Fix handling of ignored arguments in FFI wrappers for rust functions

Fixes #16441.
auto merge of #16484 : dotdash/rust/empty_struct_wrapper_arg, r=luqmana

Fixes #16441.
librustc: Tie up loose ends in unboxed closures.

This patch primarily does two things: (1) it prevents lifetimes from
leaking out of unboxed closures; (2) it allows unboxed closure type
notation, call notation, and construction notation to construct closures
matching any of the three traits.

This breaks code that looked like:

    let mut f;
    {
        let x = &5i;
        f = |&mut:| *x + 10;
    }

Change this code to avoid having a reference escape. For example:

    {
        let x = &5i;
        let mut f; // <-- move here to avoid dangling reference
        f = |&mut:| *x + 10;
    }

I believe this is enough to consider unboxed closures essentially
implemented. Further issues (for example, higher-rank lifetimes) should
be filed as followups.

Closes #14449.

[breaking-change]
auto merge of #16122 : pcwalton/rust/lifetimes-in-unboxed-closures, r=pnkfelix

This patch primarily does two things: (1) it prevents lifetimes from
leaking out of unboxed closures; (2) it allows unboxed closure type
notation, call notation, and construction notation to construct closures
matching any of the three traits.

This breaks code that looked like:

    let mut f;
    {
        let x = &5i;
        f = |&mut:| *x + 10;
    }

Change this code to avoid having a reference escape. For example:

    {
        let x = &5i;
        let mut f; // <-- move here to avoid dangling reference
        f = |&mut:| *x + 10;
    }

I believe this is enough to consider unboxed closures essentially
implemented. Further issues (for example, higher-rank lifetimes) should
be filed as followups.

Closes #14449.

[breaking-change]

r? @pnkfelix
librustc: Implement simple `where` clauses.

These `where` clauses are accepted everywhere generics are currently
accepted and desugar during type collection to the type parameter bounds
we have today.

A new keyword, `where`, has been added. Therefore, this is a breaking
change. Change uses of `where` to other identifiers.

[breaking-change]