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Allow specifying multiple bounds for same associated item (remove E0719) #143146

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66ab8f8
Remove E0719
Jules-Bertholet Jun 28, 2025
dae4341
Fix CI
Jules-Bertholet Jun 28, 2025
ba6e138
Add more tests
Jules-Bertholet Jun 28, 2025
5abbb55
Add tests and tweak error message for `dyn`
Jules-Bertholet Jun 29, 2025
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4 changes: 3 additions & 1 deletion compiler/rustc_error_codes/src/error_codes/E0719.md
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@fmease fmease Jun 28, 2025 •
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(commenting on a random file so it's threaded)

Thanks for incorporating my suggestion about adding more tests!

What are the interactions with trait object types? It's possible that lower_trait_object_ty still ends up rejecting such dyn-Traits (e.g., dyn Trait<X = i32, X = u64>) due to its intricate and delicate logic but I might not be right. For example, we currently reject:

#![feature(trait_alias)]
trait Iter = Iterator<Item = ()>;
fn f(_: Box<dyn Iter<Item = u64>>) {} //~ ERROR conflicting associated type bounds for `Item` when expanding trait alias

I'm curious about whether that gets triggered for sth. like dyn Trait<X = i32, X = u64> 🤔 If not, I wonder if we should then also lift this trait alias restriction?

I don't know the soundness implications of allowing these equality predicates in trait object types since I haven't spent much thought on it. In my mind, a trait object type with "impossible" existential projection predicates should be fine, it would just be impossible to construct a value of it. Anyhow, I'm not qualified enough to draw a proper conclusion.

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Good catch! It looks like we do trigger that error. I added tests, and tweaked the error message to be more generic.

Interestingly, it seems that if you do:

trait Sup {
    type Assoc;
}

trait Sub: Sup<Assoc = u32> {}

type Foo = dyn Sub<Assoc = i32>;

Then no error is emitted, but Foo does not implement Sub.

Original file line number Diff line number Diff line change
@@ -1,8 +1,10 @@
#### Note: this error code is no longer emitted by the compiler.

An associated type value was specified more than once.

Erroneous code example:

```compile_fail,E0719
```
trait FooTrait {}
trait BarTrait {}

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2 changes: 1 addition & 1 deletion compiler/rustc_error_codes/src/lib.rs
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Original file line number Diff line number Diff line change
Expand Up @@ -464,7 +464,7 @@ E0716: 0716,
E0711: 0711,
E0717: 0717,
E0718: 0718,
E0719: 0719,
E0719: 0719, // REMOVED: no longer an error
E0720: 0720,
E0722: 0722,
E0724: 0724,
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5 changes: 0 additions & 5 deletions compiler/rustc_hir_analysis/messages.ftl
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Original file line number Diff line number Diff line change
Expand Up @@ -600,11 +600,6 @@ hir_analysis_unused_generic_parameter_ty_alias_help =

hir_analysis_useless_impl_item = this item cannot be used as its where bounds are not satisfied for the `Self` type

hir_analysis_value_of_associated_struct_already_specified =
the value of the associated type `{$item_name}` in trait `{$def_path}` is already specified
.label = re-bound here
.previous_bound_label = `{$item_name}` bound here first

hir_analysis_variadic_function_compatible_convention = C-variadic functions with the {$convention} calling convention are not supported
.label = C-variadic function must have a compatible calling convention

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12 changes: 0 additions & 12 deletions compiler/rustc_hir_analysis/src/errors.rs
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Original file line number Diff line number Diff line change
Expand Up @@ -401,18 +401,6 @@ pub(crate) struct TypeofReservedKeywordUsed<'tcx> {
pub opt_sugg: Option<(Span, Applicability)>,
}

#[derive(Diagnostic)]
#[diag(hir_analysis_value_of_associated_struct_already_specified, code = E0719)]
pub(crate) struct ValueOfAssociatedStructAlreadySpecified {
#[primary_span]
#[label]
pub span: Span,
#[label(hir_analysis_previous_bound_label)]
pub prev_span: Span,
pub item_name: Ident,
pub def_path: String,
}

