Make semicolon_in_expressions_from_macros a hard error

[joshtriplett]

See #79813, which has tracked the evolution of this issue:

• We added it as an allow-by-default lint in December 2020.
• We made it warn-by-default in July 2021.
• We made it a future-incompat warn-in-deps in October 2022.
• We made it deny-by-default in July 2025.

Now, finally, let it become a hard error. Remove fields and parameters only tracked in order to detect the expression context and report the lint. Macro expansion now simply includes the semicolon to expand, without attempting to skip over it, and lets that produce an error.

This will allow subsequent simplifications of macro handling. I have PRs in progress for that.

---

I recommend reviewing the last commit separately. The first three commits reorganize/drop some tests in preparation, to make the last commit simpler and make it easier to see the relevant test changes. In particular, the first three commits remove a couple of tests that depend specifically on the existence of the lint and don't make sense without it, move some tests that still make sense from tests/ui/lints/ to tests/ui/macros/, and switch one test (of buffered lints) to use a different buffered lint for testing.

Lang nominated for the purposes of confirming that we're ready to take this last step. The concrete change, though, is a compiler change as usual.

r? petrochenkov

[rust-log-analyzer]

The job aarch64-gnu-llvm-21-1 failed! Check out the build log: (web) (plain enhanced) (plain)

Click to see the possible cause of the failure (guessed by this bot)

[RUSTC-TIMING] unicode_security test:false 1.068
   Compiling wasm-encoder v0.219.2
[RUSTC-TIMING] wasm_encoder test:false 2.922
   Compiling nix v0.30.1
error: macro expansion ignores `;` and any tokens following
   --> /cargo/registry/src/index.crates.io-1949cf8c6b5b557f/cfg_aliases-0.2.1/src/lib.rs:287:29
    |
287 |           ] $($current)* $tok);
    |                               ^
    |
   ::: /cargo/registry/src/index.crates.io-1949cf8c6b5b557f/nix-0.30.1/build.rs:4:5
    |
  4 | /     cfg_aliases! {
  5 | |         android: { target_os = "android" },
  6 | |         dragonfly: { target_os = "dragonfly" },
  7 | |         ios: { target_os = "ios" },
...   |
 27 | |         solarish: { any(illumos, solaris) },
 28 | |     }
    | |_____- caused by the macro expansion here
    |
    = note: the usage of `cfg_aliases!` is likely invalid in expression context
help: you might be missing a semicolon here
    |
323 |         $crate::cfg_aliases!(@parser_clause any [] [$($tokens)+]);
    |                                                                  +

error: macro expansion ignores `;` and any tokens following
   --> /cargo/registry/src/index.crates.io-1949cf8c6b5b557f/cfg_aliases-0.2.1/src/lib.rs:268:11
    |
268 |           ]);
    |             ^
    |
   ::: /cargo/registry/src/index.crates.io-1949cf8c6b5b557f/nix-0.30.1/build.rs:4:5
    |
  4 | /     cfg_aliases! {
  5 | |         android: { target_os = "android" },
  6 | |         dragonfly: { target_os = "dragonfly" },
  7 | |         ios: { target_os = "ios" },
...   |
 27 | |         solarish: { any(illumos, solaris) },
 28 | |     }
    | |_____- caused by the macro expansion here
    |
    = note: the usage of `cfg_aliases!` is likely invalid in expression context

error: macro expansion ignores `;` and any tokens following
   --> /cargo/registry/src/index.crates.io-1949cf8c6b5b557f/cfg_aliases-0.2.1/src/lib.rs:287:29
    |
287 |           ] $($current)* $tok);
    |                               ^
    |
   ::: /cargo/registry/src/index.crates.io-1949cf8c6b5b557f/nix-0.30.1/build.rs:4:5
    |
  4 | /     cfg_aliases! {
  5 | |         android: { target_os = "android" },
  6 | |         dragonfly: { target_os = "dragonfly" },
  7 | |         ios: { target_os = "ios" },
...   |
 27 | |         solarish: { any(illumos, solaris) },
 28 | |     }
    | |_____- caused by the macro expansion here
    |
    = note: the usage of `cfg_aliases!` is likely invalid in expression context

error: macro expansion ignores `;` and any tokens following
   --> /cargo/registry/src/index.crates.io-1949cf8c6b5b557f/cfg_aliases-0.2.1/src/lib.rs:305:10
    |
305 |           );
    |            ^
    |
   ::: /cargo/registry/src/index.crates.io-1949cf8c6b5b557f/nix-0.30.1/build.rs:4:5
    |
  4 | /     cfg_aliases! {
  5 | |         android: { target_os = "android" },
  6 | |         dragonfly: { target_os = "dragonfly" },
  7 | |         ios: { target_os = "ios" },
...   |
 27 | |         solarish: { any(illumos, solaris) },
 28 | |     }
    | |_____- caused by the macro expansion here
    |
    = note: the usage of `cfg_aliases!` is likely invalid in expression context

[cyrgani]

this test still has warn- in the name

View changes since the review

[joshtriplett]

AFAICT, the issue in the combination of nix and cfg_aliases here has not previously been caught by the FCW, because we always filtered out invocations of non-local macros. I'm working on fixing that now.

[joshtriplett]

I've submitted a separate PR to make the lint apply to non-local macros like the one in cfg_aliases as used in nix. Note that the test in cfg_aliases that would otherwise have caught this also suppresses the lint, because integration tests are separate crates.

#159222

[petrochenkov]

Blocked on #159222.
@rustbot blocked

[joshtriplett]

And, likely, blocked on seeing how widespread the issue with non-local macros is.

[rust-bors]

☔ The latest upstream changes (presumably #159336) made this pull request unmergeable. Please resolve the merge conflicts by rebasing.

[mati865]

Do we track non-local macros fallout somewhere? There are popular crates like katharostech/cfg_aliases#15 (over 100M downloads last 90 days) which is used by wgpu, nix, and so on.