Dangling raw pointers in constants will ICE in codegen

[oli-obk]

cc @RalfJung

#![feature(const_let)]

const FOO: *const u32 = {
    let x = 42;
    &x
};

fn main() {
    let x = FOO;
}

(Playground)

Errors:

   Compiling playground v0.0.1 (/playground)
warning: unused variable: `x`
  --> src/main.rs:10:9
   |
10 |     let x = FOO;
   |         ^ help: consider using `_x` instead
   |
   = note: #[warn(unused_variables)] on by default

error: internal compiler error: librustc_mir/monomorphize/collector.rs:1174: alloc id without corresponding allocation: 3

thread 'main' panicked at 'Box<Any>', librustc_errors/lib.rs:600:9
note: Run with `RUST_BACKTRACE=1` for a backtrace.
error: aborting due to previous error


note: the compiler unexpectedly panicked. this is a bug.

note: we would appreciate a bug report: https://github.com/rust-lang/rust/blob/master/CONTRIBUTING.md#bug-reports

note: rustc 1.31.0-nightly (f99911a4a 2018-10-23) running on x86_64-unknown-linux-gnu

note: compiler flags: -C codegen-units=1 -C debuginfo=2 --crate-type bin

note: some of the compiler flags provided by cargo are hidden

error: Could not compile `playground`.

To learn more, run the command again with --verbose.

[oli-obk]

Fix instructions:

The if in

rust/src/librustc_mir/interpret/validity.rs

Line 211 in 8ec22e7

if !ty.is_unsafe_ptr() {

doesn't handle raw pointers at all, because the assumption is that we can't know anything about raw pointers.

In fact we can know one thing, and that is whether the raw pointer is dangling.

So, expand the condition to handle the ty.is_unsafe_ptr() case and call self.memory.get(place.ptr). If that results in an Ok, then we're good, but if you get an Err, it can essentially only mean that we got a dangling pointer. So you can call validation_failure! with an appropriate error.

Remember to also add a test in src/test/ui/consts to make sure that this doesn't ICE the compiler anymore.

[RalfJung]

Why do you want to validate raw pointers not to dangle? That seems wrong, they are allowed to.

[RalfJung]

I thought you had #55262 which would bail out during interning when it sees a dangling relocation. That didn't land yet, so I am not surprised we have this ICE.