Adjust windows CRT linking for the new /defaultlib linking in core.

This more strictly follows the recipe in
rust-lang/rust#122268 (comment)

And we remove the -Zlink-directives=false for building the libc crate,
as that crate no longer provides the CRT link directives.

The link directives are now in core, and are done through /defaultlib
so that we can remove it in the command line with /nodefaultlib. This
allows us to control linking entirely through our GN linking rules,
and not during stdlib compilation, giving us support for prebuilt
Rust stdlib as well.

[email protected]

Bug: 5476668
Change-Id: I1466c4f721a9d8cc9ac3465c2e15f866b731a738
Cq-Include-Trybots: luci.chromium.try:android-rust-arm32-rel,android-rust-arm64-rel,android-rust-arm64-dbg,linux-rust-x64-dbg,linux-rust-x64-rel,mac-rust-x64-dbg,win-rust-x64-dbg,win-rust-x64-rel
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/5477889
Reviewed-by: Nico Weber <[email protected]>
Commit-Queue: danakj <[email protected]>
Cr-Commit-Position: refs/heads/main@{#1292464}
NOKEYCHECK=True
GitOrigin-RevId: 714e31ddaca456ecf74d7f6f309447d6095f7ed4

config/win/BUILD.gn

@@ -511,8 +511,9 @@ config("release_crt") {
   if (is_component_build) {
     cflags = [ "/MD" ]
 
-    # The - in front of crt-static turns the feature off.
-    rustflags = [ "-Ctarget-feature=-crt-static" ]
+    # /MD specifies msvcrt.lib as the CRT library, which is the dynamic+release
+    # version. Rust needs to agree, and its default mode is dynamic+release, so
+    # we do nothing here. See https://github.com/rust-lang/rust/issues/39016.
 
     if (use_custom_libcxx) {
       # On Windows, including libcpmt[d]/msvcprt[d] explicitly links the C++
@@ -521,6 +522,10 @@ config("release_crt") {
     }
   } else {
     cflags = [ "/MT" ]
+
+    # /MT specifies libcmt.lib as the CRT library, which is the static+release
+    # version. Rust needs to agree, so we tell it to use the static+release CRT
+    # as well. See https://github.com/rust-lang/rust/issues/39016.
     rustflags = [ "-Ctarget-feature=+crt-static" ]
 
     if (use_custom_libcxx) {
@@ -534,18 +539,13 @@ config("dynamic_crt") {
     # This pulls in the DLL debug CRT and defines _DEBUG
     cflags = [ "/MDd" ]
 
-    # /MDd specifies msvcrtd.lib as the CRT library. Rust needs to agree, so
-    # we specify it explicitly.
-    # Once https://github.com/rust-lang/rust/issues/39016 is resolved we should
-    # instead tell rustc which CRT to use (static/dynamic + release/debug). We
-    # can't support prebuilt stdlib in this path until then.
-    rustflags = [ "-Clink-arg=msvcrtd.lib" ]
-
-    # Disable libraries rustc links implicitly;
-    # see https://github.com/rust-lang/rust/pull/122268
-    rustflags += [
-      "-Clink-arg=/nodefaultlib:libcmt.lib",
+    # /MDd specifies msvcrtd.lib as the CRT library. Rust needs to agree, so we
+    # specify it explicitly. Rust defaults to the dynamic+release library, which
+    # we remove here, and then replace. See
+    # https://github.com/rust-lang/rust/issues/39016.
+    rustflags = [
       "-Clink-arg=/nodefaultlib:msvcrt.lib",
+      "-Clink-arg=msvcrtd.lib",
     ]
 
     if (use_custom_libcxx) {
@@ -554,8 +554,9 @@ config("dynamic_crt") {
   } else {
     cflags = [ "/MD" ]
 
-    # The - in front of crt-static turns the feature off.
-    rustflags = [ "-Ctarget-feature=-crt-static" ]
+    # /MD specifies msvcrt.lib as the CRT library, which is the dynamic+release
+    # version. Rust needs to agree, and its default mode is dynamic+release, so
+    # we do nothing here. See https://github.com/rust-lang/rust/issues/39016.
 
     if (use_custom_libcxx) {
       ldflags = [ "/DEFAULTLIB:msvcprt.lib" ]
@@ -568,25 +569,25 @@ config("static_crt") {
     # This pulls in the static debug CRT and defines _DEBUG
     cflags = [ "/MTd" ]
 
-    # /MTd specifies libcmtd.lib as the CRT library. Rust needs to agree, so
-    # we specify it explicitly.
-    # Once https://github.com/rust-lang/rust/issues/39016 is resolved we should
-    # instead tell rustc which CRT to use (static/dynamic + release/debug). We
-    # can't support prebuilt stdlib in this path until then.
-    rustflags = [ "-Clink-arg=libcmtd.lib" ]
-
-    # Disable libraries rustc links implicitly;
-    # see https://github.com/rust-lang/rust/pull/122268
-    rustflags += [
+    # /MTd specifies libcmtd.lib as the CRT library. Rust needs to agree, so we
+    # specify it explicitly. We tell Rust that we're using the static CRT but
+    # remove the release library that it chooses, and replace with the debug
+    # library. See https://github.com/rust-lang/rust/issues/39016.
+    rustflags = [
+      "-Ctarget-feature=+crt-static",
       "-Clink-arg=/nodefaultlib:libcmt.lib",
-      "-Clink-arg=/nodefaultlib:msvcrt.lib",
+      "-Clink-arg=libcmtd.lib",
     ]
 
     if (use_custom_libcxx) {
       ldflags = [ "/DEFAULTLIB:libcpmtd.lib" ]
     }
   } else {
     cflags = [ "/MT" ]
+
+    # /MT specifies libcmt.lib as the CRT library, which is the static+release
+    # version. Rust needs to agree, so we tell it to use the static+release CRT
+    # as well. See https://github.com/rust-lang/rust/issues/39016.
     rustflags = [ "-Ctarget-feature=+crt-static" ]
 
     if (use_custom_libcxx) {

rust/std/gnrt_config.toml

@@ -64,9 +64,7 @@ remove_deps = ['allocator-api2']
 [crate.libc]
 # This target is #[no_core] when included by std, which is incompatible with
 # profiling.
-#
-# Suppress link directives since we specify the deps in GN configs.
-extra_kv = { include_coverage = false, no_link_directives = true }
+extra_kv = { include_coverage = false }
 
 [crate.profiler_builtins]
 # The build script is used to compile a profiler-rt library, but we get

rust/std/rules/BUILD.gn

@@ -1442,7 +1442,6 @@ cargo_crate("libc") {
   rustflags = [
     "--cfg=backtrace_in_libstd",
     "-Zforce-unstable-if-unmarked",
-    "-Zlink-directives=false",
   ]
   output_dir =
       "$root_out_dir/local_rustc_sysroot/lib/rustlib/$rust_abi_target/lib/"