Add rustc wrapper shim to fix RUSTFLAGS conflicts

Fixes #18556 by using RUSTC_WRAPPER instead of setting RUSTFLAGS directly.
This allows Homebrew's optimization flags to coexist with .cargo/config.toml
settings, preventing build failures when projects have their own Rust
configuration.

- Add rustc_wrapper shim that clears RUSTFLAGS and prepends HOMEBREW_RUSTFLAGS
- Update both std and super environments to use RUSTC_WRAPPER
- Store Homebrew's rustflags in HOMEBREW_RUSTFLAGS instead of RUSTFLAGS

🤖 Generated with [Claude Code](https://claude.ai/code)

Co-Authored-By: Claude <noreply@anthropic.com>
Co-authored-by: Carlo Cabrera <github@carlo.cab>

Library/Homebrew/extend/ENV/shared.rb

@@ -113,6 +113,11 @@ module SharedEnvExtension
     self[key] = PATH.new(self[key]).append(path)
   end
 
+  sig { params(rustflags: String).void }
+  def append_to_rustflags(rustflags)
+    append("HOMEBREW_RUSTFLAGS", rustflags)
+  end
+
   # Prepends a directory to `PATH`.
   # Is the formula struggling to find the pkgconfig file? Point it to it.
   # This is done automatically for keg-only formulae.

Library/Homebrew/extend/ENV/std.rb

@@ -34,7 +34,8 @@ module Stdenv
     self["PKG_CONFIG_LIBDIR"] = determine_pkg_config_libdir
 
     self["MAKEFLAGS"] = "-j#{make_jobs}"
-    self["RUSTFLAGS"] = Hardware.rustflags_target_cpu(effective_arch)
+    self["RUSTC_WRAPPER"] = "#{HOMEBREW_SHIMS_PATH}/super/rustc_wrapper"
+    self["HOMEBREW_RUSTFLAGS"] = Hardware.rustflags_target_cpu(effective_arch)
 
     if HOMEBREW_PREFIX.to_s != "/usr/local"
       # /usr/local is already an -isystem and -L directory so we skip it

Library/Homebrew/extend/ENV/super.rb

@@ -62,7 +62,8 @@ module Superenv
 
     self["HOMEBREW_ENV"] = "super"
     self["MAKEFLAGS"] ||= "-j#{determine_make_jobs}"
-    self["RUSTFLAGS"] = Hardware.rustflags_target_cpu(effective_arch)
+    self["RUSTC_WRAPPER"] = "#{HOMEBREW_SHIMS_PATH}/super/rustc_wrapper"
+    self["HOMEBREW_RUSTFLAGS"] = Hardware.rustflags_target_cpu(effective_arch)
     self["PATH"] = determine_path
     self["PKG_CONFIG_PATH"] = determine_pkg_config_path
     self["PKG_CONFIG_LIBDIR"] = determine_pkg_config_libdir || ""

Library/Homebrew/shims/super/rustc_wrapper

@@ -0,0 +1,15 @@
+#!/bin/bash
+
+# Prepend HOMEBREW_RUSTFLAGS to rustc arguments if set.
+# This allows Homebrew to pass optimization flags while still respecting
+# .cargo/config.toml (which sets RUSTFLAGS).
+
+# rustc is passed as the first argument to RUSTC_WRAPPER
+# https://doc.rust-lang.org/cargo/reference/environment-variables.html
+RUSTC="${1}"
+shift
+
+# Prepend HOMEBREW_RUSTFLAGS to the arguments if set.
+# HOMEBREW_RUSTFLAGS is set in extend/ENV/{std,super}.rb
+# shellcheck disable=SC2086,SC2154
+exec "${RUSTC}" ${HOMEBREW_RUSTFLAGS} "$@"