Add ui regression test for new rustdoc failed_merged_doctest_compilation lint

Author: GuillaumeGomez

tests/rustdoc-ui/doctest/failed_merged_doctest_compilation-lint.rs

@@ -0,0 +1,16 @@
+// Regression test for the `failed_merged_doctest_compilation` lint.
+
+//@ edition: 2024
+//@ compile-flags:--test
+//@ normalize-stdout: "tests/rustdoc-ui/doctest" -> "$$DIR"
+//@ normalize-stdout: "finished in \d+\.\d+s" -> "finished in $$TIME"
+//@ normalize-stdout: "ran in \d+\.\d+s" -> "ran in $$TIME"
+//@ normalize-stdout: "compilation took \d+\.\d+s" -> "compilation took $$TIME"
+//@ failure-status: 101
+
+#![deny(rustdoc::failed_merged_doctest_compilation)] //~ ERROR
+
+//! ```
+//!
+//! let x
+//! ```

tests/rustdoc-ui/doctest/failed_merged_doctest_compilation-lint.stderr

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+error: failed to compile merged doctests for edition 2024. Reverting to standalone doctests.
+  --> $DIR/failed_merged_doctest_compilation-lint.rs:11:1
+   |
+LL | / #![deny(rustdoc::failed_merged_doctest_compilation)]
+LL | |
+LL | | //! ```
+LL | | //!
+LL | | //! let x
+LL | | //! ```
+   | |_______^
+   |
+note: the lint level is defined here
+  --> $DIR/failed_merged_doctest_compilation-lint.rs:11:9
+   |
+LL | #![deny(rustdoc::failed_merged_doctest_compilation)]
+   |         ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
+

tests/rustdoc-ui/doctest/failed_merged_doctest_compilation-lint.stdout

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+
+running 1 test
+test $DIR/failed_merged_doctest_compilation-lint.rs - (line 13) ... FAILED
+
+failures:
+
+---- $DIR/failed_merged_doctest_compilation-lint.rs - (line 13) stdout ----
+error: expected `;`, found `}`
+  --> $DIR/failed_merged_doctest_compilation-lint.rs:14:6
+   |
+LL | let x
+   |      ^ help: add `;` here
+LL | } _doctest_main_tests_rustdoc_ui_doctest_failed_merged_doctest_compilation_lint_rs_13_0() }
+   | - unexpected token
+
+error[E0282]: type annotations needed
+  --> $DIR/failed_merged_doctest_compilation-lint.rs:14:5
+   |
+LL | let x
+   |     ^
+   |
+help: consider giving `x` an explicit type
+   |
+LL | let x: /* Type */
+   |      ++++++++++++
+
+error: aborting due to 2 previous errors
+
+For more information about this error, try `rustc --explain E0282`.
+Couldn't compile the test.
+
+failures:
+    $DIR/failed_merged_doctest_compilation-lint.rs - (line 13)
+
+test result: FAILED. 0 passed; 1 failed; 0 ignored; 0 measured; 0 filtered out; finished in $TIME
+
+all doctests ran in $TIME; merged doctests compilation took $TIME