Inside empty sysroot, is the rustlib load-bearing?

Summary:
This code is from {D54130104}.

Prior to that, we had been using an entirely empty directory since {D34351072}.

Ensuring a `lib/rustlib/{target}/lib` subdirectory present in the directory given to `--sysroot` used to be necessary because rustc would pass `-L{sysroot}/lib/rustlib/{target}/lib` to the linker, which would cause some linkers (`ld64`) to error out if nonexistent.

Since [rust-lang/rust#128370](rust-lang/rust#128370) in Rust 1.82, rustc no longer passes that, so we no longer need to create the subdirectory.

> **linker: Pass fewer search directories to the linker**
> ...
> The main sysroot library directory shouldn't generally be passed because it shouldn't contain native libraries, except for one case which is now marked with a FIXME.

Reviewed By: capickett, JakobDegen

Differential Revision: D72823498

fbshipit-source-id: 4343ab0fa4c19a168574b57d6eed8df23630af1d

Author: David Tolnay

prelude/rust/build.bzl

@@ -152,13 +152,11 @@ def compile_context(ctx: AnalysisContext, binary: bool = False) -> CompileContex
         panic_runtime = toolchain_info.panic_runtime,
     )
 
-    # When we pass explicit sysroot deps, we need to override the default sysroot to avoid accidentally
-    # linking against the prebuilt sysroot libs provided by the toolchain. Rustc requires a specific layout
-    # for these libs, so we need to carefully recreate the directory structure below.
+    # When we pass explicit sysroot deps, we need to override the default
+    # sysroot to avoid accidentally linking against the prebuilt sysroot libs
+    # provided by the toolchain.
     if toolchain_info.explicit_sysroot_deps:
-        empty_dir = ctx.actions.copied_dir("empty_dir", {})
-        empty_sysroot = ctx.actions.copied_dir("empty_sysroot", {"lib/rustlib/" + toolchain_info.rustc_target_triple + "/lib": empty_dir})
-
+        empty_sysroot = ctx.actions.copied_dir("empty_dir", {})
         sysroot_args = cmd_args("--sysroot=", empty_sysroot, delimiter = "")
     elif toolchain_info.sysroot_path:
         sysroot_args = cmd_args("--sysroot=", toolchain_info.sysroot_path, delimiter = "")