Tracking issue for future-incompatibility lint `patterns_in_fns_without_body`

[petrochenkov]

Code using patterns in parameters of foreign functions, function pointers or trait methods without body is not accepted by the compiler.

extern {
    fn f((a, b): (u8, u16)); // ERROR
}

type T = fn((a, b): (u8, u16)); // ERROR

trait Tr {
    fn f((a, b): (u8, u16)); // ERROR
}

Previously some simple patterns like &ident, &&ident or mut ident were allowed in these positions, now they aren't. Such patterns weren't properly type checked, e.g. type A = fn(&arg: u8); compiled despite u8 not being a reference.

What are these errors for?

Full patterns don't make sense in functions without bodies, but simple identifiers may be useful for documenting argument purposes even if they aren't actually used - type Callback = fn(useful_name: u8).
By restricting patterns in body-less function signatures to ident: TYPE we can make argument names optional and accept simply a TYPE in argument position (type T = fn(u8)) without introducing parsing ambiguities.

How to fix this warning/error

Remove & or mut from the pattern and make the function parameter a single identifier or _.

Current status

• Properly enforce the "patterns aren't allowed in foreign functions" rule #35015 made patterns in foreign functions and function pointers hard errors
• Prohibit patterns in trait methods without bodies #37378 made patterns in trait methods without body warn-by-default lints
• Accept interpolated patterns in trait method parameters #45775 made patterns in trait methods without body hard errors, except for mut IDENT
• Transition future compat lints to {ERROR, DENY} - Take 2 #65785 made patterns in trait methods without body deny-by-default lints
• ? made patterns in trait methods without body hard errors
• On "pattern in arg for fn def without body" emit structured suggestion #78927 provide structured suggestions