Add const-eval support for indirects

[pvdrz]

r? @wesleywiser

[Centril]

cc @oli-obk

Shouldn't this have some tests?

[oli-obk]

Some tests would be great indeed. iirc @wesleywiser mentioned that promoteds will cause Indirects to occur. Not sure how to write a test around that, maybe something with casts of promoted values?

[oli-obk]

Unfortunately I don't understand what the current code is doing. By reading the code I would have assumed this to not work, but it does, so my understanding must be wrong.

What I thought is happening:

1. we get an Operand::Indirect, which is kind of like a C++ reference, it refers to the value, but the background logic is a pointer. So It's not a ty::Ref in the type system, but a directly e.g. a ty::Int.
2. read_immediate converts this into an Immediate (which can only be Immediate::Scalar or Immediate::ScalarPair), by actually reading from the backing memory.
3. Then, operand_from_ref is called
4. which takes a pointer out of the Immediate::Scalar (so the value, iff that value is a pointer)
5. then creates a ByRef to wherever the pointer points to, but uses the ImmTy's layout

I assumed 5. to cause us to to essentially make this operation a deref operation, which should totally not work for casting.

Looking at read_immediate (we should probably use try_read_immediate to future proof this and not cause ICEs), I don't see how we could end up doing anything but reading from the MPlace...

[wesleywiser]

I think we should probably just squash all of these commits together. You can do that by running

git rebase -i origin/master

(assuming origin is the name of the https://github.com/rust-lang/rust repo).

Then in your editor pick the first commit and squash all the others.

[pvdrz]

Ok the tests passed, I rebased and I'll wait until tomorrow for CI

[wesleywiser]

Looks good to me but I'd like @oli-obk to sign off as well.

[oli-obk]

@bors r=wesleywiser,oli-obk

[bors]

📌 Commit b253678 has been approved by wesleywiser,oli-obk

[bors]

⌛ Testing commit b253678 with merge acda261...

[bors]

☀️ Test successful - checks-travis, status-appveyor
Approved by: wesleywiser,oli-obk
Pushing acda261 to master...

[rust-highfive]

📣 Toolstate changed by #61437!

Tested on commit acda261.
Direct link to PR: #61437

🎉 rls on windows: test-fail → test-pass (cc @Xanewok, @rust-lang/infra).