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A trait's local impls are trivially coherent if there are no impls. #120834

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Feb 12, 2024
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A trait's local impls are trivially coherent if there are no impls. #120834

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4 changes: 3 additions & 1 deletion compiler/rustc_hir_analysis/src/coherence/mod.rs
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Original file line number Diff line number Diff line change
Expand Up @@ -120,11 +120,13 @@ pub fn provide(providers: &mut Providers) {
}

fn coherent_trait(tcx: TyCtxt<'_>, def_id: DefId) -> Result<(), ErrorGuaranteed> {
// If there are no impls for the trait, then "all impls" are trivially coherent and we won't check anything
// anyway. Thus we bail out even before the specialization graph, avoiding the dep_graph edge.
let Some(impls) = tcx.all_local_trait_impls(()).get(&def_id) else { return Ok(()) };
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please explain why we early return here and why this is sound

// Trigger building the specialization graph for the trait. This will detect and report any
// overlap errors.
let mut res = tcx.ensure().specialization_graph_of(def_id);

let impls = tcx.hir().trait_impls(def_id);
for &impl_def_id in impls {
let trait_ref = tcx.impl_trait_ref(impl_def_id).unwrap().instantiate_identity();

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