Add regression tests for #130681

tests/rustdoc-ui/doctest/dead-code-2024.rs

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+// This test ensures that the 2024 edition merged doctest will not use `#[allow(unused)]`.
+
+//@ compile-flags:--test -Zunstable-options --edition 2024
+//@ normalize-stdout-test: "tests/rustdoc-ui/doctest" -> "$$DIR"
+//@ normalize-stdout-test: "finished in \d+\.\d+s" -> "finished in $$TIME"
+//@ failure-status: 101
+
+#![doc(test(attr(allow(unused_variables), deny(warnings))))]
+
+/// Example
+///
+/// ```rust,no_run
+/// trait T { fn f(); }
+/// ```
+pub fn f() {}

tests/rustdoc-ui/doctest/dead-code-2024.stdout

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+
+running 1 test
+test $DIR/dead-code-2024.rs - f (line 12) - compile ... FAILED
+
+failures:
+
+---- $DIR/dead-code-2024.rs - f (line 12) stdout ----
+error: trait `T` is never used
+  --> $DIR/dead-code-2024.rs:13:7
+   |
+LL | trait T { fn f(); }
+   |       ^
+   |
+note: the lint level is defined here
+  --> $DIR/dead-code-2024.rs:11:9
+   |
+LL | #![deny(warnings)]
+   |         ^^^^^^^^
+   = note: `#[deny(dead_code)]` implied by `#[deny(warnings)]`
+
+error: aborting due to 1 previous error
+
+Couldn't compile the test.
+
+failures:
+    $DIR/dead-code-2024.rs - f (line 12)
+
+test result: FAILED. 0 passed; 1 failed; 0 ignored; 0 measured; 0 filtered out; finished in $TIME
+

tests/rustdoc-ui/doctest/dead-code.rs

@@ -0,0 +1,15 @@
+// This test ensures that the doctest will not use `#[allow(unused)]`.
+
+//@ compile-flags:--test
+//@ normalize-stdout-test: "tests/rustdoc-ui/doctest" -> "$$DIR"
+//@ normalize-stdout-test: "finished in \d+\.\d+s" -> "finished in $$TIME"
+//@ failure-status: 101
+
+#![doc(test(attr(allow(unused_variables), deny(warnings))))]
+
+/// Example
+///
+/// ```rust,no_run
+/// trait T { fn f(); }
+/// ```
+pub fn f() {}

tests/rustdoc-ui/doctest/dead-code.stdout

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+
+running 1 test
+test $DIR/dead-code.rs - f (line 12) - compile ... FAILED
+
+failures:
+
+---- $DIR/dead-code.rs - f (line 12) stdout ----
+error: trait `T` is never used
+  --> $DIR/dead-code.rs:13:7
+   |
+LL | trait T { fn f(); }
+   |       ^
+   |
+note: the lint level is defined here
+  --> $DIR/dead-code.rs:11:9
+   |
+LL | #![deny(warnings)]
+   |         ^^^^^^^^
+   = note: `#[deny(dead_code)]` implied by `#[deny(warnings)]`
+
+error: aborting due to 1 previous error
+
+Couldn't compile the test.
+
+failures:
+    $DIR/dead-code.rs - f (line 12)
+
+test result: FAILED. 0 passed; 1 failed; 0 ignored; 0 measured; 0 filtered out; finished in $TIME
+