rustc: add support for split debuginfo (aka fission)

This requires some helper logic in the process wrapper so we can move
dwo files to predictable output locations, but otherwise it's mostly
straightforward. This requires a bazel built after
d2c79821cd45b30eeaec2efc3bd5fbd26f37d3c2, but that's old enough it
landed in bazel 8.

rust/private/rustc.bzl

@@ -1224,6 +1224,13 @@ def rustc_compile_action(
     if experimental_use_cc_common_link:
         emit = ["obj"]
 
+    use_split_debuginfo = cc_common.is_enabled(feature_configuration = feature_configuration, feature_name = "per_object_debug_info") and ctx.fragments.cpp.fission_active_for_current_compilation_mode()
+    if use_split_debuginfo:
+        rust_flags = rust_flags + [
+            "--codegen=split-debuginfo=unpacked",
+            "--codegen=debuginfo=full",
+        ]
+
     args, env_from_args = construct_arguments(
         ctx = ctx,
         attr = attr,
@@ -1324,6 +1331,13 @@ def rustc_compile_action(
         elif toolchain.target_os == "darwin":
             dsym_folder = ctx.actions.declare_directory(crate_info.output.basename + ".dSYM", sibling = crate_info.output)
             action_outputs.append(dsym_folder)
+    if use_split_debuginfo:
+        fission_directory = crate_info.name + "_fission"
+        if output_hash:
+            fission_directory = fission_directory + output_hash
+        dwo_outputs = ctx.actions.declare_directory(fission_directory, sibling = crate_info.output)
+        args.process_wrapper_flags.add("--kludge-move-dwo-to", dwo_outputs.path)
+        action_outputs.append(dwo_outputs)
 
     if ctx.executable._process_wrapper:
         # Run as normal
@@ -1382,15 +1396,19 @@ def rustc_compile_action(
     else:
         fail("No process wrapper was defined for {}".format(ctx.label))
 
+    cco_args = {}
     if experimental_use_cc_common_link:
         # Wrap the main `.o` file into a compilation output suitable for
         # cc_common.link. The main `.o` file is useful in both PIC and non-PIC
         # modes.
-        compilation_outputs = cc_common.create_compilation_outputs(
-            objects = depset([output_o]),
-            pic_objects = depset([output_o]),
-        )
-
+        cco_args["objects"] = depset([output_o])
+        cco_args["pic_objects"] = depset([output_o])
+    if use_split_debuginfo:
+        cco_args["dwo_objects"] = depset([dwo_outputs])  # buildifier: disable=uninitialized
+        cco_args["pic_dwo_objects"] = depset([dwo_outputs])  # buildifier: disable=uninitialized
+    compilation_outputs = cc_common.create_compilation_outputs(**cco_args)
+    debug_context = cc_common.create_debug_context(compilation_outputs)
+    if experimental_use_cc_common_link:
         malloc_library = ctx.attr._custom_malloc or ctx.attr.malloc
 
         # Collect the linking contexts of the standard library and dependencies.
@@ -1535,7 +1553,7 @@ def rustc_compile_action(
     else:
         providers.extend([crate_info, dep_info])
 
-    providers += establish_cc_info(ctx, attr, crate_info, toolchain, cc_toolchain, feature_configuration, interface_library)
+    providers += establish_cc_info(ctx, attr, crate_info, toolchain, cc_toolchain, feature_configuration, interface_library, debug_context)
 
     output_group_info = {}
 
@@ -1617,7 +1635,7 @@ def _add_codegen_units_flags(toolchain, args):
 
     args.add("-Ccodegen-units={}".format(toolchain._codegen_units))
 
-def establish_cc_info(ctx, attr, crate_info, toolchain, cc_toolchain, feature_configuration, interface_library):
+def establish_cc_info(ctx, attr, crate_info, toolchain, cc_toolchain, feature_configuration, interface_library, debug_context = None):
     """If the produced crate is suitable yield a CcInfo to allow for interop with cc rules
 
     Args:
@@ -1628,7 +1646,7 @@ def establish_cc_info(ctx, attr, crate_info, toolchain, cc_toolchain, feature_co
         cc_toolchain (CcToolchainInfo): The current `CcToolchainInfo`
         feature_configuration (FeatureConfiguration): Feature configuration to be queried.
         interface_library (File): Optional interface library for cdylib crates on Windows.
-
+        debug_context (DebugContext): Optional debug context.
     Returns:
         list: A list containing the CcInfo provider
     """
@@ -1694,7 +1712,10 @@ def establish_cc_info(ctx, attr, crate_info, toolchain, cc_toolchain, feature_co
     )
 
     cc_infos = [
-        CcInfo(linking_context = linking_context),
+        CcInfo(
+            linking_context = linking_context,
+            debug_context = debug_context,
+        ),
         toolchain.stdlib_linkflags,
     ]
 

util/process_wrapper/main.rs

@@ -184,6 +184,24 @@ fn main() -> Result<(), ProcessWrapperError> {
                 ))
             })?;
         }
+        if let Some(dwp_destination) = opts.kludge_move_dwo_to {
+            // find all .dwo files in our out_dir (how do we get that? --out-dir?)
+            // move them all to dwp_destination
+            let dest = std::path::Path::new(&dwp_destination);
+            // Unwrap is okay because we know that --kludge-move-dwo-two will be a child of
+            // rustc's --out-dir.
+            let out_dir = dest.parent().unwrap();
+            let paths = std::fs::read_dir(out_dir).unwrap();
+            for p in paths {
+                let p = p.unwrap();
+                let fn_ = p.file_name();
+                let basename = fn_.to_string_lossy();
+                if basename.ends_with(".dwo") {
+                    let tgt = dest.join(fn_);
+                    std::fs::rename(p.path(), tgt).map_err(|e| ProcessWrapperError(format!("failed to rename dwo output: {}", e)))?;
+                }
+            }
+        }
     }
 
     exit(code)

util/process_wrapper/options.rs

@@ -49,6 +49,8 @@ pub(crate) struct Options {
     pub(crate) rustc_quit_on_rmeta: bool,
     // This controls the output format of rustc messages.
     pub(crate) rustc_output_format: Option<rustc::ErrorFormat>,
+    // If set, move any produced .dwo files into the named directory.
+    pub(crate) kludge_move_dwo_to: Option<String>,
 }
 
 pub(crate) fn options() -> Result<Options, OptionError> {
@@ -67,6 +69,7 @@ pub(crate) fn options() -> Result<Options, OptionError> {
     let mut rustc_quit_on_rmeta_raw = None;
     let mut rustc_output_format_raw = None;
     let mut flags = Flags::new();
+    let mut kludge_move_dwo_to = None;
     flags.define_repeated_flag("--subst", "", &mut subst_mapping_raw);
     flags.define_flag("--stable-status-file", "", &mut stable_status_file_raw);
     flags.define_flag("--volatile-status-file", "", &mut volatile_status_file_raw);
@@ -114,6 +117,11 @@ pub(crate) fn options() -> Result<Options, OptionError> {
         Default: `rendered`",
         &mut rustc_output_format_raw,
     );
+    flags.define_flag(
+        "--kludge-move-dwo-to",
+        "If set, move all produced .dwo files to the specified directory.",
+        &mut kludge_move_dwo_to,
+    );
 
     let mut child_args = match flags
         .parse(env::args().collect())
@@ -212,6 +220,7 @@ pub(crate) fn options() -> Result<Options, OptionError> {
         output_file,
         rustc_quit_on_rmeta,
         rustc_output_format,
+        kludge_move_dwo_to,
     })
 }