Report const index out-of-bound condition earlier

[graydon]

Currently if you try indexing into a const vector, in a const expr, we report the bounds-overrun late in compilation -- during translation. We should notice it earlier, in the const evaluation pass.

[jld]

Apparently the “error” this produces doesn't cause compilation to fail — code will be generated and rustc will return 0. LLVM will complain if the index is physically out of bounds, and this is currently always equivalent to an out-of-bounds index (I think), but that won't be the case for const slices.

This is especially unexpected during testing, which (except for check-fast, apparently?) doesn't show the output/error on success.

[catamorphism]

Not critical for 0.6; de-milestoning

[catamorphism]

Nominating for milestone 5, production-ready

[graydon]

accepted for production-ready milestone

[alexcrichton]

As an example of what I believe that this is talking about:

static a: &'static [int] = &[];
static b: int = a[1];

fn main() {}

yields

$ rustc foo.rs
foo.rs:2:16: 2:19 error: const index-expr is out of bounds
foo.rs:2 static b: int = a[1];
                         ^~~
Assertion failed: (ReqTy && "extractvalue indices invalid!"), function getExtractValue, file ../../../../src/llvm/lib/IR/Constants.cpp, line 1982.
zsh: abort      rustc foo.rs

Seems bad that we're hitting an LLVM assertion at all.

[pnkfelix]

Accepted for P-low.

[huonw]

Triage: @alexcrichton's (9 month old) example is still syntactically valid (yay!) and still fails with that assertion (boo!).

[vks]

I believe this is fixed.

[JustAPerson]

If you update @alexcrichton's example:

#![allow(dead_code)]
const A: &'static [usize] = &[];
const B: usize = A[1];

fn main() {}

It now compiles successfully without any complaints.

Only once you attempt to use the invalid value B will rustc encounter a problem.

#![allow(dead_code)]
const A: &'static [usize] = &[];
const B: usize = A[1];

fn main() {
    println!("B={}", B);
}

<anon>:3:18: 3:22 error: const index-expr is out of bounds
<anon>:3 const B: usize = A[1];
                          ^~~~
rustc: /home/rustbuild/src/rust-buildbot/slave/nightly-dist-rustc-linux/build/src/llvm/lib/IR/Constants.cpp:2174: static llvm::Constant* llvm::ConstantExpr::getExtractValue(llvm::Constant*, llvm::ArrayRef<unsigned int>, llvm::Type*): Assertion `ReqTy && "extractvalue indices invalid!"' failed.
Aborted (core dumped)
playpen: application terminated with error code 134

[steveklabnik]

error: cannot refer to other statics by value, use the address-of operator or a constant instead

Yup, seems good.

[pnkfelix]

wait, @JustAPerson 's example does not seem okay to me. Reopening.

[tamird]

I think this is now fixed for real:

$ cat foo.rs
#![allow(dead_code)]
const A: &'static [usize] = &[];
const B: usize = A[1];

fn main() {
    println!("B={}", B);
}
$ rustc foo.rs
foo.rs:3:18: 3:22 error: const index-expr is out of bounds
foo.rs:3 const B: usize = A[1];
                          ^~~~
error: aborting due to previous error
$ rustc --version
rustc 1.0.0-dev (a691f1eef 2015-04-15) (built 2015-04-15)