Generate base commit in tidy checks

Author: GuillaumeGomez

src/tools/tidy/src/main.rs

@@ -7,19 +7,48 @@
 use std::collections::VecDeque;
 use std::num::NonZeroUsize;
 use std::path::PathBuf;
+use std::process::Command;
 use std::str::FromStr;
 use std::sync::atomic::{AtomicBool, Ordering};
 use std::thread::{self, ScopedJoinHandle, scope};
 use std::{env, process};
 
 use tidy::*;
 
+// We set the `BASE_COMMIT` environment variable
+fn set_env_base_commit() {
+    if env::var("BASE_COMMIT").is_ok() {
+        // Nothing to be done!
+        return;
+    }
+    // If first commit is from `[email protected]`, then we need to skip it as it means we're in
+    // the CI and it's the merge commit generated to test the PR.
+    let output = Command::new("git")
+        .args(&["log", "-n", "1", "--pretty=format:%ae"])
+        .output()
+        .expect("Failed to run `git` command to retrieve base commit");
+    let output = String::from_utf8_lossy(&output.stdout);
+    let skip = if output.trim() == "[email protected]" { "--skip=1" } else { "--skip=0" };
+
+    // Now we find the last merge commit from bors to know what base commit to use.
+    let output = Command::new("git")
+        .args(&["log", "[email protected]", "-n", "1", "--pretty=format:%H", skip])
+        .output()
+        .expect("Failed to run `git` command to retrieve base commit");
+    let output = String::from_utf8_lossy(&output.stdout);
+
+    let base_commit = output.trim();
+    eprintln!("setting base commit: {base_commit}");
+    env::set_var("BASE_COMMIT", base_commit);
+}
+
 fn main() {
     // Running Cargo will read the libstd Cargo.toml
     // which uses the unstable `public-dependency` feature.
     //
     // `setenv` might not be thread safe, so run it before using multiple threads.
     env::set_var("RUSTC_BOOTSTRAP", "1");
+    set_env_base_commit();
 
     let root_path: PathBuf = env::args_os().nth(1).expect("need path to root of repo").into();
     let cargo: PathBuf = env::args_os().nth(2).expect("need path to cargo").into();

src/tools/tidy/src/rustdoc_json.rs

@@ -9,11 +9,7 @@ fn git_diff(base_commit: &str, extra_arg: &str) -> Option<String> {
 }
 
 pub fn check(bad: &mut bool) {
-    let Ok(base_commit) = std::env::var("BASE_COMMIT") else {
-        // Not in CI so nothing we can check here.
-        println!("not checking rustdoc JSON `FORMAT_VERSION` update");
-        return;
-    };
+    let base_commit = std::env::var("BASE_COMMIT").unwrap();
 
     // First we check that `src/rustdoc-json-types` was modified.
     match git_diff(&base_commit, "--name-status") {