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Rollup merge of #94249 - compiler-errors:better-copy-errors, r=davidtwco

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Better errors when a Copy impl on a Struct is not self-consistent

As discovered in a Zulip thread with `@nnethercote` and `@Mark-Simulacrum,` it's not immediately obvious why a field on an ADT doesn't implement `Copy`.  This PR attempts to give slightly more detailed information by spinning up a fulfillment context to try to dig down and discover transitive fulfillment errors that cause `is_copy_modulo_regions` to fail on a ADT field.

The error message still kinda sucks, but should only show up in the case that an existing error message was totally missing... so I think it's a good compromise for now?
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Matthias Krüger 2022-03-23 22:13:22 +01:00 • committed by GitHub
commit af19a50a26
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5 changed files with 100 additions and 4 deletions
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4
compiler/rustc_trait_selection/src/traits/misc.rs
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@ -11,7 +11,7 @@ use crate::traits::error_reporting::InferCtxtExt;
#[derive(Clone)]
pub enum CopyImplementationError<'tcx> {
InfrigingFields(Vec<&'tcx ty::FieldDef>),
InfrigingFields(Vec<(&'tcx ty::FieldDef, Ty<'tcx>)>),
NotAnAdt,
HasDestructor,
}
@ -67,7 +67,7 @@ pub fn can_type_implement_copy<'tcx>(
match traits::fully_normalize(&infcx, ctx, cause, param_env, ty) {
Ok(ty) => {
if !infcx.type_is_copy_modulo_regions(param_env, ty, span) {
infringing.push(field);
infringing.push((field, ty));
}
}
Err(errors) => {