4 files changed, 125 insertions, 0 deletions
diff --git a/src/tools/build_helper/Cargo.toml b/src/tools/build_helper/Cargo.toml
new file mode 100644
index 000000000..99f6fea2e
--- /dev/null
+++ b/src/tools/build_helper/Cargo.toml
@@ -0,0 +1,8 @@
+[package]
+name = "build_helper"
+version = "0.1.0"
+edition = "2021"
+
+# See more keys and their definitions at https://doc.rust-lang.org/cargo/reference/manifest.html
+
+[dependencies]
diff --git a/src/tools/build_helper/src/ci.rs b/src/tools/build_helper/src/ci.rs
new file mode 100644
index 000000000..9f113c72b
--- /dev/null
+++ b/src/tools/build_helper/src/ci.rs
@@ -0,0 +1,40 @@
+use std::process::Command;
+
+#[derive(Copy, Clone, PartialEq, Eq, Debug)]
+pub enum CiEnv {
+    /// Not a CI environment.
+    None,
+    /// The Azure Pipelines environment, for Linux (including Docker), Windows, and macOS builds.
+    AzurePipelines,
+    /// The GitHub Actions environment, for Linux (including Docker), Windows and macOS builds.
+    GitHubActions,
+}
+
+impl CiEnv {
+    /// Obtains the current CI environment.
+    pub fn current() -> CiEnv {
+        if std::env::var("TF_BUILD").map_or(false, |e| e == "True") {
+            CiEnv::AzurePipelines
+        } else if std::env::var("GITHUB_ACTIONS").map_or(false, |e| e == "true") {
+            CiEnv::GitHubActions
+        } else {
+            CiEnv::None
+        }
+    }
+
+    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 {
+            // Due to use of stamp/docker, the output stream of rustbuild is not
+            // a TTY in CI, so coloring is by-default turned off.
+            // The explicit `TERM=xterm` environment is needed for
+            // `--color always` to actually work. This env var was lost when
+            // compiling through the Makefile. Very strange.
+            cmd.env("TERM", "xterm").args(&["--color", "always"]);
+        }
+    }
+}
diff --git a/src/tools/build_helper/src/git.rs b/src/tools/build_helper/src/git.rs
new file mode 100644
index 000000000..dc62051cb
--- /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())
+}
diff --git a/src/tools/build_helper/src/lib.rs b/src/tools/build_helper/src/lib.rs
new file mode 100644
index 000000000..d3d2323db
--- /dev/null
+++ b/src/tools/build_helper/src/lib.rs
@@ -0,0 +1,2 @@
+pub mod ci;
+pub mod git;