Add CI job for checking stdlib semver compatibility

bootstrap.example.toml

@@ -935,6 +935,15 @@
 #
 #rust.parallel-frontend-threads = 1
 
+# Baseline commit SHA for comparing semver breakages in the Rust standard library.
+# The in-tree stdlib API will be evaluated for semver breakages against this commit.
+# Used for the `./x test std-semver-check` command.
+# If unset, the first upstream parent commit will be used.
+#
+# The SHA must point to a merge commit merged into the mainline rust-lang/rust `main` branch,
+# because bootstrap will attempt to download the JSON docs data for this commit from its CI.
+#rust.stdlib-semver-baseline = "<commit-sha>"
+
 # =============================================================================
 # Distribution options
 #

src/bootstrap/src/core/build_steps/test.rs

@@ -4630,6 +4630,9 @@ fn check_if_cargo_semver_checks_is_installed(builder: &Builder<'_>) -> bool {
 /// Run cargo-semver-checks on the standard library and compare its API
 /// versus a previous baseline, using rustdoc JSON data.
 ///
+/// The baseline commit can be configured using `rust.stdlib-semver-baseline`.
+/// If unset, the first upstream parent commit will be used.
+///
 /// Fails if a semver-breaking change is detected.
 #[derive(Debug, Clone, PartialEq, Eq, Hash)]
 pub struct StdSemverCheck {
@@ -4651,19 +4654,22 @@ impl CommandLineStep for StdSemverCheck {
             panic!("cargo-semver-checks was not found, please install it");
         }
 
-        let baseline_commit = match get_closest_upstream_commit(
-            Some(&run.builder.config.src),
-            &run.builder.config.git_config(),
-            run.builder.config.ci_env,
-        ) {
-            Ok(Some(commit)) => commit,
-            Ok(None) => {
-                panic!("No baseline parent commit found for std-semver-check");
-            }
-            Err(error) => {
-                panic!("Cannot get baseline parent commit for std-semver-check: {error:?}");
-            }
-        };
+        let baseline_commit =
+            run.builder.config.stdlib_semver_baseline.clone().unwrap_or_else(|| {
+                match get_closest_upstream_commit(
+                    Some(&run.builder.config.src),
+                    &run.builder.config.git_config(),
+                    run.builder.config.ci_env,
+                ) {
+                    Ok(Some(commit)) => commit,
+                    Ok(None) => {
+                        panic!("No baseline parent commit found for std-semver-check");
+                    }
+                    Err(error) => {
+                        panic!("Cannot get baseline parent commit for std-semver-check: {error:?}");
+                    }
+                }
+            });
 
         run.builder.ensure(Self {
             build_compiler: run.builder.compiler_for_std(run.builder.top_stage),
@@ -4698,7 +4704,41 @@ impl CommandLineStep for StdSemverCheck {
                 .arg(directory.join(format!("{library}.json")))
                 .arg("--baseline-rustdoc")
                 .arg(baseline_dir.join(format!("{library}.json")));
-            cmd.run(builder);
+
+            // We use run_capture to get the exit status
+            let res = cmd.allow_failure().run_capture(builder);
+            match res.status() {
+                Some(status) if status.success() => {
+                    println!("{}\n{}", res.stdout(), res.stderr());
+                }
+                // 101 marks that csc was unable to parse the JSON data, but it did not fail with a
+                // semver breakage.
+                Some(status) if status.code() == Some(101) => {
+                    eprintln!(
+                        "cargo-semver-checks was unable to process {library} (this is not a fatal error)\n{}\n{}",
+                        res.stderr(),
+                        res.stdout()
+                    );
+                }
+                // 100 marks semver breakage
+                Some(status) if status.code() == Some(100) => {
+                    let error = format!(
+                        "cargo-semver-checks found semver breakage in {library}\n{}\n{}",
+                        res.stderr(),
+                        res.stdout()
+                    );
+                    if builder.fail_fast {
+                        eprintln!("{error}",);
+                        exit!(1);
+                    } else {
+                        builder.config.exec_ctx().add_to_delay_failure(error);
+                    }
+                }
+                _ => {
+                    eprintln!("cargo-semver-checks failed.\n{}\n{}", res.stderr(), res.stdout());
+                    exit!(1);
+                }
+            }
         }
     }
 }

src/bootstrap/src/core/config/config.rs

@@ -240,6 +240,8 @@ pub struct Config {
     pub rustdoc_pgo: PgoConfig,
     pub cargo_pgo: PgoConfig,
 
+    pub stdlib_semver_baseline: Option<String>,
+
     pub llvm_libunwind_default: Option<LlvmLibunwind>,
     pub enable_bolt_settings: bool,
 
@@ -610,6 +612,7 @@ impl Config {
             std_features: rust_std_features,
             break_on_ice: rust_break_on_ice,
             rustflags: rust_rustflags,
+            stdlib_semver_baseline: rust_stdlib_semver_baseline,
         } = toml_rust.unwrap_or_default();
 
         let Llvm {
@@ -1594,6 +1597,7 @@ NOTE: Please add `--stage 2` to your command line, or if you're sure you want to
                 .or(rust_rustc_debug_assertions)
                 .unwrap_or(rust_debug == Some(true)),
             stderr_is_tty: std::io::stderr().is_terminal(),
+            stdlib_semver_baseline: rust_stdlib_semver_baseline,
             stdout_is_tty: std::io::stdout().is_terminal(),
             submodules: build_submodules,
             sysconfdir: install_sysconfdir.map(PathBuf::from),

src/bootstrap/src/core/config/toml/rust.rs

@@ -77,6 +77,7 @@ define_config! {
         std_features: Option<BTreeSet<String>> = "std-features",
         break_on_ice: Option<bool> = "break-on-ice",
         parallel_frontend_threads: Option<u32> = "parallel-frontend-threads",
+        stdlib_semver_baseline: Option<String> = "stdlib-semver-baseline",
     }
 }
 
@@ -384,6 +385,7 @@ pub fn check_incompatible_options_for_ci_rustc(
         parallel_frontend_threads: _,
         bootstrap_override_lld: _,
         rustflags: _,
+        stdlib_semver_baseline: _,
     } = ci_rust_config;
 
     // There are two kinds of checks for CI rustc incompatible options: