Unexpected semicolon_in_expressions_from_macros warning

Code

cro_rules! log {
    ($level: ident, $($t:tt)*) => {
        #[cfg(feature = "log")]
        { log::$level!($($t)*) }
        // Silence unused variables warnings.
        #[cfg(not(feature = "log"))]
        { if false { let _ = ( $($t)* ); } }
    } 
}

Current output

error: trailing semicolon in macro used in expression position
   --> src/macros.rs:93:11
    |
 93 |           { log::$level!($($t)*) }
    |             ^^^^^^^^^^^^^^^^^^^^
    |
   ::: src/poll.rs:580:9
    |
580 | /         trace!(
581 | |             "registering event source with poller: token={:?}, interests={:?}",
582 | |             token,
583 | |             interests
584 | |         );
    | |_________- in this macro invocation
    |
    = note: macro invocations at the end of a block are treated as expressions
    = note: to ignore the value produced by the macro, add a semicolon after the invocation of `trace`
    = warning: this was previously accepted by the compiler but is being phased out; it will become a hard error in a future release!
    = note: for more information, see issue #79813 <https://github.com/rust-lang/rust/issues/79813>
    = note: `#[deny(semicolon_in_expressions_from_macros)]` (part of `#[deny(future_incompatible)]`) on by default
    = note: this error originates in the macro `log::trace` which comes from the expansion of the macro `trace` (in Nightly builds, run with -Z macro-backtrace for more info)

Desired output

I wouldn't expect an error here.

Rationale and extra context

Relevant code is https://github.com/tokio-rs/mio at commit 4b055e560301477e11a140e06beaf90f5d01cb78.

• Macro definiton: https://github.com/tokio-rs/mio/blob/4b055e560301477e11a140e06beaf90f5d01cb78/src/macros.rs#L90-L98
• Usage: https://github.com/tokio-rs/mio/blob/4b055e560301477e11a140e06beaf90f5d01cb78/src/poll.rs#L580-L584

Other cases

Rust Version

rustc 1.98.0-nightly (ce9954c0c 2026-06-26)
binary: rustc
commit-hash: ce9954c0cfc4bf26b82aef16e6fd8b020c237992
commit-date: 2026-06-26
host: aarch64-apple-darwin
release: 1.98.0-nightly
LLVM version: 22.1.8

Anything else?

PR to fix/silence the warning: https://github.com/tokio-rs/mio/pull/1975.

Original comment: https://github.com/rust-lang/rust/issues/79813#issuecomment-4991879370.

Reproduce the diagnostic with the macro in mio's src/macros.rs and its invocation in src/poll.rs at commit 4b055e560301477e11a140e06beaf90f5d01cb78. Read rust-lang/rust issue #79813 and compare mio PR #1975 to understand the semicolon_in_expressions_from_macros behavior. Done means the provided case no longer produces the unexpected error, with regression coverage for the behavior.