Rollup merge of #139328 - GuillaumeGomez:fix-panic-output-137970, r=fmease

Fix 2024 edition doctest panic output

Fixes #137970.

The problem was that the output was actually displayed by rustc itself because we're exiting with `Result<(), String>`, and the display is really not great. So instead, we get the output, we print it and then we return an `ExitCode`.

r? ````@aDotInTheVoid````

src/librustdoc/doctest/runner.rs

@@ -113,6 +113,7 @@ impl DocTestRunner {
 mod __doctest_mod {{
     use std::sync::OnceLock;
     use std::path::PathBuf;
+    use std::process::ExitCode;
 
     pub static BINARY_PATH: OnceLock<PathBuf> = OnceLock::new();
     pub const RUN_OPTION: &str = \"RUSTDOC_DOCTEST_RUN_NB_TEST\";
@@ -123,16 +124,17 @@ mod __doctest_mod {{
     }}
 
     #[allow(unused)]
-    pub fn doctest_runner(bin: &std::path::Path, test_nb: usize) -> Result<(), String> {{
+    pub fn doctest_runner(bin: &std::path::Path, test_nb: usize) -> ExitCode {{
         let out = std::process::Command::new(bin)
             .env(self::RUN_OPTION, test_nb.to_string())
             .args(std::env::args().skip(1).collect::<Vec<_>>())
             .output()
             .expect(\"failed to run command\");
         if !out.status.success() {{
-            Err(String::from_utf8_lossy(&out.stderr).to_string())
+            eprint!(\"{{}}\", String::from_utf8_lossy(&out.stderr));
+            ExitCode::FAILURE
         }} else {{
-            Ok(())
+            ExitCode::SUCCESS
         }}
     }}
 }}

tests/rustdoc-ui/doctest/edition-2024-error-output.rs

@@ -0,0 +1,14 @@
+// This is a regression test for <https://github.com/rust-lang/rust/issues/137970>.
+// The output must look nice and not like a `Debug` display of a `String`.
+
+//@ edition: 2024
+//@ compile-flags: --test
+//@ normalize-stdout: "tests/rustdoc-ui/doctest" -> "$$DIR"
+//@ normalize-stdout: "panicked at .+rs:" -> "panicked at $$TMP:"
+//@ normalize-stdout: "finished in \d+\.\d+s" -> "finished in $$TIME"
+//@ rustc-env:RUST_BACKTRACE=0
+//@ failure-status: 101
+
+//! ```rust
+//! assert_eq!(2 + 2, 5);
+//! ```

tests/rustdoc-ui/doctest/edition-2024-error-output.stdout

@@ -0,0 +1,20 @@
+
+running 1 test
+test $DIR/edition-2024-error-output.rs - (line 12) ... FAILED
+
+failures:
+
+---- $DIR/edition-2024-error-output.rs - (line 12) stdout ----
+
+thread 'main' panicked at $TMP:6:1:
+assertion `left == right` failed
+  left: 4
+ right: 5
+note: run with `RUST_BACKTRACE=1` environment variable to display a backtrace
+
+
+failures:
+    $DIR/edition-2024-error-output.rs - (line 12)
+
+test result: FAILED. 0 passed; 1 failed; 0 ignored; 0 measured; 0 filtered out; finished in $TIME
+