## Major Changes
Portable Types
In #1266, we added support for transpiling portable integer types like size_t and uint64_t.  Previously, these were translated in a platform-dependent way, as C defines these portable types as typedefs of the platform-dependent primitive types like int and long.

Now we special case these portable integer types and make sure to translate them as the equivalent Rust portable types, so size_t becomes usize and uint64_t becomes u64, for example.
These still often go through multiple typedefs in C, which become multiple type aliases in Rust, so the definitions are not always textually identical in Rust, but the underlying types should always be portable now.
Const Macros
We previously had an unstable and often broken --translate-const-macros option, which translated const/object-like C macros as Rust consts.  This has now been significantly improved and --translate-const-macros conservative is enabled by default.  The option has been split into none, conservative, and experimental, so we still allow the previous more experimental translations under experimental, but conservative should generally work and thus is enabled by default.  However, some const macros are not yet translatable, like macros referencing other variables, functions, etc., which conservative skips, while some const macros can never be properly translated as a Rust const, such as when it references a variable that is not yet defined.  We're still working on improving these cases so that more const macros can be successfully translated, but the current version should capture most usages in the wild.  For tmux, 98% of const macro definitions and expansions are successfully translated, for example.
One area in particular we're still working on is the combination of const macros and portable types.  The features don't yet work together seamlessly, with const macro types using the old non-portable types and then casting to the portable type at use sites.
SrcLoc Sorting
On stable, sorting now panics when a non-total order is discovered, which was the case in fn compare_src_locs.  We had previously tried to fix this, and fixed it in a bunch of cases, but it was still cropping up sometimes, like when transpiling libxml2.
We've now fixed the root cause of incorrect SrcLoc::fileids when we calculate #include paths.  Besides fixing any panics, this should improve the sorting of items, which was sometimes buggy before, likely due to this.
cmake 4.0 Support
#1214 adds support for cmake 4.0 by increasing the minimum to cmake 3.5.  This should fix any installation issues due to using a new cmake.
LLVM/Clang Support

(#1243, #1244) LLVM 18
(#1230) LLVM 19
(#1248) LLVM 20
(#1274) LLVM 21

Misc. Bug Fixes

(#1265) Handle all cases of extra braces in string initializers.
(#1340) Remove extraneous :: separators in paths with arguments (e.g. ::core::mem::size_of::<libc::c_int>::()).
(#1359) Store alloca vector at function-level scope, matching C.
(#1360) Avoid mutable transmutes (UB) for string literals (mem::transmute::<[u8; N], [c_char; N]>(*b"") instead of *mem::transmute::<&[u8; N], &mut [c_char; N]>(b"")).
(#1361) Support anonymous struct/union/enums within structs and unions.
(#1374) Inline asm fixes for ioq3.

Misc. Improvements

(#1221, #1222, #1229, #1381) Prefer core:: over libc:: and std::, fixing --emit-no-std.
(#1218) Prefer [x; N] over [x, ..., x].
(#1344) Skip redundant expressions for ++ and -- (i += 1; i; vs. i += 1;).
(#1373) use libc only when needed.

New Instrinsics Added

(#814) C11 atomics
(#1342) _Static_assert
(#1235) __float128
(#1259, #1260) __builtin_rotateright*
(#1263, #1292) __builtin_ia32_pause
(#1263, #1292) __builtin_arm_yield
(#1129) __builtin_ia32_vperm2f128_pd256
(#1129) _mm256_permute2f128_pd

