[WIP; Crater] Use parse_cond_expr instead parse_expr in if guards

[tesuji]

This PR improves diagnostic a bit for #15980.

[rust-highfive]

r? @estebank

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[tesuji]

I added a test for unimplemented #51114 , also cc #74232 .

[estebank]

I think that the change is correct but I want a crater run in case this affects code out in the wild.

@bors try @craterbot check

[bors]

⌛ Trying commit a1a3d97 with merge 0ab79c4b48ad19260fa46623f452c1b403a360e8...

[bors]

☀️ Try build successful - checks-actions, checks-azure
Build commit: 0ab79c4b48ad19260fa46623f452c1b403a360e8 (0ab79c4b48ad19260fa46623f452c1b403a360e8)

[estebank]

@craterbot check

[craterbot]

👌 Experiment pr-74315 created and queued.
🤖 Automatically detected try build 0ab79c4b48ad19260fa46623f452c1b403a360e8
🔍 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

[eddyb]

But it is a subset of "let chains" (or rather, let expressions in more positions), at most you should change what feature-gate it points to.

Looking around at the let chains tests, you want all of these cases to emit feature-gate errors:

rust/src/test/ui/rfc-2497-if-let-chains/feature-gate.rs

Lines 111 to 136 in 6ee1b62

	fn _macros() {
	macro_rules! noop_expr { ($e:expr) => {}; }
	noop_expr!((let 0 = 1));
	//~^ ERROR `let` expressions in this position are experimental [E0658]
	macro_rules! use_expr {
	($e:expr) => {
	if $e {}
	while $e {}
	}
	}
	use_expr!((let 0 = 1 && 0 == 0));
	//~^ ERROR `let` expressions in this position are experimental [E0658]
	//~| ERROR `let` expressions are not supported here
	//~| ERROR `let` expressions are not supported here
	use_expr!((let 0 = 1));
	//~^ ERROR `let` expressions in this position are experimental [E0658]
	//~| ERROR `let` expressions are not supported here
	//~| ERROR `let` expressions are not supported here
	#[cfg(FALSE)] (let 0 = 1);
	//~^ ERROR `let` expressions in this position are experimental [E0658]
	use_expr!(let 0 = 1);
	//~^ ERROR no rules expected the token `let`
	// ^--- FIXME(53667): Consider whether `Let` can be added to `ident_can_begin_expr`.
	}

Note that noop_expr! and #[cfg(FALSE)] must emit feature-gate errors despite those AST nodes never reaching HIR lowering.

I think you should copy that entire test, remove _while, and then put every if let in a guard, e.g.:

match () { () if let ... = ... => {} }

[craterbot]

🚧 Experiment pr-74315 is now running

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

[craterbot]

🎉 Experiment pr-74315 is completed!
📊 12 regressed and 4 fixed (113052 total)
📰 Open the full report.

⚠️ If you notice any spurious failure please add them to the blacklist!
ℹ️ Crater is a tool to run experiments across parts of the Rust ecosystem. Learn more

[tesuji]

Closing in favor of #74566.
Will triage crater there.