Lean.Meta.getConst? has been renamed. We have renamed getConst? to getUnfoldableConst? (and getConstNoEx? to getUnfoldableConstNoEx?). These were not intended to be part of the public API, but downstream projects had been using them (sometimes expecting different behaviour) incorrectly instead of Lean.getConstInfo.

dsimp / simp / simp_all now fail by default if they make no progress.

This can be overridden with the (config := { failIfUnchanged := false }) option. This change was made to ease manual use of simp (with complicated goals it can be hard to tell if it was effective) and to allow easier flow control in tactics internally using simp. See the summary discussion on zulip for more details.

simp_all now preserves order of hypotheses.

In order to support the failIfUnchanged configuration option for dsimp / simp / simp_all the way simp_all replaces hypotheses has changed. In particular it is now more likely to preserve the order of hypotheses. See simp_all reorders hypotheses unnecessarily. (Previously all non-dependent propositional hypotheses were reverted and reintroduced. Now only such hypotheses which were changed, or which come after a changed hypothesis, are reverted and reintroduced. This has the effect of preserving the ordering amongst the non-dependent propositional hypotheses, but now any dependent or non-propositional hypotheses retain their position amongst the unchanged non-dependent propositional hypotheses.) This may affect proofs that use rename_i, case ... =>, or next ... =>.

New have this implementation.

this is now a regular identifier again that is implicitly introduced by anonymous have := for the remainder of the tactic block. It used to be a keyword that was visible in all scopes and led to unexpected behavior when explicitly used as a binder name.

Show typeclass and tactic names in profile output.

Make calc require the sequence of relation/proof-s to have the same indentation, and add calc alternative syntax allowing underscores _ in the first relation.

The flexible indentation in calc was often used to align the relation symbols:

example (x y : Nat) : (x + y) * (x + y) = x * x + y * x + x * y + y * y :=
  calc
      (x + y) * (x + y) = (x + y) * x + (x + y) * y       := by rw [Nat.mul_add]
                      -- improper indentation
                      _ = x * x + y * x + (x + y) * y     := by rw [Nat.add_mul]
                      _ = x * x + y * x + (x * y + y * y) := by rw [Nat.add_mul]
                      _ = x * x + y * x + x * y + y * y   := by rw [←Nat.add_assoc]

This is no longer legal. The new syntax puts the first term right after the calc and each step has the same indentation:

example (x y : Nat) : (x + y) * (x + y) = x * x + y * x + x * y + y * y :=
  calc (x + y) * (x + y)
    _ = (x + y) * x + (x + y) * y       := by rw [Nat.mul_add]
    _ = x * x + y * x + (x + y) * y     := by rw [Nat.add_mul]
    _ = x * x + y * x + (x * y + y * y) := by rw [Nat.add_mul]
    _ = x * x + y * x + x * y + y * y   := by rw [←Nat.add_assoc]

Update Lake to latest prerelease.

Make go-to-definition on a typeclass projection application go to the instance(s).

Include timings in trace messages when profiler is true.

Pretty-print signatures in hover and #check <ident>.

Introduce parser memoization to avoid exponential behavior.

feat: allow doSeq in let x <- e | seq.

Add hover/go-to-def/refs for options.

Add empty type ascription syntax (e :).

Make tokens in <|> relevant to syntax match.

Add linter.deprecated option to silence deprecation warnings.

Improve fuzzy-matching heuristics.

Implementation-detail hypotheses.

Hover information for cases/induction case names.

Prefer longer parse even if unsuccessful.

Show declaration module in hover.

New conv mode structuring tactics.

simp can track information and can print an equivalent simp only. PR #1626.

Enforce uniform indentation in tactic blocks / do blocks. See issue #1606.

Moved AssocList, HashMap, HashSet, RBMap, RBSet, PersistentArray, PersistentHashMap, PersistentHashSet to the Lean package. The standard library contains versions that will evolve independently to simplify bootstrapping process.

Standard library moved to the std4 GitHub repository.

InteractiveGoals now has information that a client infoview can use to show what parts of the goal have changed after applying a tactic. PR #1610.

Add [inheritDoc] attribute. PR #1480.

Expose that panic = default. PR #1614.

New code generator project has started.

Remove description argument from register_simp_attr. PR #1566.

Additional concurrency primitives.

Collapsible traces with messages.

Hygienic resolution of namespaces.

New Float functions.

Many new doc strings have been added to declarations at Init.