Changelog

docs/readme: Update bear arguments by @javierhonduco in docs/readme: Update bear arguments immunant/c2rust#1206
GH Actions: bump tagged versions of actions/cache by @brk in GH Actions: bump tagged versions of actions/cache immunant/c2rust#1212
build: support cmake 4.0 by @chenrui333 in build: support cmake 4.0 immunant/c2rust#1214
CI updates (Ubuntu 22, LLVM 15, pinned once_cell) by @folkertdev in CI updates (Ubuntu 22, LLVM 15, pinned once_cell) immunant/c2rust#1219
Remove some libc dependencies. by @nnethercote in Remove some libc dependencies. immunant/c2rust#1221
Fixes to rust-toolchain.toml handling by @joshtriplett in Fixes to rust-toolchain.toml handling immunant/c2rust#1225
Reduce libc usage in c2rust-transpile by @nnethercote in Reduce libc usage in c2rust-transpile immunant/c2rust#1222
Remove more unnecessary libc usage by @nnethercote in Remove more unnecessary libc usage immunant/c2rust#1229
Make test_translator.py tests pass on LLVM 19 by @nnethercote in Make test_translator.py tests pass on LLVM 19 immunant/c2rust#1230
Support __float128, which shows up in clang headers on some systems by @joshtriplett in Support __float128, which shows up in clang headers on some systems immunant/c2rust#1235
Allow running multiple transpiles in parallel by @jameysharp in Allow running multiple transpiles in parallel immunant/c2rust#1241
transpile: remove invalid Punct::new(' ') in fn convert_asm and update proc-macro2 by @kkysen in transpile: remove invalid Punct::new(' ') in fn convert_asm and update proc-macro2 immunant/c2rust#1244
Move Linux CI from Azure Pipelines to GitHub Actions by @joshtriplett in Move Linux CI from Azure Pipelines to GitHub Actions immunant/c2rust#1232
Bump bindgen to be able to handle CXCursor_LinkageSpec in LLVM18+ by @spastorino in Bump bindgen to be able to handle CXCursor_LinkageSpec in LLVM18+ immunant/c2rust#1243
Fix getBitWidthValue compilation error on LLVM 20+ by @shirok1 in Fix getBitWidthValue compilation error on LLVM 20+ immunant/c2rust#1248
Introduce some snapshot tests of c2rust-transpile by @jameysharp in Introduce some snapshot tests of c2rust-transpile immunant/c2rust#1246
transpile: some snapshot test fixes by @kkysen in transpile: some snapshot test fixes immunant/c2rust#1258
Support C11 atomics required by dav1d by @thedataking in Support C11 atomics required by dav1d immunant/c2rust#814
transpile: impl __builtin_ia32_pause and __builtin_arm_yield by @thedataking in transpile: impl __builtin_ia32_pause and __builtin_arm_yield immunant/c2rust#1263
Various small cleanups by @fw-immunant in Various small cleanups immunant/c2rust#1267
More driveby cleanups by @fw-immunant in More driveby cleanups immunant/c2rust#1268
transpile: Add SIMD intrinsic __builtin_ia32_vperm2f128_pd256/_mm256_permute2f128_pd by @Kriskras99 in transpile: Add SIMD intrinsic __builtin_ia32_vperm2f128_pd256/_mm256_permute2f128_pd immunant/c2rust#1129
transpile: fix doc tests on aarch64 (c_char is unsigned there) by @kkysen in transpile: fix doc tests on aarch64 (c_char is unsigned there) immunant/c2rust#1255
transpile: impl __builtin_rotateright* as core::intrinsics::rotate_right by @kkysen in transpile: impl __builtin_rotateright* as core::intrinsics::rotate_right immunant/c2rust#1259
transpile: emit .rotate_{left,right} instead of core::intrinsics::rotate_{left,right} by @kkysen in transpile: emit .rotate_{left,right} instead of core::intrinsics::rotate_{left,right} immunant/c2rust#1260
Add tests/requirements.txt dependencies into nix python environment by @Chobbes in Add tests/requirements.txt dependencies into nix python environment immunant/c2rust#1269
ci: allow reusing testsuite workflow by @fw-immunant in ci: allow reusing testsuite workflow immunant/c2rust#1272
Allow platform-dependent snapshot test results by @fw-immunant in Allow platform-dependent snapshot test results immunant/c2rust#1270
transpile: operators: resolve type when checking for unsignedness by @fw-immunant in transpile: operators: resolve type when checking for unsignedness immunant/c2rust#1273
fix compilation error on clang/llvm 21 by @bungcip in fix compilation error on clang/llvm 21 immunant/c2rust#1274
ci: switch python to use uv by @kkysen in ci: switch python to use uv immunant/c2rust#1276
