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When a trait isn't implemented, but another similar impl is found, point at it #145640

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fa6e2ba
When a trait isn't implemented, but another similar impl is found, po…
estebank Aug 20, 2025
3f07cda
When more than a single `impl` and less than 4 could apply, point at …
estebank Aug 20, 2025
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7 changes: 5 additions & 2 deletions compiler/rustc_errors/src/emitter.rs
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Original file line number Diff line number Diff line change
Expand Up @@ -1526,16 +1526,17 @@ impl HumanEmitter {
label_width += 2;
}
let mut line = 0;
let mut pad = false;
for (text, style) in msgs.iter() {
let text =
self.translator.translate_message(text, args).map_err(Report::new).unwrap();
// Account for newlines to align output to its label.
for text in normalize_whitespace(&text).lines() {
for text in normalize_whitespace(&text).split('\n') {
buffer.append(
line,
&format!(
"{}{}",
if line == 0 { String::new() } else { " ".repeat(label_width) },
if pad { " ".repeat(label_width) } else { String::new() },
text
),
match style {
Expand All @@ -1544,7 +1545,9 @@ impl HumanEmitter {
},
);
line += 1;
pad = true;
}
pad = false;
// We add lines above, but if the last line has no explicit newline (which would
// yield an empty line), then we revert one line up to continue with the next
// styled text chunk on the same line as the last one from the prior one. Otherwise
Expand Down
123 changes: 77 additions & 46 deletions compiler/rustc_trait_selection/src/error_reporting/traits/fulfillment_errors.rs
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Original file line number Diff line number Diff line change
Expand Up @@ -1862,13 +1862,13 @@ impl<'a, 'tcx> TypeErrCtxt<'a, 'tcx> {
// ignore `do_not_recommend` items
.filter(|def_id| !self.tcx.do_not_recommend_impl(*def_id))
// Ignore automatically derived impls and `!Trait` impls.
.filter_map(|def_id| self.tcx.impl_trait_header(def_id))
.filter_map(|header| {
.filter_map(|def_id| self.tcx.impl_trait_header(def_id).map(|h| (h, def_id)))
.filter_map(|(header, def_id)| {
(header.polarity != ty::ImplPolarity::Negative
|| self.tcx.is_automatically_derived(def_id))
.then(|| header.trait_ref.instantiate_identity())
.then(|| (header.trait_ref.instantiate_identity(), def_id))
})
.filter(|trait_ref| {
.filter(|(trait_ref, _)| {
let self_ty = trait_ref.self_ty();
// Avoid mentioning type parameters.
if let ty::Param(_) = self_ty.kind() {
Expand All @@ -1886,7 +1886,7 @@ impl<'a, 'tcx> TypeErrCtxt<'a, 'tcx> {
})
.collect();

impl_candidates.sort_by_key(|tr| tr.to_string());
impl_candidates.sort_by_key(|(tr, _)| tr.to_string());
impl_candidates.dedup();
impl_candidates
};
Expand Down Expand Up @@ -1914,7 +1914,7 @@ impl<'a, 'tcx> TypeErrCtxt<'a, 'tcx> {
let candidates = if impl_candidates.is_empty() {
alternative_candidates(trait_def_id)
} else {
impl_candidates.into_iter().map(|cand| cand.trait_ref).collect()
impl_candidates.into_iter().map(|cand| (cand.trait_ref, cand.impl_def_id)).collect()
};
let mut span: MultiSpan = self.tcx.def_span(trait_def_id).into();
span.push_span_label(self.tcx.def_span(trait_def_id), "this is the required trait");
Expand Down Expand Up @@ -1946,7 +1946,7 @@ impl<'a, 'tcx> TypeErrCtxt<'a, 'tcx> {
self.tcx.def_span(found_type),
"this type doesn't implement the required trait",
);
for trait_ref in candidates {
for (trait_ref, _) in candidates {
if let ty::Adt(def, _) = trait_ref.self_ty().peel_refs().kind()
&& let candidate_def_id = def.did()
&& let Some(name) = self.tcx.opt_item_name(candidate_def_id)
Expand Down Expand Up @@ -2133,7 +2133,7 @@ impl<'a, 'tcx> TypeErrCtxt<'a, 'tcx> {
msg.extend(types.1.0);
msg.push(StringPart::normal("`"));
}
err.highlighted_help(msg);
err.highlighted_span_help(self.tcx.def_span(single.impl_def_id), msg);

