`const` operands in `asm!` should use inline consts rather than const promotion

[bstrie]

Elevating an item from the asm! progress report from a comment to an issue so that it can be tracked:

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const operands should switch to using the new inline consts instead of explicit promotion. In theory this should have no impact on existing code, but it requires inline consts to support references to generic paramters of the surrounding function, which is not supported yet.

One aspect of asm! that I think needs further scrutiny before stabilization is const arguments, which currently are one of the few syntactic constructs that trigger promotion. Promotion is about turning a part of a function into an anonymous const (the "promoted") and using that instead of the ripped-out part of the function body. This has all sorts of "interesting" consequences (see e.g. rust-lang/rfcs#3027).

Similar to the situation for array repeat expressions in #49147, there is an alternative: we could avoid having to deal with promotion here by requiring that the const argument be either a literal (4), a named constant (CONST), or an explicit inline const (const { arbitrary_const_expr }). If that would be acceptable to y'all, I think it would be better to only support this subset upon initial stabilization. This is forward compatible with supporting more expressions by performing promotion later. The only difference this would make is one of convenience -- one could avoid having to type const { ... }.

[RalfJung]

(The syntax I suggested in the quote above is up for bikeshedding of course; the key point is that the const expression is treated separately from the start and we don't have to hot-fix the MIR later via promotion.)

[bstrie]

The syntax I suggested in the quote above is up for bikeshedding of course

Surely it would just use the same syntax as inline const in general, yes? #76001 suggests that const { $expr } is the chosen syntax for this feature, but is there any reason to suspect that this is incompatible with the syntax of the asm! macro?

[RalfJung]

asm! being a macro can do almost whatever it wants. ;)
So we'd end up with something like this?

    asm!(
        "mov {0}, {1}",
        "add {0}, {number}",
        out(reg) o,
        in(reg) i,
        number = const { 5 },
    );

[bstrie]

Sure, although just as you say in your comment it should also be possible to elide the explicit const for the case of literals and named consts. I believe this exactly mirrors your plan for const array repeat expressions, yes? I.e. [5; 2], [FOO; 2], [const { foo() }; 2]?

[RalfJung]

Well, the asm! RFC says const is a required keyword here, it just does not use curly braces.

For arrays, there's not much of a "plan", but [5; 2] already works so we'll certainly continue supporting it. In [expr; COUNT], expr is a normal expression syntactically, so the const keyword here is nothing sepcific to arrays but just the same keyword as in let x = const { 1+2 };. In contrast, asm! has a const keyword built-in (in the RFC).

[Amanieu]

I made an attempt at switching const operands to use inline consts (well, AnonConst to be precise) in #83916.