rust_bindgen_library: support cc_lib targets with only a dynamic library (#4251)

Split of from #4249

---------

Co-authored-by: Fabrice <schaubfabrice@gmail.com>
Co-authored-by: Krasimir Georgiev <krasimir@google.com>

rust/private/rust_analyzer.bzl

diff --git a/rust/private/rust_analyzer.bzl b/rust/private/rust_analyzer.bzl
index 7fa8686..76519f5 100644
--- a/rust/private/rust_analyzer.bzl
+++ b/rust/private/rust_analyzer.bzl
@@ -266,7 +266,7 @@
 
     # We're only interested in the build info for local crates as these are the
     # only ones for which we want build file watching and code lens runnables support.
-    if not is_external and not is_generated:
+    if not is_external:
         crate["build"] = {
             "build_file": _WORKSPACE_TEMPLATE + ctx.build_file_path,
             # Emit canonical `//pkg:name` form. Bazel's BEP reports action
@@ -280,8 +280,16 @@
     if is_generated:
         srcs = getattr(ctx.rule.files, "srcs", [])
         src_map = {src.short_path: src for src in srcs if src.is_source}
+
+        # Only re-anchor into the workspace when every sibling src lives there too.
+        has_generated_srcs = any([
+            not src.is_source
+            for src in srcs
+            if src.short_path != info.crate.root.short_path
+        ])
         if info.crate.root.short_path in src_map:
-            crate["root_module"] = _WORKSPACE_TEMPLATE + src_map[info.crate.root.short_path].path
+            if not has_generated_srcs:
+                crate["root_module"] = _WORKSPACE_TEMPLATE + src_map[info.crate.root.short_path].path
             crate["source"]["include_dirs"].extend([
                 _WORKSPACE_TEMPLATE + src_map[info.crate.root.short_path].dirname,
                 path_prefix + info.crate.root.dirname,

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 2c6e351..24b65f9 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
@@ -49,6 +49,9 @@
         assert!(with_gen.root_module.starts_with("/"));
         assert!(with_gen.root_module.ends_with("/lib.rs"));
 
+        // root_module isn't re-anchored to the workspace since it has a generated sibling.
+        assert!(with_gen.root_module.starts_with(output_base));
+
         let include_dirs = &with_gen.source.as_ref().unwrap().include_dirs;
         assert_eq!(include_dirs.len(), 2);