document cfg conditions on inline assembly templates and operands

[folkertdev]

Reference PR for:

• Stabilize asm_cfg rust#147736

[traviscross]

When I said to put it here, I had momentarily forgotten that this applied to operands as well.

Given that, probably it shouldn't go here. There's not really another obvious section for it, though. Probably syntax is the closest, but maybe it'd be better to just create a new asm.attributes section, even just for this one purpose, as we're describing particularly what is semantically accepted, so it's not a great fit for syntax.

Probably too, we'll want to use the phrase "accepted semantically" in this sentence to be more clear upfront about the distinction we're making.

[traviscross]

Let's also please describe the rule imposed in rust-lang/rust#147736 (comment):

operand before template

The current implementation expects at least one template string before any operands. In the example below, if the cfg condition evaluates to true, the assembly block is ill-formed. But even when it evaluates to false this block is rejected, because the parser still expects just a template (a template is parsed as an expression and then validated to ensure that it is or expands to a string literal).

Changing how this works is difficult.

// This is rejected because `a = out(reg) x` does not parse as an expresion.
asm!(
	#[cfg(false)]
	a = out(reg) x, //~ ERROR expected token: `,`
	"",
);

Let's include the example in the text.

[petrochenkov]

Again, it may be a mostly theoretical concern, but attributes are not supported here.
asm! rather have an attribute-like syntax in its DSL - rust-lang/rust#147736 (comment).

The "only cfg and cfg_attr" restriction is unique to all attribute positions and is a direct consequence of the above.

[folkertdev]

This list is about allowed positions, and with asm_cfg attributes are parsed on template strings and operands, so saying that they are a valid position for attributes feels right.

But yeah they aren't really supported like in the other places listed here.