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

@@ -4617,7 +4617,7 @@ impl CommandLineStep for RemoteTestClientTests {
 }
 
 fn check_if_cargo_semver_checks_is_installed(builder: &Builder<'_>) -> bool {
-    command("cargo")
+    command(&builder.initial_cargo)
         .allow_failure()
         .arg("semver-checks")
         .arg("--version")
@@ -4630,7 +4630,13 @@ 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.
+///
+/// If you want to allow a breaking change in a given PR, or if cargo-semver-checks has a false
+/// positive, modify the `src/bootstrap/stdlib-semver-check-stamp` file.
 #[derive(Debug, Clone, PartialEq, Eq, Hash)]
 pub struct StdSemverCheck {
     build_compiler: Compiler,
@@ -4651,19 +4657,22 @@ 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),
@@ -4673,6 +4682,15 @@ impl CommandLineStep for StdSemverCheck {
     }
 
     fn run(self, builder: &Builder<'_>) {
+        const STDLIB_SEMVER_CHECK_STAMP_PATH: &str = "src/bootstrap/stdlib-semver-check-stamp";
+
+        if builder.config.ci_env.is_running_in_ci()
+            && builder.config.has_changes_from_upstream(&[STDLIB_SEMVER_CHECK_STAMP_PATH])
+        {
+            builder.info(&format!("Skipping stdlib semver check, because {STDLIB_SEMVER_CHECK_STAMP_PATH} was modified."));
+            return;
+        }
+
         let Some(docs_dir) = builder.config.download_std_json_docs(self.target, &self.commit)
         else {
             return;
@@ -4687,7 +4705,7 @@ impl CommandLineStep for StdSemverCheck {
 
         for library in ["core", "alloc", "std"] {
             println!("Checking semver compatibility of {library}");
-            let mut cmd = command("cargo");
+            let mut cmd = command(&builder.initial_cargo);
             cmd.arg("semver-checks")
                 .arg("-Z")
                 .arg("unstable-options")
@@ -4698,7 +4716,41 @@ 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:

src/bootstrap/stdlib-semver-check-stamp

@@ -0,0 +1,5 @@
+Change this file to explicitly acknowledge making a breaking change to the Rust standard library.
+If this file is modified in the same PR as the breaking change, then CI will not fail due to the
+breaking change being detected by cargo-semver-checks.
+
+Last change is for: https://github.com/rust-lang/rust/pull/160253

src/ci/docker/host-x86_64/x86_64-gnu-stdlib-semver-check/Dockerfile

@@ -0,0 +1,31 @@
+FROM ubuntu:26.04
+
+ARG DEBIAN_FRONTEND=noninteractive
+RUN apt-get update && apt-get install -y --no-install-recommends \
+  g++ \
+  make \
+  ninja-build \
+  file \
+  curl \
+  ca-certificates \
+  python3 \
+  git \
+  cmake \
+  sudo \
+  gdb \
+  libssl-dev \
+  pkg-config \
+  xz-utils \
+  mingw-w64 \
+  zlib1g-dev \
+  libzstd-dev \
+  && rm -rf /var/lib/apt/lists/*
+
+COPY scripts/sccache.sh /scripts/
+RUN sh /scripts/sccache.sh
+
+ENV RUST_CONFIGURE_ARGS="--build=x86_64-unknown-linux-gnu"
+ENV RUSTC_WRAPPER=/usr/local/bin/sccache
+
+COPY /scripts/std-semver-check.sh /tmp/std-semver-check.sh
+ENV SCRIPT="bash /tmp/std-semver-check.sh"

src/ci/docker/scripts/std-semver-check.sh

@@ -0,0 +1,24 @@
+#!/bin/bash
+
+set -euo pipefail
+
+BUILD_DIR=$(realpath ./build/x86_64-unknown-linux-gnu)
+
+# Install the latest version of cargo-semver-checks, so that once the JSON doc format changes,
+# we will eventually get a csc version that supports it
+# Speed up compilation by reducing optimizations settings a bit
+RUSTC="${BUILD_DIR}"/stage0/bin/rustc \
+CARGO_PROFILE_RELEASE_LTO=false \
+CARGO_PROFILE_RELEASE_CODEGEN_UNITS=16 \
+"${BUILD_DIR}"/stage0/bin/cargo install cargo-semver-checks --locked
+
+# Provide path to cargo-semver-checks
+export PATH=${PATH}:/cargo/bin
+
+# Explicitly compute the baseline commit (the first git parent, which is the latest upstream main
+# commit), so that it is shown in the commit log and so that the command can be easily reproduced
+# locally.
+PARENT=$(git rev-parse HEAD^1)
+
+# Run the test
+python3 ../x.py test std-semver-check --set rust.stdlib-semver-baseline=${PARENT}

src/ci/github-actions/jobs.yml

@@ -491,6 +491,9 @@ auto:
   - name: x86_64-gnu-miri
     <<: *job-linux-4c
 
+  - name: x86_64-gnu-stdlib-semver-check
+    <<: *job-linux-4c
+
   - name: optional-x86_64-gnu-autodiff
     continue_on_error: true
     doc_url: https://rustc-dev-guide.rust-lang.org/tests/autodiff-ci-job.html