Auto merge of #90179 - Nilstrieb:lifetime-elision-mismatch-hint, r=estebank

Add beginner friendly lifetime elision hint to E0623

Address #90170

Suggest adding a new lifetime parameter when two elided lifetimes should match up but don't.

Example:

```
error[E0623]: lifetime mismatch
  --> $DIR/issue-90170-elision-mismatch.rs:2:35
   |
LL | fn foo(slice_a: &mut [u8], slice_b: &mut [u8]) {
   |                 ---------           --------- these two types are declared with different lifetimes...
LL |     core::mem::swap(&mut slice_a, &mut slice_b);
   |                                   ^^^^^^^^^^^^ ...but data from `slice_b` flows into `slice_a` here
   |
   = note: each elided lifetime in input position becomes a distinct lifetime
help: explicitly declare a lifetime and assign it to both
   |
LL | fn foo<'a>(slice_a: &'a mut [u8], slice_b: &'a mut [u8]) {
   |       ++++           ++                     ++

```

for

```rust
fn foo(slice_a: &mut [u8], slice_b: &mut [u8]) {
    core::mem::swap(&mut slice_a, &mut slice_b);
}
```

src/test/ui/lifetimes/issue-90170-elision-mismatch.fixed

@@ -0,0 +1,9 @@
+// run-rustfix
+
+pub fn foo<'a>(x: &mut Vec<&'a u8>, y: &'a u8) { x.push(y); } //~ ERROR lifetime mismatch
+
+pub fn foo2<'a>(x: &mut Vec<&'a u8>, y: &'a u8) { x.push(y); } //~ ERROR lifetime mismatch
+
+pub fn foo3<'a>(_other: &'a [u8], x: &mut Vec<&'a u8>, y: &'a u8) { x.push(y); } //~ ERROR lifetime mismatch
+
+fn main() {}

src/test/ui/lifetimes/issue-90170-elision-mismatch.nll.stderr

@@ -0,0 +1,29 @@
+error: lifetime may not live long enough
+  --> $DIR/issue-90170-elision-mismatch.rs:3:40
+   |
+LL | pub fn foo(x: &mut Vec<&u8>, y: &u8) { x.push(y); }
+   |                        -        -      ^^^^^^^^^ argument requires that `'1` must outlive `'2`
+   |                        |        |
+   |                        |        let's call the lifetime of this reference `'1`
+   |                        let's call the lifetime of this reference `'2`
+
+error: lifetime may not live long enough
+  --> $DIR/issue-90170-elision-mismatch.rs:5:44
+   |
+LL | pub fn foo2(x: &mut Vec<&'_ u8>, y: &u8) { x.push(y); }
+   |                         -           -      ^^^^^^^^^ argument requires that `'1` must outlive `'2`
+   |                         |           |
+   |                         |           let's call the lifetime of this reference `'1`
+   |                         let's call the lifetime of this reference `'2`
+
+error: lifetime may not live long enough
+  --> $DIR/issue-90170-elision-mismatch.rs:7:63
+   |
+LL | pub fn foo3<'a>(_other: &'a [u8], x: &mut Vec<&u8>, y: &u8) { x.push(y); }
+   |                                               -        -      ^^^^^^^^^ argument requires that `'1` must outlive `'2`
+   |                                               |        |
+   |                                               |        let's call the lifetime of this reference `'1`
+   |                                               let's call the lifetime of this reference `'2`
+
+error: aborting due to 3 previous errors
+

src/test/ui/lifetimes/issue-90170-elision-mismatch.rs

@@ -0,0 +1,9 @@
+// run-rustfix
+
+pub fn foo(x: &mut Vec<&u8>, y: &u8) { x.push(y); } //~ ERROR lifetime mismatch
+
+pub fn foo2(x: &mut Vec<&'_ u8>, y: &u8) { x.push(y); } //~ ERROR lifetime mismatch
+
+pub fn foo3<'a>(_other: &'a [u8], x: &mut Vec<&u8>, y: &u8) { x.push(y); } //~ ERROR lifetime mismatch
+
+fn main() {}

src/test/ui/lifetimes/issue-90170-elision-mismatch.stderr

@@ -0,0 +1,45 @@
+error[E0623]: lifetime mismatch
+  --> $DIR/issue-90170-elision-mismatch.rs:3:47
+   |
+LL | pub fn foo(x: &mut Vec<&u8>, y: &u8) { x.push(y); }
+   |                        ---      ---           ^ ...but data from `y` flows into `x` here
+   |                                 |
+   |                                 these two types are declared with different lifetimes...
+   |
+   = note: each elided lifetime in input position becomes a distinct lifetime
+help: consider introducing a named lifetime parameter
+   |
+LL | pub fn foo<'a>(x: &mut Vec<&'a u8>, y: &'a u8) { x.push(y); }
+   |           ++++              ++          ++
+
+error[E0623]: lifetime mismatch
+  --> $DIR/issue-90170-elision-mismatch.rs:5:51
+   |
+LL | pub fn foo2(x: &mut Vec<&'_ u8>, y: &u8) { x.push(y); }
+   |                         ------      ---           ^ ...but data from `y` flows into `x` here
+   |                                     |
+   |                                     these two types are declared with different lifetimes...
+   |
+   = note: each elided lifetime in input position becomes a distinct lifetime
+help: consider introducing a named lifetime parameter
+   |
+LL | pub fn foo2<'a>(x: &mut Vec<&'a u8>, y: &'a u8) { x.push(y); }
+   |            ++++              ~~          ++
+
+error[E0623]: lifetime mismatch
+  --> $DIR/issue-90170-elision-mismatch.rs:7:70
+   |
+LL | pub fn foo3<'a>(_other: &'a [u8], x: &mut Vec<&u8>, y: &u8) { x.push(y); }
+   |                                               ---      ---           ^ ...but data from `y` flows into `x` here
+   |                                                        |
+   |                                                        these two types are declared with different lifetimes...
+   |
+   = note: each elided lifetime in input position becomes a distinct lifetime
+help: consider introducing a named lifetime parameter
+   |
+LL | pub fn foo3<'a>(_other: &'a [u8], x: &mut Vec<&'a u8>, y: &'a u8) { x.push(y); }
+   |                                                ++          ++
+
+error: aborting due to 3 previous errors
+
+For more information about this error, try `rustc --explain E0623`.

