Check item bounds for projection clause wellformedness

[adwinwhite]

Experiment for #149283.
There're some fatal regressions.
Preparing to run crater to find out the range of breakages.

r? @ghost

[rust-bors]

☔ The latest upstream changes (presumably #150844) made this pull request unmergeable. Please resolve the merge conflicts.

[lcnr]

how does this folder differ from whatever.instantiate(projection.projection_term.args)? 🤔

[adwinwhite]

This folder additionally maps identity projection alias to the rhs type of the projection: Self::Assoc -> projection.term. It's needed to apply the item bounds on projection.term.

E.g.

trait Trait<T> {
    type Assoc: Bound<T>;
}

fn function<T, U, I>()
where 
    // We need to transform `Self::Assoc: Bound<T>` into `U: Bound<I>`.
    // With instantiating alone, the result would be `T::Assoc: Bound<I>`, which is pointless 
    // as `AliasBound` candidate already covers this.
    T: Trait<I, Assoc = U>
{}

[adwinwhite]

@bors try

[rust-bors]

☀️ Try build successful (CI)
Build commit: b6bca0d (b6bca0d7327839b5e3f7a7779947cb9113d08917, parent: 08cd08fbef8c6663b052d8d9e4930d5696cbb8a7)

[adwinwhite]

@craterbot run name=150662-leaf mode=check-only start=master#08cd08fbef8c6663b052d8d9e4930d5696cbb8a7 end=try#b6bca0d7327839b5e3f7a7779947cb9113d08917+rustflags=-Zstrict-projection-item-bounds

[craterbot]

👌 Experiment 150662-leaf created and queued.
🔍 You can check out the queue and this experiment's details.

ℹ️ Crater is a tool to run experiments across parts of the Rust ecosystem. Learn more