if let [TypeError::Sorts(exp_found)] = &terrs[..] {
let exp_found = self.resolve_vars_if_possible(*exp_found);
Expand All @@ -2159,12 +2159,12 @@ impl<'a, 'tcx> TypeErrCtxt<'a, 'tcx> {
}

let other = if other { "other " } else { "" };
let report = |mut candidates: Vec<TraitRef<'tcx>>, err: &mut Diag<'_>| {
candidates.retain(|tr| !tr.references_error());
let report = |mut candidates: Vec<(TraitRef<'tcx>, DefId)>, err: &mut Diag<'_>| {
candidates.retain(|(tr, _)| !tr.references_error());
if candidates.is_empty() {
return false;
}
if let &[cand] = &candidates[..] {
if let &[(cand, def_id)] = &candidates[..] {
if self.tcx.is_diagnostic_item(sym::FromResidual, cand.def_id)
&& !self.tcx.features().enabled(sym::try_trait_v2)
{
Expand All @@ -2180,56 +2180,87 @@ impl<'a, 'tcx> TypeErrCtxt<'a, 'tcx> {
};
let trait_ = self.tcx.short_string(cand.print_trait_sugared(), err.long_ty_path());
let self_ty = self.tcx.short_string(cand.self_ty(), err.long_ty_path());
err.highlighted_help(vec![
StringPart::normal(format!("the trait `{trait_}` ",)),
StringPart::highlighted("is"),
StringPart::normal(desc),
StringPart::highlighted(self_ty),
StringPart::normal("`"),
StringPart::normal(mention_castable),
]);
err.highlighted_span_help(
self.tcx.def_span(def_id),
vec![
StringPart::normal(format!("the trait `{trait_}` ",)),
StringPart::highlighted("is"),
StringPart::normal(desc),
StringPart::highlighted(self_ty),
StringPart::normal("`"),
StringPart::normal(mention_castable),
],
);
return true;
}
let trait_ref = TraitRef::identity(self.tcx, candidates[0].def_id);
let trait_ref = TraitRef::identity(self.tcx, candidates[0].0.def_id);
// Check if the trait is the same in all cases. If so, we'll only show the type.
let mut traits: Vec<_> =
candidates.iter().map(|c| c.print_only_trait_path().to_string()).collect();
candidates.iter().map(|(c, _)| c.print_only_trait_path().to_string()).collect();
traits.sort();
traits.dedup();
// FIXME: this could use a better heuristic, like just checking
// that args[1..] is the same.
let all_traits_equal = traits.len() == 1;

let candidates: Vec<String> = candidates
.into_iter()
.map(|c| {
if all_traits_equal {
format!("\n {}", self.tcx.short_string(c.self_ty(), err.long_ty_path()))
} else {
format!(
"\n `{}` implements `{}`",
self.tcx.short_string(c.self_ty(), err.long_ty_path()),
self.tcx.short_string(c.print_only_trait_path(), err.long_ty_path()),
)
}
})
.collect();

