borrowck: consolidate mut suggestions

[arielb1]

This converts all of borrowck's mut suggestions to a new
mc::ImmutabilityBlame API instead of the current mix of various hacks.

Fixes#35937.
Fixes#40823.
Fixes#40859.

cc @estebank
r? @pnkfelix

[estebank]

I like it (besides a couple of nitpicks).

[estebank]

pub fn main?

[estebank]

update comment to node_id

[estebank]

Shouldn't it suggest &mut self in that case?

[arielb1]

Nope. Binding vs. reference - the code is intentionally careful to do the right thing.

[estebank]

I see, you're right, of course.

[estebank]

What's the procedure to remove Errors that we no longer emit?

[arielb1]

there's no such procedure.

[nagisa]

We put these into register_diagnostics! {}, commented out, although a better way to annotate codes that may not be reused should be made.

[pnkfelix]

@nagisa are you saying that this modification (the commenting out here) is okay as is? Or are you saying it shouldn't stay here under register_long_diagnostics!, and should ... move to register_diagnostics!...?

I think you must mean the former, that this is okay as is... Let us know if I misunderstand.

[pnkfelix]

(well I guess deleting all this long commented-out diagnostic text and replacing it with one line in register_diagnostics! could be an attractive option...)

[ghost]

Is #35927 supposed to link somewhere else?

[arielb1]

@whataloadofwhat

Indeed. Updated & fixed.

BTW, @eddyb & @petrochenkov - is there a cleaner way of determining whether a function has an implicit self?

[eddyb]

@arielb1 Doubt it. It might be better idea to preserve the distinction in hir::Ty instead, not sure.

[pnkfelix]

@bors r+

[bors]

📌 Commit 39011f8 has been approved by pnkfelix