New lint suggestion: advise replacement of `map` + `take_while` + `map` by `map_while`

First suggestion for clippy lint, sorry if I missed something

What it does

Detects usages of this form (link):

let a = [-1i32, 4, 0, 1];

let mut iter = a.iter()
               .map(|x| 16i32.checked_div(*x))
               .take_while(|x| x.is_some())
               .map(|x| x.unwrap());

assert_eq!(iter.next(), Some(-16));
assert_eq!(iter.next(), Some(4));
assert_eq!(iter.next(), None);

Categories (optional)

• Kind: once stable, either style or complexity, though similar lints are in pedantic (but they don't involve an extra .unwrap() in their to-fix form)

Advantages:

• One method call instead of three
• Removes an unwrap call (and so a lot of panic machinery when compiling for debug, maybe even for release if the compiler doesn't see it's always true)

Drawbacks

• Will only be valid for 1.57+
• Closures may be a little harder to read/write and the transformation may not be obvious (I haven't done any research about it)

Example

let a = [-1i32, 4, 0, 1];

let mut iter = a.iter()
               .map(|x| 16i32.checked_div(*x))
               .take_while(|x| x.is_some())
               .map(|x| x.unwrap());

assert_eq!(iter.next(), Some(-16));
assert_eq!(iter.next(), Some(4));
assert_eq!(iter.next(), None);

Could be written as:

let a = [-1i32, 4, 0, 1];

let mut iter = a.iter().map_while(|x| 16i32.checked_div(*x));

assert_eq!(iter.next(), Some(-16));
assert_eq!(iter.next(), Some(4));
assert_eq!(iter.next(), None);

The examples and linked Iterator::map_while documentation define the target transformation; begin by checking the Rust 1.57 compatibility constraint and how Clippy categorizes similar lints. Done means the pattern is recognized without changing iterator behavior, with coverage for the shown cases and an agreed lint category.