Do not emit alloca for ZST local even if it is uninitialized

compiler/rustc_codegen_ssa/src/mir/analyze.rs

@@ -281,7 +281,18 @@ impl<'mir, 'a, 'tcx, Bx: BuilderMethods<'a, 'tcx>> Visitor<'tcx>
                     Some(assignment_location) => {
                         assignment_location.dominates(location, &self.dominators)
                     }
-                    None => false,
+                    None => {
+                        debug!("No first assignment found for {:?}", local);
+                        // We have not seen any assignment to the local yet,
+                        // but before marking not_ssa, check if it is a ZST,
+                        // in which case we don't need to initialize the local.
+                        let ty = self.fx.mir.local_decls[local].ty;
+                        let ty = self.fx.monomorphize(ty);
+
+                        let is_zst = self.fx.cx.layout_of(ty).is_zst();
+                        debug!("is_zst: {}", is_zst);
+                        is_zst
+                    }
                 };
                 if !ssa_read {
                     self.not_ssa(local);

src/test/codegen/naked-functions.rs

@@ -9,7 +9,6 @@
 #[naked]
 pub fn naked_empty() {
     // CHECK-NEXT: {{.+}}:
-    // CHECK-NEXT: %0 = alloca {}, align 1
     // CHECK-NEXT: ret void
 }
 
@@ -19,7 +18,6 @@ pub fn naked_empty() {
 // CHECK-NEXT: define void @naked_with_args(i{{[0-9]+( %a)?}})
 pub fn naked_with_args(a: isize) {
     // CHECK-NEXT: {{.+}}:
-    // CHECK-NEXT: %0 = alloca {}, align 1
     // CHECK: ret void
 }
 

src/test/run-make/const_fn_mir/dump.mir

@@ -12,7 +12,6 @@ fn main() -> () {
     }
 
     bb1: {
-        _0 = const ();                   // scope 0 at main.rs:8:11: 10:2
         return;                          // scope 0 at main.rs:10:2: 10:2
     }
 }