rustdoc documents `mut self` as `mut self: Self`

[tesuji]

Rustdoc on nightly documents methods have signature of fn(mut self) as fn(mut self: Self) while it only has to document it as fn(self).

On rustdoc stable, things are normal:

Meta

rustc --version --verbose: Version 1.49.0 (e1884a8 2020-12-29)

[LeSeulArtichaut]

I'd like to investigate this :)

[LeSeulArtichaut]

searched nightlies: from nightly-2020-12-29 to nightly-2021-01-22
regressed nightly: nightly-2021-01-09
searched commits: from c8915ee to 26438b4
regressed commit: 9155a9d

This regressed in #80806, so this is probably caused by #80799

[LeSeulArtichaut]

This also affects

fn bar(mut foo) {}

Where rustdoc will generate:

fn bar(mut foo: Foo)

[LeSeulArtichaut]

So a few observations:

• rustc_hir_pretty::param_to_string uses print_pat which displays mutability in patterns, causing mut to be displayed
• the mechanism for detecting and eliding : Self works by litterally checking if the pattern is self, but as param_to_string returns mut self instead of self, the comparison fails and rustdoc displays the Self type.

[LeSeulArtichaut]

I see two path to solving this bug:

1. Add an option for rustc_hir_pretty::param_to_string to not display the mutability in patterns
2. Just revert Get rid of custom pretty-printing in rustdoc #80799

cc @jyn514 what do you think?

[jyn514]

I would prefer to go for 1, there's no reason to have two completely different functions for pretty-printing.

[camelid]

Assigning P-medium and removing I-prioritize as discussed in the prioritization working group.