let end = if candidates.len() <= 9 || self.tcx.sess.opts.verbose {
candidates.len()
} else {
8
};
err.help(format!(
"the following {other}types implement trait `{}`:{}{}",
trait_ref.print_trait_sugared(),
candidates[..end].join(""),
if candidates.len() > 9 && !self.tcx.sess.opts.verbose {
format!("\nand {} others", candidates.len() - 8)
} else {
String::new()
if candidates.len() < 5 {
let spans: Vec<_> =
candidates.iter().map(|(_, def_id)| self.tcx.def_span(def_id)).collect();
let mut span: MultiSpan = spans.into();
for (c, def_id) in &candidates {
let msg = if all_traits_equal {
format!("`{}`", self.tcx.short_string(c.self_ty(), err.long_ty_path()))
} else {
format!(
"`{}` implements `{}`",
self.tcx.short_string(c.self_ty(), err.long_ty_path()),
self.tcx.short_string(c.print_only_trait_path(), err.long_ty_path()),
)
};
span.push_span_label(self.tcx.def_span(def_id), msg);
}
));
err.span_help(
span,
format!(
"the following {other}types implement trait `{}`",
trait_ref.print_trait_sugared(),
),
);
} else {
let candidate_names: Vec<String> = candidates
.iter()
.map(|(c, _)| {
if all_traits_equal {
format!(
"\n {}",
self.tcx.short_string(c.self_ty(), err.long_ty_path())
)
} else {
format!(
"\n `{}` implements `{}`",
self.tcx.short_string(c.self_ty(), err.long_ty_path()),
self.tcx
.short_string(c.print_only_trait_path(), err.long_ty_path()),
)
}
})
.collect();
err.help(format!(
"the following {other}types implement trait `{}`:{}{}",
trait_ref.print_trait_sugared(),
candidate_names[..end].join(""),
if candidates.len() > 9 && !self.tcx.sess.opts.verbose {
format!("\nand {} others", candidates.len() - 8)
} else {
String::new()
}
));
}
true
};

Expand Down Expand Up @@ -2279,7 +2310,7 @@ impl<'a, 'tcx> TypeErrCtxt<'a, 'tcx> {
.collect();
impl_candidates.sort_by_key(|cand| (cand.similarity, cand.trait_ref.to_string()));
let mut impl_candidates: Vec<_> =
impl_candidates.into_iter().map(|cand| cand.trait_ref).collect();
impl_candidates.into_iter().map(|cand| (cand.trait_ref, cand.impl_def_id)).collect();
impl_candidates.dedup();

report(impl_candidates, err)
Expand Down
10 changes: 7 additions & 3 deletions tests/ui-fulldeps/rustc-dev-remap.only-remap.stderr
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Original file line number Diff line number Diff line change
Expand Up @@ -9,9 +9,13 @@ help: the trait `VisitorResult` is not implemented for `NotAValidResultType`
|
LL | struct NotAValidResultType;
| ^^^^^^^^^^^^^^^^^^^^^^^^^^
= help: the following other types implement trait `VisitorResult`:
()
ControlFlow<T>
help: the following other types implement trait `VisitorResult`
--> /rustc-dev/xyz/compiler/rustc_ast_ir/src/visit.rs:LL:COL
|
= note: `()`
::: /rustc-dev/xyz/compiler/rustc_ast_ir/src/visit.rs:LL:COL
|
= note: `ControlFlow<T>`
note: required by a bound in `rustc_ast::visit::Visitor::Result`
--> /rustc-dev/xyz/compiler/rustc_ast/src/visit.rs:LL:COL
= note: this error originates in the macro `common_visitor_and_walkers` (in Nightly builds, run with -Z macro-backtrace for more info)
Expand Down
11 changes: 8 additions & 3 deletions tests/ui-fulldeps/rustc-dev-remap.remap-unremap.stderr
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Original file line number Diff line number Diff line change
Expand Up @@ -9,9 +9,14 @@ help: the trait `VisitorResult` is not implemented for `NotAValidResultType`
|
LL | struct NotAValidResultType;
| ^^^^^^^^^^^^^^^^^^^^^^^^^^
= help: the following other types implement trait `VisitorResult`:
()
ControlFlow<T>
help: the following other types implement trait `VisitorResult`
--> $COMPILER_DIR_REAL/rustc_ast_ir/src/visit.rs:LL:COL
|
LL | impl VisitorResult for () {
| ^^^^^^^^^^^^^^^^^^^^^^^^^ `()`
...
LL | impl<T> VisitorResult for ControlFlow<T> {
| ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^ `ControlFlow<T>`
note: required by a bound in `rustc_ast::visit::Visitor::Result`
--> $COMPILER_DIR_REAL/rustc_ast/src/visit.rs:LL:COL
|
Expand Down
12 changes: 8 additions & 4 deletions tests/ui/allocator/not-an-allocator.stderr
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Original file line number Diff line number Diff line change
Expand Up @@ -6,7 +6,8 @@ LL | #[global_allocator]
LL | static A: usize = 0;
| ^^^^^ the trait `GlobalAlloc` is not implemented for `usize`
|
= help: the trait `GlobalAlloc` is implemented for `System`
help: the trait `GlobalAlloc` is implemented for `System`
--> $SRC_DIR/std/src/sys/alloc/unix.rs:LL:COL

