Implement borrowck for &pin mut|const $place

[frank-king]

Part of Pin Ergonomics.

This forbids places to be moved or mutably borrowed ever since they are pinned until they are reassigned (even after the pinned borrows themselves expire).

#![feature(pin_ergnonmics)]
#![allow(incomplete_features)]

#[pin_v2]
struct Foo;

fn foo(mut x: Foo) {
    {
        let _x = &pin mut x; // x is pinned
    } // now the pinned reference expires
    let _x = &mut x; //~ ERROR cannot borrow `x` as mutable because it is pinned
    let _x = x; //~ ERROR cannot move out of `x` because it is pinned
    
    x = Foo; // x is reassigned
    let _x = &mut x; // ok because x is reassigned and not pinned yet
}

Tasks:

• Add Pins dataflow analysis
• Improve the diagnostics of pinning errors
• Test pin pattern matching
• Forbid &pin borrows for ADTs without #[pin_v2] (for soundness concerns)

Unresolved questions

• Should we allow &pin borrows to be applied to generic ADTs without #[pin_v2]? (related discussions: Require #[pin_v2] for direct &pin borrows of ADTs frank-king/rust#2 (comment))
• Should &pin borrowed places of Unpin types be prevented from being moved, or just don't treat Unpin types specially and emit move-after-pin as usual?

[frank-king]

I think about whether to allow the &pin borrows of the generic ADTs without #[pin_v2] is under further consideration, so I added it as an unresolved question that helps the reviewers from the lang-team or the types-team to make decisions.

[frank-king]

Let me self-review the whole PR carefully before marking it ready for review.

[rust-log-analyzer]

The job x86_64-gnu-miri failed! Check out the build log: (web) (plain enhanced) (plain)

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[P8L1]

I think they renamed it, should it not be 'StableHash' instead of 'HashStable'?

Refer to: e7d28d3

I made a PR to fix it: frank-king#7

View changes since the review

[rust-bors]

☔ The latest upstream changes (presumably #156111) made this pull request unmergeable. Please resolve the merge conflicts.