1 files changed, 75 insertions, 0 deletions
diff --git a/src/tools/build_helper/src/git.rs b/src/tools/build_helper/src/git.rs
new file mode 100644
index 00000000000..dc62051cb85
--- /dev/null
+++ b/src/tools/build_helper/src/git.rs
@@ -0,0 +1,75 @@
+use std::{path::Path, process::Command};
+
+/// Finds the remote for rust-lang/rust.
+/// For example for these remotes it will return `upstream`.
+/// ```text
+/// origin  https://github.com/Nilstrieb/rust.git (fetch)
+/// origin  https://github.com/Nilstrieb/rust.git (push)
+/// upstream        https://github.com/rust-lang/rust (fetch)
+/// upstream        https://github.com/rust-lang/rust (push)
+/// ```
+pub fn get_rust_lang_rust_remote(git_dir: Option<&Path>) -> Result<String, String> {
+    let mut git = Command::new("git");
+    if let Some(git_dir) = git_dir {
+        git.current_dir(git_dir);
+    }
+    git.args(["config", "--local", "--get-regex", "remote\\..*\\.url"]);
+
+    let output = git.output().map_err(|err| format!("{err:?}"))?;
+    if !output.status.success() {
+        return Err("failed to execute git config command".to_owned());
+    }
+
+    let stdout = String::from_utf8(output.stdout).map_err(|err| format!("{err:?}"))?;
+
+    let rust_lang_remote = stdout
+        .lines()
+        .find(|remote| remote.contains("rust-lang"))
+        .ok_or_else(|| "rust-lang/rust remote not found".to_owned())?;
+
+    let remote_name =
+        rust_lang_remote.split('.').nth(1).ok_or_else(|| "remote name not found".to_owned())?;
+    Ok(remote_name.into())
+}
+
+pub fn rev_exists(rev: &str, git_dir: Option<&Path>) -> Result<bool, String> {
+    let mut git = Command::new("git");
+    if let Some(git_dir) = git_dir {
+        git.current_dir(git_dir);
+    }
+    git.args(["rev-parse", rev]);
+    let output = git.output().map_err(|err| format!("{err:?}"))?;
+
+    match output.status.code() {
+        Some(0) => Ok(true),
+        Some(128) => Ok(false),
+        None => {
+            return Err(format!(
+                "git didn't exit properly: {}",
+                String::from_utf8(output.stderr).map_err(|err| format!("{err:?}"))?
+            ));
+        }
+        Some(code) => {
+            return Err(format!(
+                "git command exited with status code: {code}: {}",
+                String::from_utf8(output.stderr).map_err(|err| format!("{err:?}"))?
+            ));
+        }
+    }
+}
+
+/// Returns the master branch from which we can take diffs to see changes.
+/// This will usually be rust-lang/rust master, but sometimes this might not exist.
+/// This could be because the user is updating their forked master branch using the GitHub UI
+/// and therefore doesn't need an upstream master branch checked out.
+/// We will then fall back to origin/master in the hope that at least this exists.
+pub fn updated_master_branch(git_dir: Option<&Path>) -> Result<String, String> {
+    let upstream_remote = get_rust_lang_rust_remote(git_dir)?;
+    let upstream_master = format!("{upstream_remote}/master");
+    if rev_exists(&upstream_master, git_dir)? {
+        return Ok(upstream_master);
+    }
+
+    // We could implement smarter logic here in the future.
+    Ok("origin/master".into())
+}