Fix substs issues for const errors

[varkor]

Fixes#60503.

[rust-highfive]

r? @petrochenkov

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[varkor]

This currently breaks

rust/src/test/ui/const-generics/const-param-before-other-params.rs

Lines 4 to 6 in ef9a876

	fnfoo<constX:(),T>(_:&T){
	//~^ ERROR type parameters must be declared prior to const parameters
	}

because it tries to substitute a const for a type parameter. I think this was a pre-existing issue, but the bug this PR fixes was hiding it. I think that type parameters are still being ordered before const parameters (even if they appear afterwards in the source code), so there's a mismatch.

[estebank]

@varkor the parser ignores the order when making the ast node but complains about the order so an error will be emitted. Why do we depend on source ordering here?

[varkor]

@estebank: I haven't fully diagnosed the issue yet, but ideally parameters would preserve their source order throughout the compiler: there shouldn't be any reason lifetimes, types and consts should come in that order specifically. The error should essentially just be a lint. However, that hasn't always been the case and I imagine there are some broken assumptions here because of it.

[varkor]

A quick fix would be to abort as soon as misordered const/type parameters are found in a signature. It should only regress diagnostics for const generics and I can fix the issue properly later.

[estebank]

@varkor I think the order is lost because in the parser we collect independent vecs for each type of argument instead of one vec with annotated items. We could indeed FatalError.raise() for this case in particular, but would like it to have a FIXME and fixed quickly, as I wouldn't want the const experience to be bad for people starting to try the feature out.

[estebank]

I think this is a first 😄

[varkor]

I've added a temporary FatalError.raise() and a FIXME.

[petrochenkov]

@bors r+

[bors]

📌 Commit 735714af24f422dcba10d1beabf35a0816f645c2 has been approved by petrochenkov

[varkor]

@bors r=petrochenkov

[bors]

📌 Commit 3e6787c has been approved by petrochenkov

[bors]

⌛ Testing commit 3e6787c with merge c55b68a...

[bors]

☀️ Test successful - checks-travis, status-appveyor
Approved by: petrochenkov
Pushing c55b68a to master...