Refactor if-available setting to work in CI

This verifies if the HEAD sha matches with the detected LLVM SHA, and if not,
permits usage of the detected LLVM. Otherwise, we fallback on regular
non-downloaded LLVM (currently still cached with sccache, though that's still
10+ minutes on macOS).

src/bootstrap/config.rs

@@ -990,41 +990,7 @@ impl Config {
             config.llvm_from_ci = match llvm.download_ci_llvm {
                 Some(StringOrBool::String(s)) => {
                     assert!(s == "if-available", "unknown option `{}` for download-ci-llvm", s);
-                    // This is currently all tier 1 targets and tier 2 targets with host tools
-                    // (since others may not have CI artifacts)
-                    // https://doc.rust-lang.org/rustc/platform-support.html#tier-1
-                    let supported_platforms = [
-                        // tier 1
-                        "aarch64-unknown-linux-gnu",
-                        "i686-pc-windows-gnu",
-                        "i686-pc-windows-msvc",
-                        "i686-unknown-linux-gnu",
-                        "x86_64-unknown-linux-gnu",
-                        "x86_64-apple-darwin",
-                        "x86_64-pc-windows-gnu",
-                        "x86_64-pc-windows-msvc",
-                        // tier 2 with host tools
-                        "aarch64-apple-darwin",
-                        "aarch64-pc-windows-msvc",
-                        "aarch64-unknown-linux-musl",
-                        "arm-unknown-linux-gnueabi",
-                        "arm-unknown-linux-gnueabihf",
-                        "armv7-unknown-linux-gnueabihf",
-                        "mips-unknown-linux-gnu",
-                        "mips64-unknown-linux-gnuabi64",
-                        "mips64el-unknown-linux-gnuabi64",
-                        "mipsel-unknown-linux-gnu",
-                        "powerpc-unknown-linux-gnu",
-                        "powerpc64-unknown-linux-gnu",
-                        "powerpc64le-unknown-linux-gnu",
-                        "riscv64gc-unknown-linux-gnu",
-                        "s390x-unknown-linux-gnu",
-                        "x86_64-unknown-freebsd",
-                        "x86_64-unknown-illumos",
-                        "x86_64-unknown-linux-musl",
-                        "x86_64-unknown-netbsd",
-                    ];
-                    supported_platforms.contains(&&*config.build.triple)
+                    crate::native::is_ci_llvm_available(&config)
                 }
                 Some(StringOrBool::Bool(b)) => b,
                 None => false,

src/bootstrap/native.rs

@@ -114,23 +114,20 @@ pub fn prebuilt_llvm_config(
     Err(Meta { stamp, build_llvm_config, out_dir, root: root.into() })
 }
 
-pub(crate) fn maybe_download_ci_llvm(builder: &Builder<'_>) {
-    let config = &builder.config;
-    if !config.llvm_from_ci {
-        return;
-    }
+/// This retrieves the LLVM sha we *want* to use, according to git history.
+pub(crate) fn detect_llvm_sha(config: &crate::config::Config) -> String {
     let mut rev_list = config.git();
     rev_list.args(&[
         PathBuf::from("rev-list"),
-        format!("--author={}", builder.config.stage0_metadata.config.git_merge_commit_email).into(),
+        format!("--author={}", config.stage0_metadata.config.git_merge_commit_email).into(),
         "-n1".into(),
         "--first-parent".into(),
         "HEAD".into(),
         "--".into(),
-        builder.src.join("src/llvm-project"),
-        builder.src.join("src/bootstrap/download-ci-llvm-stamp"),
+        config.src.join("src/llvm-project"),
+        config.src.join("src/bootstrap/download-ci-llvm-stamp"),
         // the LLVM shared object file is named `LLVM-12-rust-{version}-nightly`
-        builder.src.join("src/version"),
+        config.src.join("src/version"),
     ]);
     let llvm_sha = output(&mut rev_list);
     let llvm_sha = llvm_sha.trim();
@@ -143,8 +140,76 @@ pub(crate) fn maybe_download_ci_llvm(builder: &Builder<'_>) {
         panic!();
     }
 
+    llvm_sha.to_owned()
+}
+
+/// Returns whether the CI-found LLVM is currently usable.
+///
+/// This checks both the build triple platform to confirm we're usable at all,
+/// and then verifies if the current HEAD matches the detected LLVM SHA head,
+/// in which case LLVM is indicated as not available.
+pub(crate) fn is_ci_llvm_available(config: &crate::config::Config) -> bool {
+    // This is currently all tier 1 targets and tier 2 targets with host tools
+    // (since others may not have CI artifacts)
+    // https://doc.rust-lang.org/rustc/platform-support.html#tier-1
+    let supported_platforms = [
+        // tier 1
+        "aarch64-unknown-linux-gnu",
+        "i686-pc-windows-gnu",
+        "i686-pc-windows-msvc",
+        "i686-unknown-linux-gnu",
+        "x86_64-unknown-linux-gnu",
+        "x86_64-apple-darwin",
+        "x86_64-pc-windows-gnu",
+        "x86_64-pc-windows-msvc",
+        // tier 2 with host tools
+        "aarch64-apple-darwin",
+        "aarch64-pc-windows-msvc",
+        "aarch64-unknown-linux-musl",
+        "arm-unknown-linux-gnueabi",
+        "arm-unknown-linux-gnueabihf",
+        "armv7-unknown-linux-gnueabihf",
+        "mips-unknown-linux-gnu",
+        "mips64-unknown-linux-gnuabi64",
+        "mips64el-unknown-linux-gnuabi64",
+        "mipsel-unknown-linux-gnu",
+        "powerpc-unknown-linux-gnu",
+        "powerpc64-unknown-linux-gnu",
+        "powerpc64le-unknown-linux-gnu",
+        "riscv64gc-unknown-linux-gnu",
+        "s390x-unknown-linux-gnu",
+        "x86_64-unknown-freebsd",
+        "x86_64-unknown-illumos",
+        "x86_64-unknown-linux-musl",
+        "x86_64-unknown-netbsd",
+    ];
+    if !supported_platforms.contains(&&*config.build.triple) {
+        return false;
+    }
+
+    if crate::util::CiEnv::is_ci() {
+        let llvm_sha = detect_llvm_sha(config);
+        let head_sha = output(config.git().arg("rev-parse").arg("HEAD"));
+        let head_sha = head_sha.trim();
+        if llvm_sha == head_sha {
+            eprintln!(
+                "Detected LLVM as non-available: running in CI and modified LLVM in this change"
+            );
+            return false;
+        }
+    }
+
+    true
+}
+
+pub(crate) fn maybe_download_ci_llvm(builder: &Builder<'_>) {
+    let config = &builder.config;
+    if !config.llvm_from_ci {
+        return;
+    }
     let llvm_root = config.ci_llvm_root();
     let llvm_stamp = llvm_root.join(".llvm-stamp");
+    let llvm_sha = detect_llvm_sha(&config);
     let key = format!("{}{}", llvm_sha, config.llvm_assertions);
     if program_out_of_date(&llvm_stamp, &key) && !config.dry_run {
         download_ci_llvm(builder, &llvm_sha);

src/bootstrap/util.rs

@@ -258,6 +258,10 @@ impl CiEnv {
         }
     }
 
+    pub fn is_ci() -> bool {
+        Self::current() != CiEnv::None
+    }
+
     /// If in a CI environment, forces the command to run with colors.
     pub fn force_coloring_in_ci(self, cmd: &mut Command) {
         if self != CiEnv::None {