revert mir inlining policy for beta-1.64

compiler/rustc_mir_transform/src/inline.rs

@@ -9,7 +9,6 @@ use rustc_middle::mir::visit::*;
 use rustc_middle::mir::*;
 use rustc_middle::ty::subst::Subst;
 use rustc_middle::ty::{self, ConstKind, Instance, InstanceDef, ParamEnv, Ty, TyCtxt};
-use rustc_session::config::OptLevel;
 use rustc_span::{hygiene::ExpnKind, ExpnData, LocalExpnId, Span};
 use rustc_target::spec::abi::Abi;
 
@@ -44,15 +43,8 @@ impl<'tcx> MirPass<'tcx> for Inline {
             return enabled;
         }
 
-        match sess.mir_opt_level() {
-            0 | 1 => false,
-            2 => {
-                (sess.opts.optimize == OptLevel::Default
-                    || sess.opts.optimize == OptLevel::Aggressive)
-                    && sess.opts.incremental == None
-            }
-            _ => true,
-        }
+        // rust-lang/rust#101004: reverted to old inlining decision logic
+        sess.mir_opt_level() >= 3
     }
 
     fn run_pass(&self, tcx: TyCtxt<'tcx>, body: &mut Body<'tcx>) {

src/test/codegen/mem-replace-direct-memcpy.rs

@@ -1,3 +1,15 @@
+// ignore-test
+
+// WHY IS THIS TEST BEING IGNORED:
+//
+// This test depends on characteristics of how the stdlib was compiled,
+// namely that sufficient inlining occurred to ensure that the call to
+// `std::mem::replace` boils down to just two calls of `llvm.memcpy`.
+//
+// But the MIR inlining policy is in flux as of 1.64-beta, and the intermittent
+// breakage of this test that results is causing problems for people trying to
+// do development.
+
 // This test ensures that `mem::replace::<T>` only ever calls `@llvm.memcpy`
 // with `size_of::<T>()` as the size, and never goes through any wrapper that
 // may e.g. multiply `size_of::<T>()` with a variable "count" (which is only