Rewrite dest prop.

This fixes a number of correctness issues from the previous version. Additionally, we use a new
strategy which has much better performance charactersitics and also finds more opportunities to
apply the optimization.

compiler/rustc_mir_transform/src/dead_store_elimination.rs

@@ -70,6 +70,8 @@ pub fn eliminate<'tcx>(tcx: TyCtxt<'tcx>, body: &mut Body<'tcx>, borrowed: &BitS
     for Location { block, statement_index } in patch {
         bbs[block].statements[statement_index].make_nop();
     }
+
+    crate::simplify::SimplifyLocals.run_pass(tcx, body)
 }
 
 pub struct DeadStoreElimination;

compiler/rustc_mir_transform/src/deduce_param_attrs.rs

@@ -110,15 +110,16 @@ impl<'tcx> Visitor<'tcx> for DeduceReadOnly {
 
         if let TerminatorKind::Call { ref args, .. } = terminator.kind {
             for arg in args {
-                if let Operand::Move(_) = *arg {
-                    // ArgumentChecker panics if a direct move of an argument from a caller to a
-                    // callee was detected.
-                    //
-                    // If, in the future, MIR optimizations cause arguments to be moved directly
-                    // from callers to callees, change the panic to instead add the argument in
-                    // question to `mutating_uses`.
-                    ArgumentChecker::new(self.mutable_args.domain_size())
-                        .visit_operand(arg, location)
+                if let Operand::Move(place) = *arg {
+                    let local = place.local;
+                    if place.is_indirect()
+                        || local == RETURN_PLACE
+                        || local.index() > self.mutable_args.domain_size()
+                    {
+                        continue;
+                    }
+
+                    self.mutable_args.insert(local.index() - 1);
                 }
             }
         };
@@ -127,35 +128,6 @@ impl<'tcx> Visitor<'tcx> for DeduceReadOnly {
     }
 }
 
-/// A visitor that simply panics if a direct move of an argument from a caller to a callee was
-/// detected.
-struct ArgumentChecker {
-    /// The number of arguments to the calling function.
-    arg_count: usize,
-}
-
-impl ArgumentChecker {
-    /// Creates a new ArgumentChecker.
-    fn new(arg_count: usize) -> Self {
-        Self { arg_count }
-    }
-}
-
-impl<'tcx> Visitor<'tcx> for ArgumentChecker {
-    fn visit_local(&mut self, local: Local, context: PlaceContext, _: Location) {
-        // Check to make sure that, if this local is an argument, we didn't move directly from it.
-        if matches!(context, PlaceContext::NonMutatingUse(NonMutatingUseContext::Move))
-            && local != RETURN_PLACE
-            && local.index() <= self.arg_count
-        {
-            // If, in the future, MIR optimizations cause arguments to be moved directly from
-            // callers to callees, change this panic to instead add the argument in question to
-            // `mutating_uses`.
-            panic!("Detected a direct move from a caller's argument to a callee's argument!")
-        }
-    }
-}
-
 /// Returns true if values of a given type will never be passed indirectly, regardless of ABI.
 fn type_will_always_be_passed_directly<'tcx>(ty: Ty<'tcx>) -> bool {
     matches!(