Suggest adding a lifetime parameter on "data from X flows into Y"

[Aaron1011]

The following code:

fn foo(mut first: &u8, second: &u8) {
    first = second;
}

fn main() {}

produces the following error:

error[E0623]: lifetime mismatch
 --> src/lib.rs:2:13
  |
1 | fn foo(mut first: &u8, second: &u8) {
  |                   ---          --- these two types are declared with different lifetimes...
2 |     first = second;
  |             ^^^^^^ ...but data from `second` flows into `first` here

We could suggest changing the signature to fn foo<'a>(mut first: &'a u8, second: &'a u8)

[BGR360]

Current output on nightly:

error[E0623]: lifetime mismatch
 --> src/main.rs:2:13
  |
1 | fn foo(mut first: &u8, second: &u8) {
  |                   ---          --- these two types are declared with different lifetimes...
2 |     first = second;
  |             ^^^^^^ ...but data from `second` flows into `first` here
  |
  = note: each elided lifetime in input position becomes a distinct lifetime
help: consider introducing a named lifetime parameter
  |
1 | fn foo<'a>(mut first: &'a u8, second: &'a u8) {
  |       ++++             ++              ++

Send your kudos to @Nilstrieb : #90179

[estebank]

The current output is from NLL, but the suggestion is still there :)

error: lifetime may not live long enough
 --> src/main.rs:2:5
  |
1 | fn foo(mut first: &u8, second: &u8) {
  |                   -            - let's call the lifetime of this reference `'1`
  |                   |
  |                   let's call the lifetime of this reference `'2`
2 |     first = second;
  |     ^^^^^^^^^^^^^^ assignment requires that `'1` must outlive `'2`
  |
help: consider introducing a named lifetime parameter
  |
1 | fn foo<'a>(mut first: &'a u8, second: &'a u8) {
  |       ++++             ++              ++