ci: move macOS azure-pipelines.yml to GitHub Actions with a matrix by @kkysen in ci: move macOS azure-pipelines.yml to GitHub Actions with a matrix immunant/c2rust#1275
ci: add arch to the matrix for clarity by @kkysen in ci: add arch to the matrix for clarity immunant/c2rust#1284
Fix skipping of Paren expressions in typed AST iterators by @fw-immunant in Fix skipping of Paren expressions in typed AST iterators immunant/c2rust#1286
ci: run on both Ubuntu 22.04 and ubuntu-latest by @fw-immunant in ci: run on both Ubuntu 22.04 and ubuntu-latest immunant/c2rust#1288
Revert "transpile: snapshot: build to stdout rather than creating+deleting rlib" by @kkysen in Revert "transpile: snapshot: build to stdout rather than creating+deleting rlib" immunant/c2rust#1287
ci: reorder some steps to cache more (rustup) and run fast checks earlier by @kkysen in ci: reorder some steps to cache more (rustup) and run fast checks earlier immunant/c2rust#1289
transpile: handle all cases of extra braces in string initializers by @kkysen in transpile: handle all cases of extra braces in string initializers immunant/c2rust#1265
transpile: do not allow f128::new in const contexts by @fw-immunant in transpile: do not allow f128::new in const contexts immunant/c2rust#1294
A couple testing tweaks by @fw-immunant in A couple testing tweaks immunant/c2rust#928
test_translator: remove #![feature(nll)], stabilized in 1.63 by @kkysen in test_translator: remove #![feature(nll)], stabilized in 1.63 immunant/c2rust#1290
scripts: restore provision_mac.sh (removed in #1275) and remove docker scripts by @kkysen in scripts: restore provision_mac.sh (removed in #1275) and remove docker scripts immunant/c2rust#1299
transpile: save platform-specific .rs files, too by @kkysen in transpile: save platform-specific .rs files, too immunant/c2rust#1291
ci: test on aarch64 macOS by @kkysen in ci: test on aarch64 macOS immunant/c2rust#1280
transpile: add arch-specific snapshots by @kkysen in transpile: add arch-specific snapshots immunant/c2rust#1293
transpile: revert __builtin_ia32_pause/__builtin_arm_yield to emit _mm_pause and __yield instead of cross-platform spin_loop by @kkysen in transpile: revert __builtin_ia32_pause/__builtin_arm_yield to emit _mm_pause and __yield instead of cross-platform spin_loop immunant/c2rust#1292
Upgrade to Syn 2 by @bungcip in Upgrade to Syn 2 immunant/c2rust#1271
ci: use awalsh128/cache-apt-pkgs-action@latest to cache apt install by @kkysen in ci: use awalsh128/cache-apt-pkgs-action@latest to cache apt install immunant/c2rust#1277
Remove snapshot .rs files by @fw-immunant in Remove snapshot .rs files immunant/c2rust#1300
transpile: use a repeat expression for arrays with a single zero initializer by @folkertdev in transpile: use a repeat expression for arrays with a single zero initializer immunant/c2rust#1218
transpile: syntax highlight *transpile*.c.snaps as Rust by @kkysen in transpile: syntax highlight *transpile*.c.snaps as Rust immunant/c2rust#1301
transpile: add docs and some renames/refactors for code related to macros by @kkysen in transpile: add docs and some renames/refactors for code related to macros immunant/c2rust#1303
Translate C integer types to portable Rust types by @fw-immunant in Translate C integer types to portable Rust types immunant/c2rust#1266
transpile: tests: snapshots: hard-code nightly toolchain for now by @kkysen in transpile: tests: snapshots: hard-code nightly toolchain for now immunant/c2rust#1310
tests: fix out-of-bounds UB in test_casts.rs and cast_funptr.c by @kkysen in tests: fix out-of-bounds UB in test_casts.rs and cast_funptr.c immunant/c2rust#1309
transpile: add macros.c snapshot by @kkysen in transpile: add macros.c snapshot immunant/c2rust#1304
transpile: add --translate-const-macros conservative by @kkysen in transpile: add --translate-const-macros conservative immunant/c2rust#1305
Revert "transpile: make --translate-{const,fn}-macros conservative the default for the CLI, too" by @kkysen in Revert "transpile: make --translate-{const,fn}-macros conservative the default for the CLI, too" immunant/c2rust#1313
ci: run c2rust-testsuite on all PRs, not just those into master by @kkysen in ci: run c2rust-testsuite on all PRs, not just those into master immunant/c2rust#1312
