Solver and symbolic reasoning got a big upgrade
Lean spent much of the month hardening grind and expanding the newer SymM stack. grind got multiple invariant fixes, better proof-hint construction, clearer local theorem diagnostics, tighter generation limits, and improved cache-key handling; SymM gained full dsimp support, binder-aware simplification, reusable reduction simprocs, and better universe matching. On top of that, premise selection was improved via MePo fixes and better filtering/ordering.
mvcgen' is maturing into a first-class VC generator
The experimental mvcgen' tactic was extended repeatedly: it now integrates with sym => blocks, handles local specs per subgoal, supports typeclass-method projections, and picked up several fixes around goal rebuilding and spec migration. In parallel, Lean added verifiable loop support via whileM/forIn specifications, making the verification story for control flow substantially stronger.
Elaboration, diagnostics, and linting became more precise
Several user-facing elaboration issues were fixed, including leaked metavariables in docstrings, private-import instance leakage, incorrect termination call-site reporting, structural recursion regressions, and transparency-related tactic failures. Diagnostics also improved in a few places: try? became info-tree based and extensible, simpa using behavior was tightened with an opt-in escape hatch, #where now reports module scope state, and multiple linters were upstreamed or improved (unusedVariables, unusedDecidableInType, checkUnivs, defProp, dupNamespace, unreachableTactic).
Lake, plugin loading, and module handling were cleaned up
Lake gained more correct dependency/header ordering, transitive meta import artifact handling, better shake --explain output, workspace-hoisted compiled configs, and more stable cache/test behavior. Plugin loading was generalized with explicit init functions and safer CLI syntax, while module loading and export handling were adjusted for interrupted reads, OLeanEntries, and public-vs-private scope correctness.
Runtime, serialization, and platform safety improved
Lean upgraded LLVM to 22, added faster string comparison and specialized dec lowering, and tightened runtime allocator failure handling to panic safely instead of corrupting state. Serialization also moved forward with a compacted save/read API and experimental closure serialization for CompactedRegion.save, plus assorted fixes for module data and cross-file sharing.
Other misc changes
Docstring rendering gained blockquote support; time/date APIs were expanded with wall time, locales, and week-aware formatting; do elaboration now supports indexed monads; impossible was added; UInt8.ofNatTruncate was renamed to ofNatClamp; and CI/test infrastructure saw many stability and tooling cleanups.