Better closure error message
Use tracked data introduced in #42196 to provide a better closure
error message by showing why a closure implements `FnOnce`.
```
error[E0525]: expected a closure that implements the `Fn` trait, but
this closure only implements `FnOnce`
--> $DIR/issue_26046.rs:4:19
|
4 | let closure = move || {
| ___________________^
5 | | vec
6 | | };
| |_____^
|
note: closure is `FnOnce` because it moves the variable `vec` out of
its environment
--> $DIR/issue_26046.rs:5:9
|
5 | vec
| ^^^
error: aborting due to previous error(s)
```
Fixes#26046
r? @nikomatsakis
cc @doomrobo
f09576c4a4