`unused_patterns` lint

[nikomatsakis]

As @scottmcm pointed out here, the unused_parens lints should be extended to also catch unused parentheses in patterns:

    match (1) { // Warns, as expected
        (_) => {} // Doesn't warn, but should for consistency
    }

Play

[nikomatsakis]

Here are some tips for how to update that lint. These aren't full mentoring notes but they might get you started.

The lint itself is declared here:

rust/src/librustc_lint/unused.rs

Lines 258 to 262 in 3bc2ca7

	declare_lint! {
	pub(super) UNUSED_PARENS,
	Warn,
	"`if`, `match`, `while` and `return` do not need parentheses"
	}

The meat of it is found in the check_expr callback here, which is invoked for every expression:

rust/src/librustc_lint/unused.rs

Line 326 in 3bc2ca7

fn check_expr(&mut self, cx: &EarlyContext, e: &ast::Expr) {

As you can see, it checks for various places where a lint would not be needed, such as in the body if an If, and then invokes this routine check_unused_patterns_core to issue a warning.

For patterns, I imagine we'll want to do something similar, but adding a check_pat method (this is the definition from the trait).

That said, I think this is quite a bit harder than the stabilization itself. If the person who handles this issue wants to do it, seems great, but if they are not familiar with the compiler, I think we could leave it out from the first PR, and then move this to a distinct issue to be added at some later time.

[kleimkuhler]

I can pick this up. The mentoring notes provide enough for me to get started. I will reach out if I need some help or clarification!

[nikomatsakis]

@kleimkuhler Great! If you have any further problems, please don't hestitate to ping on Zulip or Discord.

[nikomatsakis]

I started a thread on Zulip, for future reference.