[rust] Enable mutable-noalias

We haven't seen any regressions upstream since it was last enabled again
in March 2021: rust-lang/rust#82834.

This results in a negligible increase in binary size of 24-56 kiB,
depending on build configuration.

Runtime perf only changed on x64 builds. 46 test cases got faster and
18 test cases got slower, with a couple of significant regressions in
FIDL microbenchmarks. Overall the results look positive.

Fixed: 76297
Change-Id: Id4a2b643e30e748e8d200f9d88c54ecc0ea02b2c
Reviewed-on: https://fuchsia-review.googlesource.com/c/fuchsia/+/674307
Commit-Queue: Tyler Mandry <tmandry@google.com>
Reviewed-by: Dan Johnson <computerdruid@google.com>

build/config/BUILDCONFIG.gn

@@ -369,11 +369,7 @@ if (in_default_toolchain) {
 }
 
 # Prepend the prefix configs, if any, before the default ones.
-default_common_binary_configs =
-    toolchain_variant.prefix_configs + [
-      # TODO(fxbug.dev/76297) Remove this to re-enable mutable-noalias.
-      "//build/config/rust:no_mutable_noalias",
-    ]
+default_common_binary_configs = toolchain_variant.prefix_configs
 
 # Note that Zircon toolchains and non-Zircon ones have a very different set
 # of default configs for all target types.

build/config/rust/BUILD.gn

@@ -119,11 +119,6 @@ config("v0_symbol_mangling") {
   }
 }
 
-# TODO(fxbug.dev/76297) Remove this to re-enable mutable-noalias.
-config("no_mutable_noalias") {
-  rustflags = [ "-Zmutable-noalias=off" ]
-}
-
 config("allow_unknown_lints") {
   rustflags = [ "-Aunknown-lints" ]
 }