Improve error message when _ is used for in/inout asm operands

[Amanieu]

As suggested by @Commeownist in #72016 (comment).

[rust-highfive]

r? @matthewjasper

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[Amanieu]

r? @nagisa

[nagisa]

This seems like a more general problem plaguing _ uses in an expression position. Like

error[E0658]: destructuring assignments are unstable
 --> src/main.rs:3:5
  |
3 |     _;
  |     ^
  |
  = note: see issue #71126 <https://github.com/rust-lang/rust/issues/71126> for more information

error: in expressions, `_` can only be used on the left-hand side of an assignment
 --> src/main.rs:3:5
  |
3 |     _;
  |     ^ `_` not allowed here

cc @estebank might be of interest to you.

---

Overall this seems like a good place to provide asm! specific diagnostic. I would perhaps have chosen to word this as "_ is not a valid input operand" or somesuch, but this seems good regardless.

@bors r+

[bors]

📌 Commit 4d6bfde has been approved by nagisa

[bors]

🌲 The tree is currently closed for pull requests below priority 100. This pull request will be tested once the tree is reopened.

[nagisa]

Ah well, another thing that would be nice is some deduplication. I would perhaps extract

            if p.eat_keyword(kw::Underscore) {
                let err = ecx.struct_span_err(p.token.span, "_ cannot be used for input operands");
                return Err(err);
            }
            let expr = p.parse_expr()?;

into a parse_asm_input_operand method or somesuch.

[estebank]

@nagisa weird

rust/compiler/rustc_parse/src/parser/expr.rs

Lines 1182 to 1184 in 77ac329

	} else if self.eat_keyword(kw::Underscore) {
	self.sess.gated_spans.gate(sym::destructuring_assignment, self.prev_token.span);
	Ok(self.mk_expr(self.prev_token.span, ExprKind::Underscore, attrs))