#[derive(Diagnostic)]
#[diag(hir_analysis_unconstrained_opaque_type)]
#[note]
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17 changes: 2 additions & 15 deletions compiler/rustc_hir_analysis/src/hir_ty_lowering/bounds.rs
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Original file line number Diff line number Diff line change
@@ -1,6 +1,6 @@
use std::ops::ControlFlow;

use rustc_data_structures::fx::{FxIndexMap, FxIndexSet};
use rustc_data_structures::fx::FxIndexSet;
use rustc_errors::codes::*;
use rustc_errors::struct_span_code_err;
use rustc_hir as hir;
Expand Down Expand Up @@ -524,14 +524,13 @@ impl<'tcx> dyn HirTyLowerer<'tcx> + '_ {
/// the `trait_ref` here will be `for<'a> T: Iterator`.
/// The `constraint` data however is from *inside* the binder
/// (e.g., `&'a u32`) and hence may reference bound regions.
#[instrument(level = "debug", skip(self, bounds, duplicates, path_span))]
#[instrument(level = "debug", skip(self, bounds, path_span))]
pub(super) fn lower_assoc_item_constraint(
&self,
hir_ref_id: hir::HirId,
trait_ref: ty::PolyTraitRef<'tcx>,
constraint: &hir::AssocItemConstraint<'tcx>,
bounds: &mut Vec<(ty::Clause<'tcx>, Span)>,
duplicates: &mut FxIndexMap<DefId, Span>,
path_span: Span,
predicate_filter: PredicateFilter,
) -> Result<(), ErrorGuaranteed> {
Expand Down Expand Up @@ -587,18 +586,6 @@ impl<'tcx> dyn HirTyLowerer<'tcx> + '_ {
)
.expect("failed to find associated item");

duplicates
.entry(assoc_item.def_id)
.and_modify(|prev_span| {
self.dcx().emit_err(errors::ValueOfAssociatedStructAlreadySpecified {
span: constraint.span,
prev_span: *prev_span,
item_name: constraint.ident,
def_path: tcx.def_path_str(assoc_item.container_id(tcx)),
});
})
.or_insert(constraint.span);

let projection_term = if let ty::AssocTag::Fn = assoc_tag {
let bound_vars = tcx.late_bound_vars(constraint.hir_id);
ty::Binder::bind_with_vars(
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5 changes: 1 addition & 4 deletions compiler/rustc_hir_analysis/src/hir_ty_lowering/dyn_compatibility.rs
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Original file line number Diff line number Diff line change
Expand Up @@ -151,10 +151,7 @@ impl<'tcx> dyn HirTyLowerer<'tcx> + '_ {
self.dcx()
.struct_span_err(
span,
format!(
"conflicting associated type bounds for `{item}` when \
expanding trait alias"
),
format!("conflicting associated type bounds for `{item}`"),
)
.with_span_label(
old_proj_span,
Expand Down
5 changes: 1 addition & 4 deletions compiler/rustc_hir_analysis/src/hir_ty_lowering/mod.rs
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Original file line number Diff line number Diff line change
Expand Up @@ -24,7 +24,7 @@ use std::assert_matches::assert_matches;
use std::slice;

use rustc_ast::TraitObjectSyntax;
use rustc_data_structures::fx::{FxHashSet, FxIndexMap, FxIndexSet};
use rustc_data_structures::fx::{FxHashSet, FxIndexSet};
use rustc_errors::codes::*;
use rustc_errors::{
Applicability, Diag, DiagCtxtHandle, ErrorGuaranteed, FatalError, struct_span_code_err,
Expand Down Expand Up @@ -801,7 +801,6 @@ impl<'tcx> dyn HirTyLowerer<'tcx> + '_ {
poly_trait_ref,
constraint,
&mut Default::default(),
&mut Default::default(),
constraint.span,
predicate_filter,
);
Expand Down Expand Up @@ -907,7 +906,6 @@ impl<'tcx> dyn HirTyLowerer<'tcx> + '_ {
}
}

let mut dup_constraints = FxIndexMap::default();
for constraint in trait_segment.args().constraints {
// Don't register any associated item constraints for negative bounds,
// since we should have emitted an error for them earlier, and they
Expand All @@ -926,7 +924,6 @@ impl<'tcx> dyn HirTyLowerer<'tcx> + '_ {
poly_trait_ref,
constraint,
bounds,
&mut dup_constraints,
constraint.span,
predicate_filter,
);
Expand Down
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