Suggest dereferencing on assignment to mutable borrow

[estebank]

Fix #33570

[rust-highfive]

r? @zackmdavis

(rust_highfive has picked a reviewer for you, use r? to override)

[estebank]

cc @Centril on wording

[zackmdavis]

@bors r+

[bors]

📌 Commit 1e3302d has been approved by zackmdavis

[mark-i-m]

Won't this suggestion trigger even if the type of the pointee and the assignee are different?

let mut x = 0;

(&mut x) = None; // suggestion to consider dereferencing does not work

[estebank]

I'll add a negative check to the test to avoid regressions, but by the time we hit this code we have already checked that the two types can be unified and that the left is a mutable borrow.

[Centril]

@bors r- Until that test is added then :)

[Centril]

I'm a big, big fan of having a diagnostic for this as I make this mistake on a regular basis.

[Centril]

With this addition the function is ~200+ LOC -- can we avoid making it bigger? =P

[estebank]

I'm focusing on slimming/splitting Parser now. I'll move to other parts of the compiler once in done with that.

[Centril]

Cool, just don't forget about it... =)

[estebank]

@bors r=zackmdavis added the test

[bors]

🙀 added is not a valid commit SHA. Please try again with 7fbbcfaafd08023cc2296481425e16a02ccd3f1a.

[estebank]

@bors r=zackmdavis

[bors]

📌 Commit 7fbbcfa has been approved by zackmdavis

[estebank]

For the case in @mark-i-m's comment, the output is:

error[E0308]: mismatched types
 --> file2.rs:4:12
  |
4 | (&mut x) = None;
  |            ^^^^ expected mutable reference, found enum `std::option::Option`
  |
  = note: expected type `&mut {integer}`
             found type `std::option::Option<_>`

error[E0070]: invalid left-hand side expression
 --> file2.rs:4:1
  |
4 | (&mut x) = None;
  | ^^^^^^^^^^^^^^^ left-hand of expression not valid