tidy: skip triagebot.toml path checks for unchecked out submodules locally

I say "unchecked out", but really it's an extremely naive heuristic of
"don't bother checking paths to or under a submodule if we can't even
read the submodule dir".

Author: jieyouxu

src/tools/tidy/src/triagebot.rs

@@ -1,11 +1,13 @@
 //! Tidy check to ensure paths mentioned in triagebot.toml exist in the project.
 
-use std::path::Path;
+use std::collections::HashMap;
+use std::path::{Path, PathBuf};
 
+use build_helper::ci::CiEnv;
 use toml::Value;
 
-pub fn check(path: &Path, bad: &mut bool) {
-    let triagebot_path = path.join("triagebot.toml");
+pub fn check(checkout_root: &Path, bad: &mut bool) {
+    let triagebot_path = checkout_root.join("triagebot.toml");
     if !triagebot_path.exists() {
         tidy_error!(bad, "triagebot.toml file not found");
         return;
@@ -14,73 +16,91 @@ pub fn check(path: &Path, bad: &mut bool) {
     let contents = std::fs::read_to_string(&triagebot_path).unwrap();
     let config: Value = toml::from_str(&contents).unwrap();
 
-    // Check [mentions."*"] sections, i.e. [mentions."compiler/rustc_const_eval/src/"]
+    // Cache mapping between submodule path <-> whether submodule is checked out. This cache is to
+    // avoid excessive filesystem accesses.
+    let submodule_checked_out_status = cache_submodule_checkout_status(checkout_root);
+
+    // Check `[mentions."*"]` sections, i.e.
+    //
+    // ```
+    // [mentions."compiler/rustc_const_eval/src/"]
+    // ```
     if let Some(Value::Table(mentions)) = config.get("mentions") {
         for path_str in mentions.keys() {
             // Remove quotes from the path
             let clean_path = path_str.trim_matches('"');
-            let full_path = path.join(clean_path);
+            let full_path = checkout_root.join(clean_path);
 
-            if !full_path.exists() {
+            if !check_path_exists_if_required(&submodule_checked_out_status, &full_path) {
                 tidy_error!(
                     bad,
-                    "triagebot.toml [mentions.*] contains path '{}' which doesn't exist",
+                    "triagebot.toml `[mentions.*]` contains path `{}` which doesn't exist",
                     clean_path
                 );
             }
         }
     } else {
         tidy_error!(
             bad,
-            "triagebot.toml missing [mentions.*] section, this wrong for rust-lang/rust repo."
+            "`triagebot.toml` is missing the `[mentions.*]` section; this is wrong for the \
+            `rust-lang/rust` repo."
         );
     }
 
-    // Check [assign.owners] sections, i.e.
+    // Check `[assign.owners]` sections, i.e.
+    //
+    // ```
     // [assign.owners]
     // "/.github/workflows" = ["infra-ci"]
+    // ```
     if let Some(Value::Table(assign)) = config.get("assign") {
         if let Some(Value::Table(owners)) = assign.get("owners") {
             for path_str in owners.keys() {
                 // Remove quotes and leading slash from the path
                 let clean_path = path_str.trim_matches('"').trim_start_matches('/');
-                let full_path = path.join(clean_path);
+                let full_path = checkout_root.join(clean_path);
 
-                if !full_path.exists() {
+                if !check_path_exists_if_required(&submodule_checked_out_status, &full_path) {
                     tidy_error!(
                         bad,
-                        "triagebot.toml [assign.owners] contains path '{}' which doesn't exist",
+                        "`triagebot.toml` `[assign.owners]` contains path `{}` which doesn't exist",
                         clean_path
                     );
                 }
             }
         } else {
             tidy_error!(
                 bad,
-                "triagebot.toml missing [assign.owners] section, this wrong for rust-lang/rust repo."
+                "`triagebot.toml` is missing the `[assign.owners]` section; this is wrong for the \
+                `rust-lang/rust` repo."
             );
         }
     }
 
-    // Verify that trigger_files in [autolabel."*"] exist in the project, i.e.
+    // Verify that `trigger_files` paths in `[autolabel."*"]` exists, i.e.
+    //
+    // ```
     // [autolabel."A-rustdoc-search"]
     // trigger_files = [
     //    "src/librustdoc/html/static/js/search.js",
     //    "tests/rustdoc-js",
     //    "tests/rustdoc-js-std",
     // ]
+    // ```
     if let Some(Value::Table(autolabels)) = config.get("autolabel") {
         for (label, content) in autolabels {
             if let Some(trigger_files) = content.get("trigger_files").and_then(|v| v.as_array()) {
                 for file in trigger_files {
                     if let Some(file_str) = file.as_str() {
-                        let full_path = path.join(file_str);
+                        let full_path = checkout_root.join(file_str);
 
                         // Handle both file and directory paths
-                        if !full_path.exists() {
+                        if !check_path_exists_if_required(&submodule_checked_out_status, &full_path)
+                        {
                             tidy_error!(
                                 bad,
-                                "triagebot.toml [autolabel.{}] contains trigger_files path '{}' which doesn't exist",
+                                "`triagebot.toml` `[autolabel.{}]` contains `trigger_files` path \
+                                `{}` which doesn't exist",
                                 label,
                                 file_str
                             );
@@ -91,3 +111,61 @@ pub fn check(path: &Path, bad: &mut bool) {
         }
     }
 }
+
+/// Very naive heuristics for whether a submodule is checked out.
+fn cache_submodule_checkout_status(checkout_root: &Path) -> HashMap<PathBuf, bool> {
+    let mut cache = HashMap::default();
+
+    // NOTE: can't assume `git` exists.
+    let submodule_paths = build_helper::util::parse_gitmodules(&checkout_root);
+
+    for submodule in submodule_paths {
+        let full_submodule_path = checkout_root.join(submodule);
+
+        let is_checked_out = if CiEnv::is_ci() {
+            // In CI, require all submodules to be checked out and thus don't skip checking any
+            // paths.
+            true
+        } else {
+            // NOTE: for our purposes, just skip checking paths to and under a submodule if we can't
+            // read its dir locally (even if this can miss broken paths).
+            std::fs::read_dir(&full_submodule_path).is_ok_and(|entry| {
+                // NOTE: de-initializing a submodule can leave an empty folder behind
+                entry.count() > 0
+            })
+        };
+
+        if let Some(_) = cache.insert(full_submodule_path.clone(), is_checked_out) {
+            panic!(
+                "unexpected duplicate submodule paths in `deps::WORKSPACES`: {} already in \
+                submodule checkout cache",
+                full_submodule_path.display()
+            );
+        }
+    }
+
+    cache
+}
+
+/// Check that a path exists. This is:
+///
+/// - Unconditionally checked under CI environment.
+/// - Only checked under local environment if submodule is checked out (if candidate path points
+///   under or to a submodule).
+fn check_path_exists_if_required(
+    submodule_checkout_status: &HashMap<PathBuf, bool>,
+    candidate: &Path,
+) -> bool {
+    for (submodule_path, is_checked_out) in submodule_checkout_status {
+        if candidate.starts_with(submodule_path) {
+            if *is_checked_out {
+                return candidate.exists();
+            } else {
+                // Not actually checked, but just skipped.
+                return true;
+            }
+        }
+    }
+
+    candidate.exists()
+}