borrowck: do not suggest to change "&mut self" to "&mut mut self"

[birkenfeld]

Matching the snippet string might not be the cleanest, but matching
the AST node instead seems to end in a lot of nested if lets, so I
don't know what's better.

Of course it's entirely possible that there is another API altogether
that I just don't know of?

Fixes #31424.

[rust-highfive]

r? @Aatch

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[jseyfried]

This would prevent the following correct suggestion:

struct Struct;

impl Struct {
    fn foo(self: &mut Self) {
        (&mut self).bar();
        //~^ ERROR cannot borrow immutable argument `self` as mutable
        //~| HELP to make the argument mutable, use `mut` as shown:
        //~|    fn foo(mut self: &mut Self) {
    }

    fn bar(&mut self) {}
}

It might be a good idea to match the AST node so that we can continue providing the suggestion in the above case. It would be even better if we could provide the above HELP message even when self is declared as &mut self instead of self: &mut Self.

That being said, I think it's more important to not provide a bad suggestion in the common case of &mut self, so r=me unless you would like to work one of the above improvements.

[birkenfeld]

I've looked a bit at what would be involved, and it's tricky: the "pat" node I can get only ever spans the "self" (it's the special pseudo node added by Arg::new_self). The next parent is the whole Item, and the individual Args have no spans. So it'd be a lot of work for this edge case (which is probably quite obscure, since a &mut &mut T is not often necessary).

But I've rebased and added a test, will push that now.

[jseyfried]

Nice -- I didn't know about forbid-output

[jseyfried]

@bors r+

[bors]

📌 Commit fef8276 has been approved by jseyfried

[bors]

⌛ Testing commit fef8276 with merge 6dbb0e8...

[bors]

☀️ Test successful - auto-linux-32-nopt-t, auto-linux-32-opt, auto-linux-32cross-opt, auto-linux-64-cargotest, auto-linux-64-cross-armhf, auto-linux-64-cross-armsf, auto-linux-64-cross-freebsd, auto-linux-64-cross-netbsd, auto-linux-64-debug-opt, auto-linux-64-nopt-t, auto-linux-64-opt, auto-linux-64-opt-mir, auto-linux-64-opt-rustbuild, auto-linux-64-x-android-t, auto-linux-cross-opt, auto-linux-musl-64-opt, auto-mac-32-opt, auto-mac-64-nopt-t, auto-mac-64-opt, auto-mac-64-opt-rustbuild, auto-mac-cross-ios-opt, auto-win-gnu-32-nopt-t, auto-win-gnu-32-opt, auto-win-gnu-32-opt-rustbuild, auto-win-gnu-64-nopt-t, auto-win-gnu-64-opt, auto-win-msvc-32-cross-opt, auto-win-msvc-32-opt, auto-win-msvc-64-cargotest, auto-win-msvc-64-opt, auto-win-msvc-64-opt-mir, auto-win-msvc-64-opt-rustbuild