Parse pinned local variable declarations

[frank-king]

This PR implements part of #130494, parsing let pin mut x or let pin const x in local variables that cannot be moved.

[rustbot]

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[rustbot]

Some changes occurred in match checking

cc @Nadrieril

[Noratrieb]

The tracking issue only mentions constructors and reborrowing, and not pinned locals, so I don't think this should be added.
I don't even know what this should do, it is very non-obvious to me. Can you explain to me what you'd expect this to do?

[frank-king]

Compared to mut locals, pin mut locals can be borrowed by &pin mut, but cannot be moved by assignment, passed to a function, or mutable borrow &mut.

Similarly, pin const locals cannot be passed to a function, or something like Cell::take.

For example:

struct Foo;

impl Foo {
    fn by_pin_const(&pin const self) {}
    fn by_pin_mut(&pin mut self) {}
    fn by_ref(&self) {}
    fn by_mut(&mut self) {}
    fn by_value(self) {}
}

fn bar() {
    let pin mut foo = Foo; // define a pinned mutable local (cannot be moved but can be mutated)
    foo.by_pin(); // ok
    foo.by_pin_mut(); // ok
    foo.by_ref(); // ok because `Pin<Foo>` impls Deref<Target = Foo>
    foo.by_mut(); //~ ERROR cannot borrow `foo` mutably because it is pinned
    foo.by_value();  //~ ERROR cannot move `foo` because it is pinned
    foo = Foo; //~ ERROR cannot assign to `foo` because the previous value it bound has be pinned

    let pin const foo = Foo; // define a pinned immutable local (cannot be moved nor mutated)
    foo.by_pin(); // ok
    foo.by_pin_mut(); // ERROR cannot borrow `foo` mutably because it is immutably pinned
    foo.by_ref(); // ok because `Pin<Foo>` impls Deref<Target = Foo>
    foo.by_mut(); //~ ERROR cannot borrow `foo` mutably because it is immutably pinned
    foo.by_value();  //~ ERROR cannot move `foo` because it is pinned
    foo = Foo; //~ ERROR cannot assign to `foo` because the `foo` is immuable
}

[compiler-errors]

This feature needs to be gated properly so that we reject this in #[cfg]'d out code.

[rustbot]

Some changes occurred in src/tools/rustfmt

cc @rust-lang/rustfmt

Some changes occurred in src/tools/clippy

cc @rust-lang/clippy