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: Stop assuming that implementations and traits only contain
methods.

This paves the way to associated items by introducing an extra level of
abstraction ("impl-or-trait item") between traits/implementations and
methods. This new abstraction is encoded in the metadata and used
throughout the compiler where appropriate.

There are no functional changes; this is purely a refactoring.
auto merge of #16286 : pcwalton/rust/associated-items-groundwork, r=nikomatsakis

methods.

This paves the way to associated items by introducing an extra level of
abstraction ("impl-or-trait item") between traits/implementations and
methods. This new abstraction is encoded in the metadata and used
throughout the compiler where appropriate.

There are no functional changes; this is purely a refactoring.

r? @nick29581
libsyntax: Accept `use foo as bar;` in lieu of `use bar as foo;`

The old syntax will be removed after a snapshot.

RFC #47.

Issue #16461.
auto merge of #16468 : pcwalton/rust/as-renaming-import, r=alexcrichton

The old syntax will be removed after a snapshot.

RFC #47.

Issue #16461.

r? @brson
librustc: Allow trait bounds on structures and enumerations, and check
them during kind checking.

This implements RFC #11.

Closes #15759.