`unreachable_patterns` not emitted on macros generated by another crate's macro

I tried this code:

crate_a:

#[macro_export]
macro_rules! create_pats {
    () => {
        macro_rules! pat_a {
            () => {
                _
            };
        }
        macro_rules! pat_b {
            () => {
                _
            };
        }
    };
}

crate_b:

use crate_a::create_pats;

create_pats!();

#[deny(unreachable_patterns)]
fn demo(value: u8) -> i32 {
    match value {
        pat_a!() => 42,
        pat_b!() => 24,
    }
}

I expected to see this happen: unreachable_pattern error

error: unreachable pattern
  --> crate_a/src/lib.rs:11:17
   |
 6 |                 _
   |                 - matches any value
...
11 |                 _
   |                 ^ no value can reach this

  --> crate_b/src/main.rs:9:17
 9 |         pat_b!() => 24,
   |         -------- in this macro invocation
   |
note: the lint level is defined here
  --> src/main.rs:5:8
   |
 5 | #[deny(unreachable_patterns)]
   |        ^^^^^^^^^^^^^^^^^^^^
   = note: this error originates in the macro `pat_b` (in Nightly builds, run with -Z macro-backtrace for more info)

Instead, this happened: successful compilation

When the create_pats! macro is defined in crate_b, the lint is emitted as expected.

Meta

rustc --version --verbose:

rustc 1.98.0-nightly (b30f3df3b 2026-06-11)
binary: rustc
commit-hash: b30f3df3ba3c4c9de2f58f1a75dd9500b79b3f8d
commit-date: 2026-06-11
host: x86_64-unknown-linux-gnu
release: 1.98.0-nightly
LLVM version: 22.1.6

@rustbot label +A-diagnostics +A-lints +A-patterns +L-unreachable_patterns +T-compiler +L-false-negative

まず crate_a/src/lib.rs と crate_b/src/main.rs を使って 2 つの crate の例を再現し、次に create_pats! が crate_b で定義されている場合と比較します。crate をまたいで生成されたマクロに対して unreachable_patterns lint がどのように処理されるかを追跡します。lint を deny したときに pat_b!() アームが到達不能として診断されれば完了です。