Lint: useless/unidiomatic match statement on bool

[codybloemhard]

What it does

Catch code that flips a boolean with a match statement, as well as code that applies the identity function over a boolean with a match statement.
Lints for code like this exists:

let a = true;
let b = if a { false } else { true };
let c = if b == true { false } else { true };
let d = if c { true } else { false };

but there seems to be no lint for code like this:

let a = true;
let b = match a{
    true => false,
    false => true,
};

I saw someone writing that piece of code, so it could be useful if there was a lint for it.

Categories (optional)

• clippy::style

What is the advantage of the recommended code over the original code

• The recommended is code is shorter and more readable.

Drawbacks

None.

Example 0

let a = true;
let b = match a{
    true => false,
    false => true,
};

Could be written as:

let a = true;
let b = !a;

Example 1

let a = true;
let b = match a{
    true => true,
    false => false,
};

Could be written as:

let a = true;
let b = a;

[giraffate]

It would be better to enhance the needless_bool and bool_comparison lint.

[ada4a]

I believe this does get addressed thanks to match_bool -- it suggests the following:

warning: `match` on a boolean expression
  --> src/main.rs:7:9
   |
7  |   let b = match a{
   |  _________^
8  | |     true => false,
9  | |     false => true,
10 | | };
   | |_^ help: consider using an `if`/`else` expression: `if a { false } else { true }`
   |
   = help: for further information visit https://rust-lang.github.io/rust-clippy/master/index.html#match_bool

and the resulting if-else then gets linted by needlees_bool.

One caveat is that match_bool is pedantic and thus allow-by-default, unlike needless_bool which is warn-by-default. I do feel like the former is uncontroversial and complexity-reducing enough that it could be moved to complexity as well.

Still, I'd (propose to) close this issue -- feel free to reopen if you disagree though.
@rustbot label S-fixed