fix external crate paths for gen_rust_project (#4200)

This addresses part 1 of #4057

test/rust_analyzer/generated_srcs_test/rust_project_json_test.rs

diff --git a/test/rust_analyzer/generated_srcs_test/rust_project_json_test.rs b/test/rust_analyzer/generated_srcs_test/rust_project_json_test.rs
index d6a2c5b..2c6e351 100644
--- a/test/rust_analyzer/generated_srcs_test/rust_project_json_test.rs
+++ b/test/rust_analyzer/generated_srcs_test/rust_project_json_test.rs
@@ -22,23 +22,23 @@
         include_dirs: Vec<String>,
     }
 
+    /// `{output_base}/external/.../library` → `{output_base}`
+    fn output_base(p: &Project) -> &str {
+        p.sysroot_src
+            .rsplitn(2, "/external/")
+            .last()
+            .expect("sysroot_src should contain /external/")
+    }
+
     #[test]
     fn test_generated_srcs() {
         let rust_project_path = PathBuf::from(env::var("RUST_PROJECT_JSON").unwrap());
-        let content = std::fs::read_to_string(&rust_project_path)
-            .unwrap_or_else(|_| panic!("couldn't open {:?}", rust_project_path));
+        let content = std::fs::read_to_string(&rust_project_path).unwrap();
         let project: Project =
-            serde_json::from_str(&content).expect("Failed to deserialize project JSON");
+            serde_json::from_str(&content).expect("project JSON should deserialize correctly");
 
-        // /tmp/_bazel/12345678/external/tools/rustlib/library => /tmp/_bazel
-        let output_base = project
-            .sysroot_src
-            .rsplitn(2, "/external/")
-            .last()
-            .unwrap()
-            .rsplitn(2, '/')
-            .last()
-            .unwrap();
+        // /tmp/_bazel/12345678/external/tools/rustlib/library => /tmp/_bazel/12345678
+        let output_base = output_base(&project);
         println!("output_base: {output_base}");
 
         let with_gen = project
@@ -58,4 +58,28 @@
         // The second entry is the output base, where the generated files are located.
         assert!(include_dirs[1].starts_with(output_base));
     }
+
+    #[test]
+    fn test_external_root_paths() {
+        let rust_project_path = PathBuf::from(env::var("RUST_PROJECT_JSON").unwrap());
+        let content = std::fs::read_to_string(&rust_project_path).unwrap();
+        let project: Project =
+            serde_json::from_str(&content).expect("project JSON should deserialize correctly");
+        let output_base = output_base(&project);
+        let mut external_roots = project
+            .crates
+            .iter()
+            .map(|c| &c.root_module)
+            .filter(|p| p.contains("/external/"))
+            .peekable();
+
+        assert!(external_roots.peek().is_some());
+
+        let output_base_external = format!("{output_base}/external/");
+
+        for root in external_roots {
+            assert!(root.starts_with(&output_base_external));
+            assert!(!root.contains("execroot"));
+        }
+    }
 }

tools/rust_analyzer/lib.rs

diff --git a/tools/rust_analyzer/lib.rs b/tools/rust_analyzer/lib.rs
index 6733cae..092a2e7 100644
--- a/tools/rust_analyzer/lib.rs
+++ b/tools/rust_analyzer/lib.rs
@@ -496,7 +496,14 @@
         .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())
+        // exec_root/external contains symlinks into the output_base/external
+        // but those symlinks are ephemeral. if we do not replace here, goto
+        // definition can fail because those symlinks would no longer be found.
+        .replace(
+            &format!("{execution_root}/external/"),
+            &format!("{output_base}/external/"),
+        );
 
     serde_json::from_str(&content).context("failed to deserialize after template substitution")
 }