NLL fails to suggest "try removing &mut here"

[pnkfelix]

For the following tests, AST-borrowck suggests removing &mut, while NLL fails to make such a suggestion:

• did_you_mean/issue-31424.rs / NLL output (partially fixed in NLL diagnostics: revise fn check_access_permissions #51275)
• did_you_mean/issue-34337.rs / NLL output (fixed in NLL diagnostics: revise fn check_access_permissions #51275)
• did_you_mean/issue-37139.rs / NLL output (fixed in NLL diagnostics: revise fn check_access_permissions #51275)

[pnkfelix]

We might be able to incorporate a fix for this into #51275 ; I haven't quite dug into how we detect this case yet. But its worth at least looking into as part of that PR.

[pnkfelix]

(at this point I have indeed incorporated a fix for this into #51275 )

[pnkfelix]

(except that my fix, in the case of did_you_mean/issue-31424.rs, is suggesting to the user that they write mut self rather than suggesting to the user that they remove the &mut borrow in the method call receiver. Not sure whether that is worth addressing. Probably shouldn't close this issue without at least looking into that.

[nikomatsakis]

To clarify the current situation. For issue-31424.rs, the AST-based checker gives this error:

error[E0596]: cannot borrow immutable argument `self` as mutable
  --> src/main.rs:19:15
   |
19 |         (&mut self).bar(); //~ ERROR cannot borrow
   |               ^^^^
   |               |
   |               cannot reborrow mutably
   |               try removing `&mut` here

and the NLL one gives this one:

error[E0596]: cannot borrow immutable item `self` as mutable
  --> src/main.rs:19:9
   |
19 |         (&mut self).bar(); //~ ERROR cannot borrow
   |         ^^^^^^^^^^^ cannot borrow as mutable

Note that the hint is missing from the NLL one.

[nikomatsakis]

This doesn't seem to me to rise to the cut of a Edition Preview blocker, or even a RC blocker.

[nikomatsakis]

I'll tag with the Release milestone for now, though I think it could plausibly be cut.