Tell rustc that we will always be providing clangrt libraries

This prevents rustc from trying to link sanitizer runtimes, so we don't
need to make fake ones in our sysroot anymore. And this avoids rustc
trying to link the Linux runtime when we're targeting Windows due to
rust-lang/rust#131363.

When this flag is specified, rustc avoids adding sanitizer libraries
here:
https://github.com/rust-lang/rust/blob/7caad6925314911eefe54b040d4bc5be940e8f92/compiler/rustc_codegen_ssa/src/back/link.rs#L1263-L1267

[email protected]

Bug: 371512562
Cq-Include-Trybots: luci.chromium.try:win-asan,android-rust-arm32-rel,android-rust-arm64-dbg,android-rust-arm64-rel,linux-rust-x64-rel,linux-rust-x64-dbg,mac-rust-x64-dbg,win-rust-x64-dbg,win-rust-x64-rel,mac_chromium_asan_rel_ng,linux_chromium_asan_rel_ng
Change-Id: Ifcc6cb841af2697c6ca5ab3b64adcc995683fbc6
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/5913992
Commit-Queue: danakj <[email protected]>
Reviewed-by: Arthur Eubanks <[email protected]>
Cr-Commit-Position: refs/heads/main@{#1365077}
NOKEYCHECK=True
GitOrigin-RevId: 7d029cad43d3bf03b0464d9e3c55cc0f8b5474e2

Author: danakj

Diff for: config/compiler/BUILD.gn

@@ -1041,6 +1041,9 @@ config("compiler") {
     # For deterministic builds, keep the local machine's current working
     # directory from appearing in build outputs.
     "-Zremap-cwd-prefix=.",
+
+    # We use clang-rt sanitizer runtimes.
+    "-Zexternal-clangrt",
   ]
 
   if (!is_win || force_rustc_color_output) {

Diff for: rust/std/BUILD.gn

@@ -236,51 +236,6 @@ if (toolchain_has_rust) {
       visibility = [ ":*" ]
     }
 
-    # When given -Zsanitize=..., rustc insists on passing a sanitizer runtime to
-    # the linker it invokes. Unfortunately, our C++ ldflags already tell the
-    # linker to link against a C++ sanitizer runtime - which contains the same
-    # symbols. So, make a blank library.
-    # The list of relevant sanitizers here is taken from
-    # https://github.com/rust-lang/rust/blob/7e7483d26e3cec7a44ef00cf7ae6c9c8c918bec6/compiler/rustc_codegen_ssa/src/back/link.rs#L1148
-    template("rustc_sanitizer_runtime") {
-      rt_name = target_name
-      not_needed([ "invoker" ])
-      static_library("sanitizer_rt_$rt_name") {
-        sources = []
-        output_name = "librustc-${rust_channel}_rt.$rt_name"
-        output_dir = "$local_rustc_sysroot/$sysroot_lib_subdir"
-        if (is_win) {
-          arflags = [ "/llvmlibempty" ]
-        }
-      }
-    }
-    rustc_sanitizer_runtimes = []
-    if (is_asan) {
-      rustc_sanitizer_runtime("asan") {
-      }
-      rustc_sanitizer_runtimes += [ ":sanitizer_rt_asan" ]
-    }
-    if (is_lsan) {
-      rustc_sanitizer_runtime("lsan") {
-      }
-      rustc_sanitizer_runtimes += [ ":sanitizer_rt_lsan" ]
-    }
-    if (is_msan) {
-      rustc_sanitizer_runtime("msan") {
-      }
-      rustc_sanitizer_runtimes += [ ":sanitizer_rt_msan" ]
-    }
-    if (is_tsan) {
-      rustc_sanitizer_runtime("tsan") {
-      }
-      rustc_sanitizer_runtimes += [ ":sanitizer_rt_tsan" ]
-    }
-    if (is_hwasan) {
-      rustc_sanitizer_runtime("hwasan") {
-      }
-      rustc_sanitizer_runtimes += [ ":sanitizer_rt_hwasan" ]
-    }
-
     # Builds and links against the Rust stdlib. Both Rust and C++ targets should
     # depend on this, as it provides the path to the library and includes the
     # allocator hooks.
@@ -299,7 +254,6 @@ if (toolchain_has_rust) {
       foreach(libname, stdlib_files + skip_stdlib_files) {
         deps += [ "rules:$libname" ]
       }
-      deps += rustc_sanitizer_runtimes
 
       public_deps = [ ":remap_alloc" ]
     }