src/test/ui/lifetimes/lifetime-errors/ex3-both-anon-regions-2.stderr

@@ -5,6 +5,12 @@ LL | fn foo(&mut (ref mut v, w): &mut (&u8, &u8), x: &u8) {
    |                                   ---           --- these two types are declared with different lifetimes...
 LL |     *v = x;
    |          ^ ...but data from `x` flows here
+   |
+   = note: each elided lifetime in input position becomes a distinct lifetime
+help: consider introducing a named lifetime parameter
+   |
+LL | fn foo<'a>(&mut (ref mut v, w): &mut (&'a u8, &u8), x: &'a u8) {
+   |       ++++                             ++               ++
 
 error: aborting due to previous error
 

src/test/ui/lifetimes/lifetime-errors/ex3-both-anon-regions-3.stderr

@@ -5,6 +5,12 @@ LL | fn foo(z: &mut Vec<(&u8,&u8)>, (x, y): (&u8, &u8)) {
    |                     ---                 --- these two types are declared with different lifetimes...
 LL |     z.push((x,y));
    |             ^ ...but data flows into `z` here
+   |
+   = note: each elided lifetime in input position becomes a distinct lifetime
+help: consider introducing a named lifetime parameter
+   |
+LL | fn foo<'a>(z: &mut Vec<(&'a u8,&u8)>, (x, y): (&'a u8, &u8)) {
+   |       ++++               ++                     ++
 
 error[E0623]: lifetime mismatch
   --> $DIR/ex3-both-anon-regions-3.rs:2:15
@@ -13,6 +19,12 @@ LL | fn foo(z: &mut Vec<(&u8,&u8)>, (x, y): (&u8, &u8)) {
    |                         ---                  --- these two types are declared with different lifetimes...
 LL |     z.push((x,y));
    |               ^ ...but data flows into `z` here
+   |
+   = note: each elided lifetime in input position becomes a distinct lifetime
+help: consider introducing a named lifetime parameter
+   |
+LL | fn foo<'a>(z: &mut Vec<(&u8,&'a u8)>, (x, y): (&u8, &'a u8)) {
+   |       ++++                   ++                      ++
 
 error: aborting due to 2 previous errors
 

src/test/ui/lifetimes/lifetime-errors/ex3-both-anon-regions-using-fn-items.stderr

@@ -5,6 +5,12 @@ LL | fn foo(x:fn(&u8, &u8), y: Vec<&u8>, z: &u8) {
    |                               ---      --- these two types are declared with different lifetimes...
 LL |   y.push(z);
    |          ^ ...but data from `z` flows into `y` here
+   |
+   = note: each elided lifetime in input position becomes a distinct lifetime
+help: consider introducing a named lifetime parameter
+   |
+LL | fn foo<'a>(x:fn(&u8, &u8), y: Vec<&'a u8>, z: &'a u8) {
+   |       ++++                         ++          ++
 
 error: aborting due to previous error
 

src/test/ui/lifetimes/lifetime-errors/ex3-both-anon-regions-using-trait-objects.stderr

@@ -5,6 +5,12 @@ LL | fn foo(x:Box<dyn Fn(&u8, &u8)> , y: Vec<&u8>, z: &u8) {
    |                     ---  --- these two types are declared with different lifetimes...
 LL |   y.push(z);
    |          ^ ...but data from `z` flows into `y` here
+   |
+   = note: each elided lifetime in input position becomes a distinct lifetime
+help: consider introducing a named lifetime parameter
+   |
+LL | fn foo<'a>(x:Box<dyn Fn(&'a u8, &'a u8)> , y: Vec<&u8>, z: &u8) {
+   |       ++++               ++      ++
 
 error: aborting due to previous error
 

src/test/ui/lifetimes/lifetime-errors/ex3-both-anon-regions.stderr

@@ -5,6 +5,12 @@ LL | fn foo(x: &mut Vec<&u8>, y: &u8) {
    |                    ---      --- these two types are declared with different lifetimes...
 LL |     x.push(y);
    |            ^ ...but data from `y` flows into `x` here
+   |
+   = note: each elided lifetime in input position becomes a distinct lifetime
+help: consider introducing a named lifetime parameter
+   |
+LL | fn foo<'a>(x: &mut Vec<&'a u8>, y: &'a u8) {
+   |       ++++              ++          ++
 
 error: aborting due to previous error
 

src/test/ui/underscore-lifetime/underscore-lifetime-elison-mismatch.stderr

@@ -5,6 +5,12 @@ LL | fn foo(x: &mut Vec<&'_ u8>, y: &'_ u8) { x.push(y); }
    |                    ------      ------           ^ ...but data from `y` flows into `x` here
    |                                |
    |                                these two types are declared with different lifetimes...
+   |
+   = note: each elided lifetime in input position becomes a distinct lifetime
+help: consider introducing a named lifetime parameter
+   |
+LL | fn foo<'a>(x: &mut Vec<&'a u8>, y: &'a u8) { x.push(y); }
+   |       ++++              ~~          ~~
 
 error: aborting due to previous error