Provide pdb file for cydlib/bin on Windows in a pdb_file output group

I've chose the `pdb_file` name for the output group since it matches
what `cc_binary` uses
(https://cs.opensource.google/bazel/bazel/+/master:src/main/java/com/google/devtools/build/lib/rules/cpp/CcBinary.java;l=707)

There is one edge case that is not covered here: if the user passes
`/DEBUG:NONE` in the `rustc_flags` no pdb will be generated and I
imagine the build will fail since it's now missing an output. I guess we
could check the `rustc_flags` for it but I don't know if it's the right
place to do so?

rust/private/rust.bzl

@@ -375,6 +375,23 @@ def _create_test_launcher(ctx, toolchain, output, env, providers):
     if not default_info:
         fail("No DefaultInfo provider returned from `rustc_compile_action`")
 
+    output_group_info = OutputGroupInfo(
+        launcher_files = depset(launcher_files),
+        output = depset([output]),
+    )
+
+    # Binaries on Windows might provide a pdb file via an `OutputGroupInfo` that we need to merge
+    if toolchain.os == "windows":
+        for i in range(len(providers)):
+            if type(providers[i]) == "OutputGroupInfo":
+                output_group_info = OutputGroupInfo(
+                    launcher_files = output_group_info.launcher_files,
+                    output = output_group_info.output,
+                    pdb_file = providers[i].pdb_file,
+                )
+                providers.pop(i)
+                break
+
     providers.extend([
         DefaultInfo(
             files = default_info.files,
@@ -384,10 +401,7 @@ def _create_test_launcher(ctx, toolchain, output, env, providers):
             ),
             executable = launcher,
         ),
-        OutputGroupInfo(
-            launcher_files = depset(launcher_files),
-            output = depset([output]),
-        ),
+        output_group_info,
     ])
 
     return providers

rust/private/rustc.bzl

@@ -794,19 +794,26 @@ def rustc_compile_action(
 
     outputs = [crate_info.output]
 
-    # For a cdylib that might be added as a dependency to a cc_* target on Windows, it is important to include the
-    # interface library that rustc generates in the output files.
     interface_library = None
-    if toolchain.os == "windows" and crate_info.type == "cdylib":
-        # Rustc generates the import library with a `.dll.lib` extension rather than the usual `.lib` one that msvc
-        # expects (see https://github.com/rust-lang/rust/pull/29520 for more context).
-        interface_library = ctx.actions.declare_file(crate_info.output.basename + ".lib")
-        outputs.append(interface_library)
+    pdb_file = None
+    if toolchain.os == "windows":
+        # For a cdylib that might be added as a dependency to a cc_* target on Windows, it is important to include the
+        # interface library that rustc generates in the output files.
+        if crate_info.type == "cdylib":
+            # Rustc generates the import library with a `.dll.lib` extension rather than the usual `.lib` one that msvc
+            # expects (see https://github.com/rust-lang/rust/pull/29520 for more context).
+            interface_library = ctx.actions.declare_file(crate_info.output.basename + ".lib")
+            outputs.append(interface_library)
+
+        # Rustc always generates a pdb file so provide it in an output group for crate types that benefit from having
+        # debug information in a separate file. Note that test targets do really need a pdb we skip them.
+        if crate_info.type in ("cdylib", "bin") and not crate_info.is_test:
+            pdb_file = ctx.actions.declare_file(crate_info.output.basename[:-len(crate_info.output.basename.extension)] + "pdb")
 
     ctx.actions.run(
         executable = ctx.executable._process_wrapper,
         inputs = compile_inputs,
-        outputs = outputs,
+        outputs = outputs + [pdb_file] if pdb_file else outputs,
         env = env,
         arguments = args.all,
         mnemonic = "Rustc",
@@ -839,6 +846,8 @@ def rustc_compile_action(
     ]
     if toolchain.target_arch != "wasm32":
         providers += establish_cc_info(ctx, attr, crate_info, toolchain, cc_toolchain, feature_configuration, interface_library)
+    if pdb_file:
+        providers.append(OutputGroupInfo(pdb_file = depset([pdb_file])))
 
     return providers
 

test/unit/win_interface_library/win_interface_library_analysis_test.bzl

@@ -36,6 +36,12 @@ def win_interface_library_analysis_test_suite(name):
         target_compatible_with = ["@platforms//os:windows"],
     )
 
+    native.filegroup(
+        name = "mylib.pdb",
+        srcs = [":mylib"],
+        output_group = "pdb_file",
+    )
+
     cc_binary(
         name = "mybin",
         srcs = ["bin.cc"],