update error messages -- in some cases maybe we should investigate

Author: nikomatsakis

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

@@ -1,30 +1,24 @@
 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
+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:20:25
    |
-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
+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:25:11

src/test/ui/nll/get_default.nll.stderr

@@ -63,9 +63,18 @@ LL |         match map.get() {
 LL |             Some(v) => {
 LL |                 map.set(String::new()); // Both AST and MIR error here
    |                 ^^^^^^^^^^^^^^^^^^^^^^ mutable borrow occurs here
-...
-LL |                 return v;
-   |                        - borrow later used here
+   |
+note: borrowed value must be valid for the anonymous lifetime #1 defined on the function body at 41:1...
+  --> $DIR/get_default.rs:41:1
+   |
+LL | / fn err(map: &mut Map) -> &String {
+LL | |     loop {
+LL | |         match map.get() {
+LL | |             Some(v) => {
+...  |
+LL | |     }
+LL | | }
+   | |_^
 
 error[E0502]: cannot borrow `*map` as mutable because it is also borrowed as immutable (Mir)
   --> $DIR/get_default.rs:51:17

src/test/ui/nll/get_default.stderr

@@ -63,9 +63,18 @@ LL |         match map.get() {
 LL |             Some(v) => {
 LL |                 map.set(String::new()); // Both AST and MIR error here
    |                 ^^^ mutable borrow occurs here
-...
-LL |                 return v;
-   |                        - borrow later used here
+   |
+note: borrowed value must be valid for the anonymous lifetime #1 defined on the function body at 41:1...
+  --> $DIR/get_default.rs:41:1
+   |
+LL | / fn err(map: &mut Map) -> &String {
+LL | |     loop {
+LL | |         match map.get() {
+LL | |             Some(v) => {
+...  |
+LL | |     }
+LL | | }
+   | |_^
 
 error[E0502]: cannot borrow `*map` as mutable because it is also borrowed as immutable (Mir)
   --> $DIR/get_default.rs:51:17

src/test/ui/nll/return-ref-mut-issue-46557.stderr

@@ -1,16 +1,13 @@
 error[E0597]: borrowed value does not live long enough
   --> $DIR/return-ref-mut-issue-46557.rs:17:21
    |
-LL |   fn gimme_static_mut() -> &'static mut u32 {
-   |  ___________________________________________-
-LL | |     let ref mut x = 1234543; //~ ERROR borrowed value does not live long enough [E0597]
-   | |                     ^^^^^^^ temporary value does not live long enough
-LL | |     x
-LL | | }
-   | | -
-   | | |
-   | |_temporary value only lives until here
-   |   borrow later used here
+LL |     let ref mut x = 1234543; //~ ERROR borrowed value does not live long enough [E0597]
+   |                     ^^^^^^^ 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: aborting due to previous error