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Rollup merge of #137728 - Darksonn:no-tuple-unsize, r=oli-obk

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Remove unsizing coercions for tuples

See https://github.com/rust-lang/rust/issues/42877#issuecomment-2686010847 and below comments for justification.

Tracking issue: #42877
Fixes: #135217
This commit is contained in:
许杰友 Jieyou Xu (Joe) 2025-03-05 21:46:44 +08:00 • committed by GitHub
commit 257b4947ed
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47 changed files with 56 additions and 878 deletions
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49
compiler/rustc_next_trait_solver/src/solve/trait_goals.rs
View file

@ -786,13 +786,6 @@ where
)
}
// `(A, B, T)` -> `(A, B, U)` where `T: Unsize<U>`
(ty::Tuple(a_tys), ty::Tuple(b_tys))
if a_tys.len() == b_tys.len() && !a_tys.is_empty() =>
{
result_to_single(ecx.consider_builtin_tuple_unsize(goal, a_tys, b_tys))
}
_ => vec![],
}
})
@ -1084,48 +1077,6 @@ where
.enter(|ecx| ecx.evaluate_added_goals_and_make_canonical_response(Certainty::Yes))
}
/// We generate the following builtin impl for tuples of all sizes.
///
/// This impl is still unstable and we emit a feature error when it
/// when it is used by a coercion.
/// ```ignore (builtin impl example)
/// impl<T: ?Sized, U: ?Sized, V: ?Sized> Unsize<(T, V)> for (T, U)
/// where
/// U: Unsize<V>,
/// {}
/// ```
fn consider_builtin_tuple_unsize(
&mut self,
goal: Goal<I, (I::Ty, I::Ty)>,
a_tys: I::Tys,
b_tys: I::Tys,
) -> Result<Candidate<I>, NoSolution> {
let cx = self.cx();
let Goal { predicate: (_a_ty, b_ty), .. } = goal;
let (&a_last_ty, a_rest_tys) = a_tys.split_last().unwrap();
let b_last_ty = b_tys.last().unwrap();
// Instantiate just the tail field of B., and require that they're equal.
let unsized_a_ty = Ty::new_tup_from_iter(cx, a_rest_tys.iter().copied().chain([b_last_ty]));
self.eq(goal.param_env, unsized_a_ty, b_ty)?;
// Similar to ADTs, require that we can unsize the tail.
self.add_goal(
GoalSource::ImplWhereBound,
goal.with(
cx,
ty::TraitRef::new(
cx,
cx.require_lang_item(TraitSolverLangItem::Unsize),
[a_last_ty, b_last_ty],
),
),
);
self.probe_builtin_trait_candidate(BuiltinImplSource::TupleUnsizing)
.enter(|ecx| ecx.evaluate_added_goals_and_make_canonical_response(Certainty::Yes))
}
// Return `Some` if there is an impl (built-in or user provided) that may
// hold for the self type of the goal, which for coherence and soundness
// purposes must disqualify the built-in auto impl assembled by considering