We don't need NonNull::as_ptr debuginfo

Stop pessimizing the use of local variables in core by skipping debug info for MIR temporaries in tiny (single-BB) functions.

For functions as simple as this -- `Pin::new`, etc -- nobody every actually wants debuginfo for them in the first place.  They're more like intrinsics than real functions, and stepping over them is good.

compiler/rustc_mir_transform/src/lib.rs

@@ -189,6 +189,7 @@ declare_passes! {
     mod simplify_comparison_integral : SimplifyComparisonIntegral;
     mod single_use_consts : SingleUseConsts;
     mod sroa : ScalarReplacementOfAggregates;
+    mod strip_debuginfo : StripDebugInfo;
     mod unreachable_enum_branching : UnreachableEnumBranching;
     mod unreachable_prop : UnreachablePropagation;
     mod validate : Validator;
@@ -699,6 +700,8 @@ fn run_optimization_passes<'tcx>(tcx: TyCtxt<'tcx>, body: &mut Body<'tcx>) {
             &o1(simplify_branches::SimplifyConstCondition::Final),
             &o1(remove_noop_landing_pads::RemoveNoopLandingPads),
             &o1(simplify::SimplifyCfg::Final),
+            // After the last SimplifyCfg, because this wants one-block functions.
+            &strip_debuginfo::StripDebugInfo,
             &copy_prop::CopyProp,
             &dead_store_elimination::DeadStoreElimination::Final,
             &nrvo::RenameReturnPlace,

compiler/rustc_mir_transform/src/strip_debuginfo.rs

@@ -0,0 +1,34 @@
+use rustc_middle::mir::*;
+use rustc_middle::ty::TyCtxt;
+use rustc_session::config::MirStripDebugInfo;
+
+/// Conditionally remove some of the VarDebugInfo in MIR.
+///
+/// In particular, stripping non-parameter debug info for tiny, primitive-like
+/// methods in core saves work later, and nobody ever wanted to use it anyway.
+pub(super) struct StripDebugInfo;
+
+impl<'tcx> crate::MirPass<'tcx> for StripDebugInfo {
+    fn is_enabled(&self, sess: &rustc_session::Session) -> bool {
+        sess.opts.unstable_opts.mir_strip_debuginfo != MirStripDebugInfo::None
+    }
+
+    fn run_pass(&self, tcx: TyCtxt<'tcx>, body: &mut Body<'tcx>) {
+        match tcx.sess.opts.unstable_opts.mir_strip_debuginfo {
+            MirStripDebugInfo::None => return,
+            MirStripDebugInfo::AllLocals => {}
+            MirStripDebugInfo::LocalsInTinyFunctions
+                if let TerminatorKind::Return { .. } =
+                    body.basic_blocks[START_BLOCK].terminator().kind => {}
+            MirStripDebugInfo::LocalsInTinyFunctions => return,
+        }
+
+        body.var_debug_info.retain(|vdi| {
+            matches!(
+                vdi.value,
+                VarDebugInfoContents::Place(place)
+                    if place.local.as_usize() <= body.arg_count && place.local != RETURN_PLACE,
+            )
+        });
+    }
+}