Apply cpu-optimisation to Rust projects

Library/Homebrew/extend/ENV/shared.rb

@@ -28,6 +28,7 @@ module SharedEnvExtension
     CMAKE_PREFIX_PATH CMAKE_INCLUDE_PATH CMAKE_FRAMEWORK_PATH
     GOBIN GOPATH GOROOT PERL_MB_OPT PERL_MM_OPT
     LIBRARY_PATH LD_LIBRARY_PATH LD_PRELOAD LD_RUN_PATH
+    RUSTFLAGS
   ].freeze
   private_constant :SANITIZED_VARS
 

Library/Homebrew/extend/ENV/std.rb

@@ -36,6 +36,7 @@ def setup_build_environment(formula: nil, cc: nil, build_bottle: false, bottle_a
     self["PKG_CONFIG_LIBDIR"] = determine_pkg_config_libdir
 
     self["MAKEFLAGS"] = "-j#{make_jobs}"
+    self["RUSTFLAGS"] = Hardware.rustflags_target_cpu
 
     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

@@ -63,6 +63,7 @@ def setup_build_environment(formula: nil, cc: nil, build_bottle: false, bottle_a
 
     self["HOMEBREW_ENV"] = "super"
     self["MAKEFLAGS"] ||= "-j#{determine_make_jobs}"
+    self["RUSTFLAGS"] = Hardware.rustflags_target_cpu
     self["PATH"] = determine_path
     self["PKG_CONFIG_PATH"] = determine_pkg_config_path
     self["PKG_CONFIG_LIBDIR"] = determine_pkg_config_libdir

Library/Homebrew/hardware.rb

@@ -41,6 +41,20 @@ def optimization_flags
       end
       alias generic_optimization_flags optimization_flags
 
+      # Rust already defaults to the oldest supported cpu for each target-triple
+      # so it's safe to ignore generic archs such as :armv6, etc., here.
+      # Rust defaults to apple-m1 since Rust 1.71 for aarch64-apple-darwin.
+      def rust_optimisation_flags
+        @rust_optimisation_flags ||= {
+          native:      "--codegen target-cpu=native",
+          ivybridge:   "--codegen target-cpu=ivybridge",
+          sandybridge: "--codegen target-cpu=sandybridge",
+          nehalem:     "--codegen target-cpu=nehalem",
+          core2:       "--codegen target-cpu=core2",
+          core:        "--codegen target-cpu=prescott",
+        }.freeze
+      end
+
       sig { returns(Symbol) }
       def arch_32_bit
         if arm?
@@ -214,6 +228,13 @@ def oldest_cpu(_version = nil)
       end
     end
     alias generic_oldest_cpu oldest_cpu
+
+    # Returns a _full_ rustflag to set target cpu, if necessary;
+    # Defaults to empty string
+    sig { returns(T.nilable(String)) }
+    def rustflags_target_cpu
+      CPU.rust_optimisation_flags.fetch(oldest_cpu, nil)
+    end
   end
 end