Make tuple constructors real const fns

[matthewjasper]

Mir construction special cases Ctor(...) to be lowered as Ctor { 0: ... }, which means this doesn't come up much in practice, but it seems inconsistent not to allow this.

Tracking issue: #61456

r? @oli-obk

[oli-obk]

This change makes me wonder whether it would be possible to generalize build_adt_ctor to the point where we can scrap mir::AggregateKind::Adt and just call the part of build_adt_ctor that generates the field and discr initialization.

Additionally, if I'm reading this correctly, build_adt_ctor could generate a shim that allows initializing arbitrary ty::Adt. So there's nothing implementation wise that would speak against the lang team figuring out a way to obtain initialization functions for struct variants or just plain structs. cc @Centril

[oli-obk]

cc @rust-lang/lang Not suggesting that we add this or anything, but wanted to let you know that we can now generate a function

fn foo(a: A, b: B, c: C) -> Foo {
    Foo { a, b, c }
}

from a struct definition

struct Foo {
    a: A,
    b: B,
    c: C,
}

In case this ever comes up as something that may be useful

[eddyb]

@oli-obk FWIW we already have an optimization that expands Rvalue::Aggregate into this instruction sequence so this code is already duplicated, I think?

[oli-obk]

iirc that optimization is very expensive to run. I know we want to do little work at MIR building time, but in this case I believe it's equivalent in complexity. We can scrap that optimization if we just do it all at MIR building time.

[eddyb]

I meant that we could put the code generating this in that optimization until we get rid of it (rather than relying on running the optimization).

[eddyb]

So I guess this is pre-optimized? As opposed to Rvalue::Aggregate?

[oli-obk]

r=me with feature gates cleaned up

[matthewjasper]

@bors r=oli-obk

[bors]

📌 Commit bcf8365 has been approved by oli-obk

[bors]

⌛ Testing commit bcf8365 with merge c1c60d2...

[bors]

☀️ Test successful - checks-travis, status-appveyor
Approved by: oli-obk
Pushing c1c60d2 to master...