Document if-let guards

[LeSeulArtichaut]

cc @pnkfelixrust-lang/rfcs#2294rust-lang/rust#51114

[ghost]

Suggested change

	`if let` guards are introduced by the `if let` keywords and are composed of a pattern, an `=` and a scrutinee expression.
	`if let` guards are composed following [The Rust Reference `if let` keywords syntax](https://doc.rust-lang.org/stable/reference/expressions/if-expr.html#if-let-expressions).

[ehuss]

@kw-fn, I think the original is correct here, as it is just describing the syntax, and is a style used throughout the reference as a way to anchor the description of what the text is talking about. Also, we do not use external links, or refer to the reference by name within itself.

[ehuss]

Thanks @LeSeulArtichaut, this looks good to me! It will need to wait for it to be stabilized on nightly, though happy to discuss any feedback from the lang team in the meantime.

[Fishrock123]

This text reads well to me.

[ghost]

I think we can close this with the documentation we have currently in the reference now at time of writing

[matthewjasper]

Suggested change

	> &nbsp;&nbsp;`if``let`[_Pattern_]`=`_Scrutinee_<sub>_except lazy boolean operator expression_</sub>
	> &nbsp;&nbsp;`if``let`[_Pattern_]`=`_Scrutinee_

After rust#115371

[rustbot]

☔ The latest upstream changes (possibly 4249fb4) made this pull request unmergeable. Please resolve the merge conflicts.

[ehuss]

Closing as superseded by #1957.