transpile: fix some mistakes in macros.c snapshot by @kkysen in transpile: fix some mistakes in macros.c snapshot immunant/c2rust#1314
Apply clippy fixes to stable crates by @brk in Apply clippy fixes to stable crates immunant/c2rust#1308
Add libzstd-dev to internal testsuite dependencies by @thedataking in Add libzstd-dev to internal testsuite dependencies immunant/c2rust#1318
transpile: update fn convert_expr docs and remove unused code by @kkysen in transpile: update fn convert_expr docs and remove unused code immunant/c2rust#1323
transpile: insert casts at const macro expansion sites to their portable override_tys by @kkysen in transpile: insert casts at const macro expansion sites to their portable override_tys immunant/c2rust#1322
readme: add known uses of c2rust transpile by @kkysen in readme: add known uses of c2rust transpile immunant/c2rust#1327
transpile: remove unused function in builder by @bungcip in transpile: remove unused function in builder immunant/c2rust#1316
ci: remove x86_64 macOS by @kkysen in ci: remove x86_64 macOS immunant/c2rust#1337
transpile: some doc improvements from #1306 by @kkysen in transpile: some doc improvements from #1306 immunant/c2rust#1336
transpile: make TypedAstContext::macro_{invocations,expansions,expansion_test} into IndexMaps instead of HashMaps by @kkysen in transpile: make TypedAstContext::macro_{invocations,expansions,expansion_test} into IndexMaps instead of HashMaps immunant/c2rust#1334
transpile: refactor single_attr and call_attr to just use mk().meta_namevalue() and mk().meta_path() instead of creating structure itself by @bungcip in transpile: refactor single_attr and call_attr to just use mk().meta_namevalue() and mk().meta_path() instead of creating structure itself immunant/c2rust#1343
ci: don't install cmake on macOS as it's already installed by @kkysen in ci: don't install cmake on macOS as it's already installed immunant/c2rust#1351
Remove extraneous separator from paths with arguments by @brk in Remove extraneous separator from paths with arguments immunant/c2rust#1340
transpile: unconditionally make consts use unsafe blocks for --translate-const-macros conservative by @kkysen in transpile: unconditionally make consts use unsafe blocks for --translate-const-macros conservative immunant/c2rust#1335
properly handle ASTEntryTag::TagStaticAssertDecl conversion by @bungcip in properly handle ASTEntryTag::TagStaticAssertDecl conversion immunant/c2rust#1342
transpile: expand --translate-const-macros conservative to a lot more CExprKinds by @kkysen in transpile: expand --translate-const-macros conservative to a lot more CExprKinds immunant/c2rust#1306
Dont write unused unary expr when operator is increment/decrement by @bungcip in Dont write unused unary expr when operator is increment/decrement immunant/c2rust#1344
scripts/test_translator.py: restore '[ OK ]' prints by @fw-immunant in scripts/test_translator.py: restore '[ OK ]' prints immunant/c2rust#1358
Change VisitEnumDecl to TraverseDecl by @bungcip in Change VisitEnumDecl to TraverseDecl immunant/c2rust#1357
Run rustfmt on the tests by @Rua in Run rustfmt on the tests immunant/c2rust#1363
transpile: expand --translate-const-macros conservative to handle statements by @kkysen in transpile: expand --translate-const-macros conservative to handle statements immunant/c2rust#1338
transpile: add support for CTypeKind::UnaryTypes in --translate-const-macros conservative by @kkysen in transpile: add support for CTypeKind::UnaryTypes in --translate-const-macros conservative immunant/c2rust#1346
transpile: support CastKind::ArrayToPointerDecays in --translate-const-macros conservative by @kkysen in transpile: support CastKind::ArrayToPointerDecays in --translate-const-macros conservative immunant/c2rust#1349
transpile: some simplifications before adding override_tys to const macros by @kkysen in transpile: some simplifications before adding override_tys to const macros immunant/c2rust#1370
Include libc dependency only when needed by @Rua in Include libc dependency only when needed immunant/c2rust#1373
Avoid unsafe in const macro translations except where necessary by @fw-immunant in Avoid unsafe in const macro translations except where necessary immunant/c2rust#1339
