add support for reading git-commit-info

Author: pietroalbini

Diff for: build.rs

@@ -80,6 +80,37 @@ fn commit_info_from_git() -> Option<CommitInfo> {
     })
 }
 
+// The rustc source tarball is meant to contain all the source code to build an exact copy of the
+// toolchain, but it doesn't include the git repository itself. It wouldn't thus be possible to
+// populate the version information with the commit hash and the commit date.
+//
+// To work around this, the rustc build process obtains the git information when creating the
+// source tarball and writes it to the `git-commit-info` file. The build process actually creates
+// at least *two* of those files, one for Rust as a whole (in the root of the tarball) and one
+// specifically for Cargo (in src/tools/cargo). This function loads that file.
+//
+// The file is a newline-separated list of full commit hash, short commit hash, and commit date.
+fn commit_info_from_rustc_source_tarball() -> Option<CommitInfo> {
+    let path = Path::new("git-commit-info");
+    if !path.exists() {
+        return None;
+    }
+
+    // Dependency tracking is a nice to have for this (git doesn't do it), so if the path is not
+    // valid UTF-8 just avoid doing it rather than erroring out.
+    if let Some(utf8) = path.to_str() {
+        println!("cargo:rerun-if-changed={utf8}");
+    }
+
+    let content = std::fs::read_to_string(&path).ok()?;
+    let mut parts = content.split('\n').map(|s| s.to_string());
+    Some(CommitInfo {
+        hash: parts.next()?,
+        short_hash: parts.next()?,
+        date: parts.next()?,
+    })
+}
+
 fn commit_info() {
     // Var set by bootstrap whenever omit-git-hash is enabled in rust-lang/rust's config.toml.
     println!("cargo:rerun-if-env-changed=CFG_OMIT_GIT_HASH");
@@ -89,7 +120,7 @@ fn commit_info() {
         return;
     }
 
-    let Some(git) = commit_info_from_git() else {
+    let Some(git) = commit_info_from_git().or_else(commit_info_from_rustc_source_tarball) else {
         return;
     };