Tweak fn pointer suggestion span

[estebank]

Use a more targeted span when suggesting casting an fn item to an fn pointer.

error[E0308]: cannot coerce functions which must be inlined to function pointers
  --> $DIR/cast.rs:10:33
   |
LL |     let _: fn(isize) -> usize = callee;
   |            ------------------   ^^^^^^ cannot coerce functions which must be inlined to function pointers
   |            |
   |            expected due to this
   |
   = note: expected fn pointer `fn(_) -> _`
                 found fn item `fn(_) -> _ {callee}`
   = note: fn items are distinct from fn pointers
help: consider casting to a fn pointer
   |
LL |     let _: fn(isize) -> usize = callee as fn(isize) -> usize;
   |                                        +++++++++++++++++++++

error[E0308]: mismatched types
  --> $DIR/fn-pointer-mismatch.rs:42:30
   |
LL |     let d: &fn(u32) -> u32 = foo;
   |            ---------------   ^^^ expected `&fn(u32) -> u32`, found fn item
   |            |
   |            expected due to this
   |
   = note: expected reference `&fn(_) -> _`
                found fn item `fn(_) -> _ {foo}`
help: consider using a reference
   |
LL |     let d: &fn(u32) -> u32 = &foo;
   |                              +

Previously we'd point at the whole expression for replacement, instead of marking what was being added.

We could also modify the suggestions for &(name as fn()), but for that we require storing more accurate spans than we have now.

[rustbot]

r? @petrochenkov

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[jieyouxu]

Thanks

[jieyouxu]

r? jieyouxu @bors r+ rollup

[bors]

📌 Commit 028a920 has been approved by jieyouxu

It is now in the queue for this repository.