Some errors only show up with cargo build not cargo check

[oli-obk]

This is partially due to the fact that our analysis query

rust/src/librustc_interface/passes.rs

Line 775 in 537ccdf

fn analysis(tcx: TyCtxt<'_>, cnum: CrateNum) -> Result<()> {

does not invoke the optimized_mir query on all items in the current crate, and also because we don't run monomorphization when building bin crates.

• run optimized_mir query in analysis
  • this may cause slowdowns for cargo check because we'd be doing optimizations now
  • All const propagation lints are emitted in optimized_mir, but we may add more in the future.
• run monomorphization where possible.
  • Not sure how useful this is, it will only give us additional lints in very specific situations. (e.g. when we subtract something form an associated constant, we may get an overflow error depending on the value of the constant).
  • This will also be significantly more expensive than just running optimized_mir

cc @Centril

[RalfJung]

Or maybe it is a bad idea to emit lints during optimization, and we should move the lints from const-prop to elsewhere?

[ehuss]

This gets reported occasionally, like #49292 and #61330.

[oli-obk]

the reason the lints are part of const prop is to ensure we don't do the work twice, once for linting and once for the const prop optimization. I'm not sure how we could do that differently.

Also I was envisioning adding more lints to the optimization pipeline instead of the other way around.

[RalfJung]

Do we have a concrete example / test case demonstrating this?

[RalfJung]

@ehuss has an example of this: it passes with cargo check, but fails cargo build, latest nightly rustc 1.45.0-nightly (fa51f810e 2020-04-29).

fn main() {
    let a: u8 = 42;
    let b = ((a | 0x0F) - 1) >> 31 > 0;
    println!("{}", b);
}

https://play.rust-lang.org/?version=nightly&mode=debug&edition=2018&gist=4998a76829fb44657cd8ed814fb2a191

[RalfJung]

Closing as a duplicate of #49292 (for the diagnostics emitted in MIR passes) and #99682 (for errors during monomorphization).