Add single_option_map lint

[yusufraji]

Checks for functions with method calls to .map(_) on an arg of type Option as the outermost expression.

Fixes #774

changelog: [`single_option_map`]: Checks for functions with method calls to `.map(_)` on an arg of type `Option` as the outermost expression.

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[yusufraji]

r? @llogiq

[samueltardieu]

It is also necessary to check that the receiver of map() is the function parameter. Otherwise, the lint will trigger on code such as:

fn f(x: &str) -> Option<u32> {
    x.parse::<u32>().ok().map(|x| x+1)
}

[samueltardieu]

This test case should be added, to reflect one problem the lint had in its earlier form:

// No lint: maps other than the receiver
fn map_not_arg(arg: Option<u32>) -> Option<u32> {
    maps_static_option().map(|_| arg.unwrap())
}

Also, wrapping a function can be done more explicitly than with just the function name:

// No lint: wrapper function with η-expanded form
fn manipulate_opt_explicit(opt_i: Option<i32>) -> Option<i32> {
    opt_i.map(|x| manipulate(x))
}

It would be great not to lint here too. But it can always be done later if this proves cumbersome.

[yusufraji]

This test case should be added, to reflect one problem the lint had in its earlier form:

// No lint: maps other than the receiver
fn map_not_arg(arg: Option<u32>) -> Option<u32> {
    maps_static_option().map(|_| arg.unwrap())
}

Also, wrapping a function can be done more explicitly than with just the function name:

// No lint: wrapper function with η-expanded form
fn manipulate_opt_explicit(opt_i: Option<i32>) -> Option<i32> {
    opt_i.map(|x| manipulate(x))
}

It would be great not to lint here too. But it can always be done later if this proves cumbersome.

Thanks, I've added it.

[llogiq]

I think we may want to look into the the closure and not lint when it's anything but a function call.

This also applies to function arguments, which should be paths to lint. We shouldn't lint arg1.map(|a1| f(a1, arg2 + 1)). Edit: I'm confused, we should certainly lint that.