don't go through llvm for const indexing

[oli-obk]

closes #3170

[rust-highfive]

r? @alexcrichton

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[pnkfelix]

... I would imagine you do need to eval the elements ...

(... well, maybe not ... i guess you are effectively doing some sort of lazy-evaluation in the constant evaluator, which may be okay. But what happens if no one ever indexes the element in a constant expression?)

[oli-obk]

But what happens if no one ever indexes the element in a constant expression?

Then they get evaluated in https://github.com/oli-obk/rust/blob/const_indexing/src/librustc_trans/trans/consts.rs#L694-717

[oli-obk]

I have updated this to simply memorize the ast::NodeId like tuples and structs do. Note: tuples and structs also do lazy evaluation for the field accesses.

[oli-obk]

hmmm... getting quite a few test failures. I guess I broke creating references to const arrays and their elements. I'm evaluating this.

    [run-pass] run-pass/check-static-slice.rs
    [run-pass] run-pass/const-autoderef.rs
    [run-pass] run-pass/const-enum-vec-index.rs
    [run-pass] run-pass/const-fields-and-indexing.rs
    [run-pass] run-pass/const-str-ptr.rs
    [run-pass] run-pass/issue-17233.rs
    [run-pass] run-pass/syntax-extension-source-utils.rs

[alexcrichton]

r? @pnkfelix or @nrc

[oli-obk]

this isn't as trivial as I thought. I'm closing this until I've got a working version. sorry about that.