Fix #54224 (const promotion regression)

Author: oli-obk

src/librustc_mir/transform/qualify_consts.rs

@@ -496,10 +496,11 @@ impl<'a, 'tcx> Visitor<'tcx> for Qualifier<'a, 'tcx, 'tcx> {
                     this.super_place(place, context, location);
                     match proj.elem {
                         ProjectionElem::Deref => {
-                            if let Mode::Fn = this.mode {
                             this.add(Qualif::NOT_CONST);
-                            } else {
                             let base_ty = proj.base.ty(this.mir, this.tcx).to_ty(this.tcx);
+                            match this.mode {
+                                Mode::Fn => {},
+                                _ => {
                                     if let ty::RawPtr(_) = base_ty.sty {
                                         if !this.tcx.sess.features_untracked().const_raw_ptr_deref {
                                             emit_feature_err(
@@ -514,6 +515,7 @@ impl<'a, 'tcx> Visitor<'tcx> for Qualifier<'a, 'tcx, 'tcx> {
                                     }
                                 }
                             }
+                        }
 
                         ProjectionElem::Field(..) |
                         ProjectionElem::Index(_) => {

src/test/ui/consts/issue-54224.rs

@@ -0,0 +1,14 @@
+#![feature(nll)]
+
+const FOO: Option<&[[u8; 3]]> = Some(&[*b"foo"]); //~ ERROR temporary value dropped while borrowed
+
+use std::borrow::Cow;
+
+pub const X: [u8; 3] = *b"ABC";
+pub const Y: Cow<'static, [ [u8; 3] ]> = Cow::Borrowed(&[X]);
+
+
+pub const Z: Cow<'static, [ [u8; 3] ]> = Cow::Borrowed(&[*b"ABC"]);
+//~^ ERROR temporary value dropped while borrowed
+
+fn main() {}

src/test/ui/consts/issue-54224.stderr

@@ -0,0 +1,23 @@
+error[E0716]: temporary value dropped while borrowed
+  --> $DIR/issue-54224.rs:3:39
+   |
+LL | const FOO: Option<&[[u8; 3]]> = Some(&[*b"foo"]); //~ ERROR temporary value dropped while borrowed
+   |                                       ^^^^^^^^^- temporary value is freed at the end of this statement
+   |                                       |
+   |                                       creates a temporary which is freed while still in use
+   |
+   = note: borrowed value must be valid for the static lifetime...
+
+error[E0716]: temporary value dropped while borrowed
+  --> $DIR/issue-54224.rs:11:57
+   |
+LL | pub const Z: Cow<'static, [ [u8; 3] ]> = Cow::Borrowed(&[*b"ABC"]);
+   |                                                         ^^^^^^^^^- temporary value is freed at the end of this statement
+   |                                                         |
+   |                                                         creates a temporary which is freed while still in use
+   |
+   = note: borrowed value must be valid for the static lifetime...
+
+error: aborting due to 2 previous errors
+
+For more information about this error, try `rustc --explain E0716`.