Duplicate errors for overflowing division / remainder

@oli-obk

When there is an overflowing division or remainder operation that const_prop can detect, the error is duplicated in release mode only:

#![deny(const_err)]

use std::i32;

fn main() {
    let _ = i32::MIN / -1;
}

shows

error: attempt to divide with overflow
 --> src/main.rs:6:13
  |
6 |     let _ = i32::MIN / -1;
  |             ^^^^^^^^^^^^^
  |
note: the lint level is defined here
 --> src/main.rs:1:9
  |
1 | #![deny(const_err)]
  |         ^^^^^^^^^

error: this expression will panic at runtime
 --> src/main.rs:6:13
  |
6 |     let _ = i32::MIN / -1;
  |             ^^^^^^^^^^^^^ attempt to divide with overflow

This is because !overflow_check is true in release mode but divison and remainder still get overflow checks.

(This test case already covers the problem, so a fix does not need a new test case, it just needs to adjust the existing test case to no longer expect two ERROR per line.)

Cc @oli-obk @wesleywiser

Basically the same issue also leads to duplicate errors for Neg, though that requires associated consts to actually trigger:

use std::i32;

pub trait Foo {
    const N: i32;
}

impl<T: Foo> Foo for Vec<T> {
    const N: i32 = -i32::MIN + T::N;
}

shows

error: attempt to negate with overflow
 --> src/lib.rs:8:20
  |
8 |     const N: i32 = -i32::MIN + T::N;
  |                    ^^^^^^^^^
  |
  = note: `#[deny(const_err)]` on by default

error: this expression will panic at runtime
 --> src/lib.rs:8:20
  |
8 |     const N: i32 = -i32::MIN + T::N;
  |                    ^^^^^^^^^ attempt to negate with overflow

Here, the cause is that even in release builds, const bodies use checked arithmetic. (The assoc const is needed because otherwise the const body just gets evaluated, not analyzed statically by const-prop.)

Rollup merge of rust-lang#69185 - RalfJung:const-prop-lints, r=oli-obk

Extend rust-lang#69020 test to include reversed order.

Move rust-lang#69020 test to ui/associated-const + rebless.

Rollup merge of rust-lang#70696 - jumbatm:extend-issue-69020-test, r=…

Auto merge of rust-lang#70734 - Dylan-DPC:rollup-xmncatq, r=Dylan-DPC

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Duplicate errors for overflowing division / remainder #69020

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Duplicate errors for overflowing division / remainder #69020

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RalfJung commented on Feb 10, 2020

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