From a962df4d0e0e57c6967d2ec5be8bfaf4957f6297 Mon Sep 17 00:00:00 2001
From: =?UTF-8?q?Lars=20Sj=C3=B6str=C3=B6m?= <lars@radicore.se>
Date: Sat, 8 Aug 2026 23:19:33 +0200
Subject: [PATCH] fix(rust_analyzer): resolve external crate sources via
 output_base, not execroot

gen_rust_project emits external crate paths through the execution root
(__EXEC_ROOT__/external/...). The execroot's external/ symlinks are
ephemeral -- only guaranteed to exist while a build action is running --
so once the build finishes those paths dangle and rust-analyzer cannot
read any external crate sources. The result is no completions, no
go-to-definition, and no docs for third-party dependencies.

The real, stable location of external repository sources is
{output_base}/external/. Rewrite {execution_root}/external/ prefixes to
{output_base}/external/ during template substitution, while leaving
bazel-out/ paths (generated files) under the execution root where they
belong.

Fixes the first bug described in #4057.

Assisted-by: Claude (Anthropic AI assistant)
---
 tools/rust_analyzer/lib.rs | 41 +++++++++++++++++++++++++++++++++++++-
 1 file changed, 40 insertions(+), 1 deletion(-)

diff --git a/tools/rust_analyzer/lib.rs b/tools/rust_analyzer/lib.rs
index 6733caedf3..dc3238ea9f 100644
--- a/tools/rust_analyzer/lib.rs
+++ b/tools/rust_analyzer/lib.rs
@@ -496,7 +496,16 @@ where
         .replace("__WORKSPACE__", workspace.as_str())
         .replace("${pwd}", execution_root.as_str())
         .replace("__EXEC_ROOT__", execution_root.as_str())
-        .replace("__OUTPUT_BASE__", output_base.as_str());
+        .replace("__OUTPUT_BASE__", output_base.as_str())
+        // External repository sources live at `{output_base}/external/`, not inside
+        // the execution root. The `{execution_root}/external/` symlinks are ephemeral
+        // (only guaranteed to exist while an action is running), so paths through the
+        // execution root break once the build finishes, leaving rust-analyzer unable
+        // to read external crate sources.
+        .replace(
+            &format!("{execution_root}/external/"),
+            &format!("{output_base}/external/"),
+        );
 
     serde_json::from_str(&content).context("failed to deserialize after template substitution")
 }
@@ -557,6 +566,36 @@ pub struct ToolchainInfo {
 mod tests {
     use super::*;
 
+    #[test]
+    fn substitution_rewrites_external_paths_to_output_base() {
+        let output_base = Utf8Path::new("/output_base");
+        let workspace = Utf8Path::new("/workspace");
+        let execution_root = Utf8Path::new("/output_base/execroot/_main");
+
+        let content = r#"[
+            "__EXEC_ROOT__/external/rules_rust+crate+anyhow-1.0.0/src/lib.rs",
+            "__EXEC_ROOT__/bazel-out/k8-fastbuild/bin/proto/gen.rs",
+            "__OUTPUT_BASE__/external/toolchain/lib/rustlib/src",
+            "__WORKSPACE__/mylib/src/lib.rs"
+        ]"#;
+
+        let paths: Vec<String> =
+            deserialize_with_substitution(content, output_base, workspace, execution_root).unwrap();
+
+        assert_eq!(
+            paths,
+            vec![
+                // `external/` paths must resolve via the output base; the
+                // execroot symlinks are gone after the build.
+                "/output_base/external/rules_rust+crate+anyhow-1.0.0/src/lib.rs",
+                // `bazel-out/` paths stay under the execution root.
+                "/output_base/execroot/_main/bazel-out/k8-fastbuild/bin/proto/gen.rs",
+                "/output_base/external/toolchain/lib/rustlib/src",
+                "/workspace/mylib/src/lib.rs",
+            ],
+        );
+    }
+
     #[test]
     fn dir_to_bazel_package_normalizes_backslashes() {
         // Windows-shaped input (post-strip_prefix.parent()): backslashes.