Ensure crate name is a valid identifier by @Rua in Ensure crate name is a valid identifier immunant/c2rust#1375
Store alloca vectors in top function scope by @Rua in Store alloca vectors in top function scope immunant/c2rust#1359
Avoid mutable transmute in string literals by @Rua in Avoid mutable transmute in string literals immunant/c2rust#1360
Fix inline assembly translation in ioq3 by @fw-immunant in Fix inline assembly translation in ioq3 immunant/c2rust#1374
transpile: tests: snapshots: fix vm_x86.c snapshot test by @fw-immunant in transpile: tests: snapshots: fix vm_x86.c snapshot test immunant/c2rust#1378
transpile: update insta to 1.43.2 for the fix to INSTA_GLOB_FILTER so that the snapshot file is correctly resolved by @kkysen in transpile: update insta to 1.43.2 for the fix to INSTA_GLOB_FILTER so that the snapshot file is correctly resolved immunant/c2rust#1377
transpile: const macros: strip implicit casts from macro expansion sites by @fw-immunant in transpile: const macros: strip implicit casts from macro expansion sites immunant/c2rust#1311
transpile: support anonymous struct/union/enums within structs and unions by @bungcip in transpile: support anonymous struct/union/enums within structs and unions immunant/c2rust#1361
cleanup AST builder functionality by @bungcip in cleanup AST builder functionality immunant/c2rust#1350
pdg: use mutex around cargo runs to avoid file race conditions by @kkysen in pdg: use mutex around cargo runs to avoid file race conditions immunant/c2rust#1380
transpile: use core::ffi::c_* instead of std::ffi::c_* by @kkysen in transpile: use core::ffi::c_* instead of std::ffi::c_* immunant/c2rust#1381
transpile: update records.c snapshot test to include #1381 after #1361 merged by @kkysen in transpile: update records.c snapshot test to include #1381 after #1361 merged immunant/c2rust#1383
ci: enable libxml2 by @kkysen in ci: enable libxml2 immunant/c2rust#1385
transpile: const macros: skip DeclRefs by @kkysen in transpile: const macros: skip DeclRefs immunant/c2rust#1387
ci: don't cache apt pkgs for c2rust-testsuite since dpkg-query doesn't work correctly by @kkysen in ci: don't cache apt pkgs for c2rust-testsuite since dpkg-query doesn't work correctly immunant/c2rust#1389
disable warning from clang in snapshot test by @bungcip in disable warning from clang in snapshot test immunant/c2rust#1393
ci: run testsuite python2 transpilation by @fw-immunant in ci: run testsuite python2 transpilation immunant/c2rust#1384
transpile: separate top-level decl converting and emitting by @kkysen in transpile: separate top-level decl converting and emitting immunant/c2rust#1371
transpile: some docs/refactoring improvements made while fixing SrcLoc sorting by @kkysen in transpile: some docs/refactoring improvements made while fixing SrcLoc sorting immunant/c2rust#1401
transpile: actually fix SrcLoc sorting by fixing include path fileids by @kkysen in transpile: actually fix SrcLoc sorting by fixing include path fileids immunant/c2rust#1400
Prepare for 0.21.0 release by @kkysen in Prepare for 0.21.0 release immunant/c2rust#1402

New Contributors

@javierhonduco made their first contribution in docs/readme: Update bear arguments immunant/c2rust#1206
@brk made their first contribution in GH Actions: bump tagged versions of actions/cache immunant/c2rust#1212
@chenrui333 made their first contribution in build: support cmake 4.0 immunant/c2rust#1214
@nnethercote made their first contribution in Remove some libc dependencies. immunant/c2rust#1221
@joshtriplett made their first contribution in Fixes to rust-toolchain.toml handling immunant/c2rust#1225
@spastorino made their first contribution in Bump bindgen to be able to handle CXCursor_LinkageSpec in LLVM18+ immunant/c2rust#1243
@shirok1 made their first contribution in Fix getBitWidthValue compilation error on LLVM 20+ immunant/c2rust#1248
@Chobbes made their first contribution in Add tests/requirements.txt dependencies into nix python environment immunant/c2rust#1269
@Rua made their first contribution in Run rustfmt on the tests immunant/c2rust#1363

Also, many thanks to @Rua and @bungcip for helping us fix a ton of bugs!
Full Changelog: immunant/c2rust@v0.20.0...v0.21.0

View the full release notes at https://github.com/immunant/c2rust/releases/tag/v0.21.0.