Rollup merge of rust-lang#159248 - workingjubilee:remove-mutable-noalias, r=Urgau

compiler: Remove `-Zmutable-noalias`

Let us mark the end of an era. The `noalias` optimization has been working relatively well for some time now. There may still be issues to sort out with `noalias`. If so, those are mostly on LLVM's side, where it may need to have a better model of what `noalias` means as that is necessary for soundly inferring it. We also may need to not add `noalias` somewhere we currently do, if our semantics do not actually match that. Either way, it doesn't seem useful to configure on a compilation-wide basis.

Author: jhpratt

compiler/rustc_interface/src/tests.rs

@@ -852,7 +852,6 @@ fn test_unstable_options_tracking_hash() {
     tracked!(mir_opt_level, Some(4));
     tracked!(mir_preserve_ub, true);
     tracked!(move_size_limit, Some(4096));
-    tracked!(mutable_noalias, false);
     tracked!(next_solver, NextSolverConfig { coherence: true, globally: true });
     tracked!(no_generate_arange_section, true);
     tracked!(no_link, true);

compiler/rustc_session/src/options.rs

@@ -2568,8 +2568,6 @@ options! {
         "Whether to remove some of the MIR debug info from methods.  Default: None"),
     move_size_limit: Option<usize> = (None, parse_opt_number, [TRACKED],
         "the size at which the `large_assignments` lint starts to be emitted"),
-    mutable_noalias: bool = (true, parse_bool, [TRACKED],
-        "emit noalias metadata for mutable references (default: yes)"),
     namespaced_crates: bool = (false, parse_bool, [TRACKED],
         "allow crates to be namespaced by other crates (default: no)"),
     next_solver: NextSolverConfig = (NextSolverConfig::default(), parse_next_solver_config, [TRACKED],

compiler/rustc_ty_utils/src/abi.rs

@@ -366,11 +366,6 @@ fn arg_attrs_for_rust_scalar<'tcx>(
             // See https://github.com/rust-lang/unsafe-code-guidelines/issues/326
             let noalias_for_box = tcx.sess.opts.unstable_opts.box_noalias;
 
-            // LLVM prior to version 12 had known miscompiles in the presence of noalias attributes
-            // (see #54878), so it was conditionally disabled, but we don't support earlier
-            // versions at all anymore. We still support turning it off using -Zmutable-noalias.
-            let noalias_mut_ref = tcx.sess.opts.unstable_opts.mutable_noalias;
-
             // `&T` where `T` contains no `UnsafeCell<U>` is immutable, and can be marked as both
             // `readonly` and `noalias`, as LLVM's definition of `noalias` is based solely on memory
             // dependencies rather than pointer equality. However this only applies to arguments,
@@ -379,7 +374,7 @@ fn arg_attrs_for_rust_scalar<'tcx>(
             // `&mut T` and `Box<T>` where `T: Unpin` are unique and hence `noalias`.
             let no_alias = match kind {
                 PointerKind::SharedRef { frozen } => frozen,
-                PointerKind::MutableRef { unpin } => unpin && noalias_mut_ref,
+                PointerKind::MutableRef { unpin } => unpin,
                 PointerKind::Box { unpin, global } => unpin && global && noalias_for_box,
             };
             // We can never add `noalias` in return position; that LLVM attribute has some very surprising semantics

tests/codegen-llvm/noalias-flag.rs

@@ -1,4 +1,4 @@
-//@ compile-flags: -Copt-level=3 -C no-prepopulate-passes -Z mutable-noalias=yes
+//@ compile-flags: -Copt-level=3 -C no-prepopulate-passes
 
 #![crate_type = "lib"]
 

tests/codegen-llvm/noalias-refcell.rs

@@ -1,4 +1,4 @@
-//@ compile-flags: -Copt-level=3 -C no-prepopulate-passes -Z mutable-noalias=yes
+//@ compile-flags: -Copt-level=3 -C no-prepopulate-passes
 
 #![crate_type = "lib"]
 

tests/codegen-llvm/noalias-rwlockreadguard.rs

@@ -1,4 +1,4 @@
-//@ compile-flags: -Copt-level=3 -Z mutable-noalias=yes
+//@ compile-flags: -Copt-level=3
 
 #![crate_type = "lib"]