-off threads, and also applies if
  the main-thread has not terminated.
* The user-chosen functor used to instantiate the Iris logic now goes from
  COFEs to Cameras (it was OFEs to Cameras).

**Changes in heap_lang:**

* CAS (compare-and-set) got replaced by CmpXchg (compare-exchange).  The
  difference is that CmpXchg returns a pair consisting of the old value and a
  boolean indicating whether the comparison was successful and hence the
  exchange happened.  CAS can be obtained by simply projecting to the second
  component, but also providing the old value more closely models the primitive
  typically provided in systems languages (C, C++, Rust).
  The comparison by this operation also got weakened to be efficiently
  implementable: CmpXchg may only be used to compare "unboxed" values that can
  be represented in a single machine word. It is sufficient if one of the two
  compared values is unboxed.
* For consistency, the restrictions CmpXchg imposes on comparison also apply to
  the `=` binary operator. This also fixes the long-standing problem that that
  operator allowed compared closures with each other.
* Implement prophecy variables using the new support for "observations". The
  erasure theorem (showing that prophecy variables do not alter program
  behavior) can be found [in the iris/examples repository][prophecy-erasure].
* heap_lang now uses right-to-left evaluation order. This makes it
  significantly easier to write specifications of curried functions.
* heap_lang values are now injected in heap_lang expressions via a specific
  constructor of the expr inductive type. This simplifies much the tactical
  infrastructure around the language. In particular, this allow us to get rid
  the reflection mechanism that was needed for proving closedness, atomicity and
  "valueness" of a term. The price to pay is the addition of new
  "administrative" reductions in the operational semantics of the language.
* heap_lang now has support for allocating, accessing and reasoning about arrays
  (continuously allocated regions of memory).
* One can now assign "meta" data to heap_lang locations.

[prophecy-erasure]: https://gitlab.mpi-sws.org/iris/examples/blob/3f33781fe6e19cfdb25259c8194d34403f1134d5/theories/logatom/proph_erasure.v

**Changes in Coq:**

* An all-new generalized proof mode that abstracts away from Iris!  Major new
  features:
  - The proof mode can now be used with logics derived from Iris (like iGPS),
    with non-step-indexed logics and even with non-affine (i.e., linear) logics.
  - `iModIntro` is more flexible and more powerful, it now also subsumes
    `iNext` and `iAlways`.
  - General infrastructure for deriving a logic for monotone predicates over
    an existing logic (see the paper for more details).
    Developments instantiating the proof mode typeclasses may need significant
    changes.  For developments just using the proof mode tactics, porting should
    not be too much effort.  Notable things to port are:
  - All the BI laws moved from the `uPred` module to the `bi` module.  For
    example, `uPred.later_equivI` became `bi.later_equivI`.
  - Big-ops are automatically imported, imports of `iris.base_logic.big_op` have
    to be removed.
  - The ⊢ notation can sometimes infer different (but convertible) terms when
    searching for the BI to use, which (due to Coq limitations) can lead to
    failing rewrites, in particular when rewriting at function types.
* The `iInv` tactic can now be used without the second argument (the name for
  the closing update).  It will then instead add the obligation to close the
  invariant to the goal.
* The new `iEval` tactic can be used to execute a simplification or rewriting
  tactic on some specific part(s) of the proofmode goal.
* Added support for defining derived connectives involving n-ary binders using
  telescopes.
* The proof mode now more consistently "prettifies" the goal after each tactic.
  Prettification also simplifies some BI connectives, like conditional
  modalities and telescope quantifiers.
* Improved pretty-printing of Iris connectives (in particular WP and fancy
  updates) when Coq has to line-wrap the output.  This goes hand-in-hand with an
  improved test suite that also tests pretty-printing.
* Added a `gmultiset` RA.
* Rename `timelessP` → `timeless` (projection of the `Timeless` class)
* The CMRA axiom `cmra_extend` is now stated in `Type`, using `sigT` instead of
  in `Prop` using `exists`. This makes it possible to define the function space
  CMRA even for an infinite domain.
* Rename proof mode type classes for laters:
  - `IntoLaterN` → `MaybeIntoLaterN` (this one _may_ strip a later)
  - `IntoLaterN'` → `IntoLaterN` (this one _should_ strip a later)
  - `IntoLaterNEnv` → `MaybeIntoLaterNEnv`
  - `IntoLaterNEnvs` → `MaybeIntoLaterNEnvs`
* Rename:
  - `frag_auth_op` → `frac_auth_frag_op`
  - `cmra_opM_assoc` → `cmra_op_opM_assoc`
  - `cmra_opM_assoc_L` → `cmra_op_opM_assoc_L`
  - `cmra_opM_assoc'` → `cmra_opM_opM_assoc`
* `namespaces` has been moved to std++.
* Changed `IntoVal` to be directly usable for rewriting `e` into `of_val v`, and
  changed `AsVal` to be usable for rewriting via the `[v <-]` destruct pattern.
* `wp_fork` is now written in curried form.
* `PureExec`/`wp_pure` now supports taking multiple steps at once.
* A new tactic, `wp_pures`, executes as many pure steps as possible, excluding
  steps that would require unlocking subterms. Every impure wp_ tactic executes
  this tactic before doing anything else.
* Add `big_sepM_insert_acc`.
* Add big separating conjunctions that operate on pairs of lists (`big_sepL2`)
  and on pairs of maps (`big_sepM2`). In the former case the lists are required
  to have the same length, and in the latter case the maps are required to
  have the same domains.
* The `_strong` lemmas (e.g. `own_alloc_strong`) work for all infinite
  sets, instead of just for cofinite sets. The versions with cofinite
  sets have been renamed to use the `_cofinite` suffix.
* Remove locked value lambdas. The value scope notations `rec: f x := e` and
  `(λ: x, e)` no longer add a `locked`. Instead, we made the `wp_` tactics
  smarter to no longer unfold lambdas/recs that occur behind definitions.
* Export the fact that `iPreProp` is a COFE.
* The CMRA `auth` now can have fractional authoritative parts. So now `auth` has
  3 types of elements: the fractional authoritative `●{q} a`, the full
  authoritative `● a ≡ ●{1} a`, and the non-authoritative `◯ a`. Updates are
  only possible with the full authoritative element `● a`, while fractional
  authoritative elements have agreement: `✓ (●{p} a ⋅ ●{q} b) ⇒ a ≡ b`. As a
  consequence, `auth` is no longer a COFE and does not preserve Leibniz
  equality.
* Add a COFE construction (and functor) on dependent pairs `sigTO`, dual to
  `discrete_funO`.
* Rename in `auth`:
  - Use `auth_auth_proj`/`auth_frag_proj` for the projections of `auth`:
    `authoritative` → `auth_auth_proj` and `auth_own` → `auth_frag_proj`.
  - Use `auth_auth` and `auth_frag` for the injections into authoritative
    elements and non-authoritative elements respectively.
  - Lemmas for the projections and injections are renamed accordingly.
    For examples:
    + `authoritative_validN` → `auth_auth_proj_validN`
    + `auth_own_validN` → `auth_frag_proj_validN`
    + `auth_auth_valid` was not renamed because it was already used for the
      authoritative injection.
  - `auth_both_valid` → `auth_both_valid_2`
  - `auth_valid_discrete_2` → `auth_both_valid`
* Add the camera `ufrac` for unbounded fractions (i.e. without fractions that