Auto merge of #51274 - nikomatsakis:issue-46557-promote-ref-mut, r=eddyb

also check `let` arms and nested patterns for mutable borrows

Fixes #46557

r? @eddyb

src/test/ui/borrowck/promote-ref-mut-in-let-issue-46557.nll.stderr

@@ -0,0 +1,63 @@
+error[E0597]: borrowed value does not live long enough
+  --> $DIR/promote-ref-mut-in-let-issue-46557.rs:15:21
+   |
+LL |   fn gimme_static_mut_let() -> &'static mut u32 {
+   |  _______________________________________________-
+LL | |     let ref mut x = 1234543; //~ ERROR
+   | |                     ^^^^^^^ temporary value does not live long enough
+LL | |     x
+LL | | }
+   | | -
+   | | |
+   | |_temporary value only lives until here
+   |   borrow later used here
+
+error[E0597]: borrowed value does not live long enough
+  --> $DIR/promote-ref-mut-in-let-issue-46557.rs:20:25
+   |
+LL |   fn gimme_static_mut_let_nested() -> &'static mut u32 {
+   |  ______________________________________________________-
+LL | |     let (ref mut x, ) = (1234543, ); //~ ERROR
+   | |                         ^^^^^^^^^^^ temporary value does not live long enough
+LL | |     x
+LL | | }
+   | | -
+   | | |
+   | |_temporary value only lives until here
+   |   borrow later used here
+
+error[E0597]: borrowed value does not live long enough
+  --> $DIR/promote-ref-mut-in-let-issue-46557.rs:25:11
+   |
+LL |     match 1234543 {
+   |           ^^^^^^^ temporary value does not live long enough
+...
+LL | }
+   | - temporary value only lives until here
+   |
+   = note: borrowed value must be valid for the static lifetime...
+
+error[E0597]: borrowed value does not live long enough
+  --> $DIR/promote-ref-mut-in-let-issue-46557.rs:31:11
+   |
+LL |     match (123443,) {
+   |           ^^^^^^^^^ temporary value does not live long enough
+...
+LL | }
+   | - temporary value only lives until here
+   |
+   = note: borrowed value must be valid for the static lifetime...
+
+error[E0597]: borrowed value does not live long enough
+  --> $DIR/promote-ref-mut-in-let-issue-46557.rs:37:10
+   |
+LL |     &mut 1234543 //~ ERROR
+   |          ^^^^^^^ temporary value does not live long enough
+LL | }
+   | - temporary value only lives until here
+   |
+   = note: borrowed value must be valid for the static lifetime...
+
+error: aborting due to 5 previous errors
+
+For more information about this error, try `rustc --explain E0597`.

src/test/ui/borrowck/promote-ref-mut-in-let-issue-46557.rs

@@ -0,0 +1,41 @@
+// Copyright 2014 The Rust Project Developers. See the COPYRIGHT
+// file at the top-level directory of this distribution and at
+// http://rust-lang.org/COPYRIGHT.
+//
+// Licensed under the Apache License, Version 2.0 <LICENSE-APACHE or
+// http://www.apache.org/licenses/LICENSE-2.0> or the MIT license
+// <LICENSE-MIT or http://opensource.org/licenses/MIT>, at your
+// option. This file may not be copied, modified, or distributed
+// except according to those terms.
+
+// Test that we fail to promote the constant here which has a `ref
+// mut` borrow.
+
+fn gimme_static_mut_let() -> &'static mut u32 {
+    let ref mut x = 1234543; //~ ERROR
+    x
+}
+
+fn gimme_static_mut_let_nested() -> &'static mut u32 {
+    let (ref mut x, ) = (1234543, ); //~ ERROR
+    x
+}
+
+fn gimme_static_mut_match() -> &'static mut u32 {
+    match 1234543 {
+        ref mut x => x //~ ERROR
+    }
+}
+
+fn gimme_static_mut_match_nested() -> &'static mut u32 {
+    match (123443,) {
+        (ref mut x,) => x, //~ ERROR
+    }
+}
+
+fn gimme_static_mut_ampersand() -> &'static mut u32 {
+    &mut 1234543 //~ ERROR
+}
+
+fn main() {
+}

src/test/ui/borrowck/promote-ref-mut-in-let-issue-46557.stderr

@@ -0,0 +1,57 @@
+error[E0597]: borrowed value does not live long enough
+  --> $DIR/promote-ref-mut-in-let-issue-46557.rs:15:9
+   |
+LL |     let ref mut x = 1234543; //~ ERROR
+   |         ^^^^^^^^^ temporary value does not live long enough
+LL |     x
+LL | }
+   | - temporary value only lives until here
+   |
+   = note: borrowed value must be valid for the static lifetime...
+
+error[E0597]: borrowed value does not live long enough
+  --> $DIR/promote-ref-mut-in-let-issue-46557.rs:20:10
+   |
+LL |     let (ref mut x, ) = (1234543, ); //~ ERROR
+   |          ^^^^^^^^^ borrowed value does not live long enough
+LL |     x
+LL | }
+   | - borrowed value only lives until here
+   |
+   = note: borrowed value must be valid for the static lifetime...
+
+error[E0597]: borrowed value does not live long enough
+  --> $DIR/promote-ref-mut-in-let-issue-46557.rs:26:9
+   |
+LL |         ref mut x => x //~ ERROR
+   |         ^^^^^^^^^ temporary value does not live long enough
+LL |     }
+LL | }
+   | - temporary value only lives until here
+   |
+   = note: borrowed value must be valid for the static lifetime...
+
+error[E0597]: borrowed value does not live long enough
+  --> $DIR/promote-ref-mut-in-let-issue-46557.rs:32:10
+   |
+LL |         (ref mut x,) => x, //~ ERROR
+   |          ^^^^^^^^^ borrowed value does not live long enough
+LL |     }
+LL | }
+   | - borrowed value only lives until here
+   |
+   = note: borrowed value must be valid for the static lifetime...
+
+error[E0597]: borrowed value does not live long enough
+  --> $DIR/promote-ref-mut-in-let-issue-46557.rs:37:10
+   |
+LL |     &mut 1234543 //~ ERROR
+   |          ^^^^^^^ temporary value does not live long enough
+LL | }
+   | - temporary value only lives until here
+   |
+   = note: borrowed value must be valid for the static lifetime...
+
+error: aborting due to 5 previous errors
+
+For more information about this error, try `rustc --explain E0597`.