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False positive of lint missing_debug_implementations if Debug is implemented for a free alias type that expands to the relevant type #157757
Description
#![feature(lazy_type_alias)]
#![deny(missing_debug_implementations)]
pub struct Local; //~ ERROR type does not implement `Debug`; consider adding `#[derive(Debug)]` or a manu
pub type Alias = Local;
impl std::fmt::Debug for Alias {
fn fmt(&self, _: &mut std::fmt::Formatter<'_>) -> std::fmt::Result { Ok(()) }
}For some reason missing_debug_implementations doesn't actually query the trait solver (!) (if the fast path doesn't apply), it literally does
let has_impl = cx
.tcx
.non_blanket_impls_for_ty(
debug,
cx.tcx.type_of(item.owner_id).instantiate_identity().skip_norm_wip(),
)
.next()
.is_some();but I don't think that function should be used for "correctness" (well, it's a lint), ideally it would only be used as a fast path (barring more specific use cases).
For comparison, lint missing_copy_implementations calls tcx.type_is_copy_modulo_regions() which has a fast path but then queries the trait solver.
A temporarily solution would be to call expand_free_alias_tys inside non_blanket_impls_for_ty and friends? I haven't thought about correctness yet, so not sure.
I don't think we should call expand_free_alias_tys in fast_reject::simplify_type; that ideal feels very iffy and it would probably trash perf.
Activity
[-]False positive for lint `missing_debug_implementations` if `Debug` is implemented for a free alias type that expands to the relevant type[/-][+]False positive of lint `missing_debug_implementations` if `Debug` is implemented for a free alias type that expands to the relevant type[/+]missing_debug_implementationsifDebugis implemented for an alias type (e.g., projection) that can get normalized to the relevant type #157758fmease commentedon Jun 11, 2026
Why does that have its own issue instead of being part of parent issue #157758?
Well, it's because this way it's easier to track the potential blockers for LTA_unchecked's stabilization:
Stable code that currently uses eager type aliases in this position would regress if LTA_unchecked became the default.
Rollup merge of rust-lang#157810 - mu001999-contrib:fix-157758, r=mat…
Rollup merge of rust-lang#157810 - mu001999-contrib:fix-157758, r=mat…
Unrolled build for #157810
Rollup merge of #157810 - mu001999-contrib:fix-157758, r=mati865