error[E0277]: the trait bound `usize: GlobalAlloc` is not satisfied
--> $DIR/not-an-allocator.rs:2:11
Expand All @@ -16,7 +17,8 @@ LL | #[global_allocator]
LL | static A: usize = 0;
| ^^^^^ the trait `GlobalAlloc` is not implemented for `usize`
|
= help: the trait `GlobalAlloc` is implemented for `System`
help: the trait `GlobalAlloc` is implemented for `System`
--> $SRC_DIR/std/src/sys/alloc/unix.rs:LL:COL
= note: duplicate diagnostic emitted due to `-Z deduplicate-diagnostics=no`

error[E0277]: the trait bound `usize: GlobalAlloc` is not satisfied
Expand All @@ -27,7 +29,8 @@ LL | #[global_allocator]
LL | static A: usize = 0;
| ^^^^^ the trait `GlobalAlloc` is not implemented for `usize`
|
= help: the trait `GlobalAlloc` is implemented for `System`
help: the trait `GlobalAlloc` is implemented for `System`
--> $SRC_DIR/std/src/sys/alloc/unix.rs:LL:COL
= note: duplicate diagnostic emitted due to `-Z deduplicate-diagnostics=no`

error[E0277]: the trait bound `usize: GlobalAlloc` is not satisfied
Expand All @@ -38,7 +41,8 @@ LL | #[global_allocator]
LL | static A: usize = 0;
| ^^^^^ the trait `GlobalAlloc` is not implemented for `usize`
|
= help: the trait `GlobalAlloc` is implemented for `System`
help: the trait `GlobalAlloc` is implemented for `System`
--> $SRC_DIR/std/src/sys/alloc/unix.rs:LL:COL
= note: duplicate diagnostic emitted due to `-Z deduplicate-diagnostics=no`

error: aborting due to 4 previous errors
Expand Down
24 changes: 20 additions & 4 deletions tests/ui/associated-types/associated-types-path-2.stderr
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Expand Up @@ -25,7 +25,11 @@ LL | f1(2u32, 4u32);
| |
| required by a bound introduced by this call
|
= help: the trait `Foo` is implemented for `i32`
help: the trait `Foo` is implemented for `i32`
--> $DIR/associated-types-path-2.rs:11:1
|
LL | impl Foo for i32 {
| ^^^^^^^^^^^^^^^^
note: required by a bound in `f1`
--> $DIR/associated-types-path-2.rs:15:14
|
Expand All @@ -38,7 +42,11 @@ error[E0277]: the trait bound `u32: Foo` is not satisfied
LL | f1(2u32, 4u32);
| ^^^^ the trait `Foo` is not implemented for `u32`
|
= help: the trait `Foo` is implemented for `i32`
help: the trait `Foo` is implemented for `i32`
--> $DIR/associated-types-path-2.rs:11:1
|
LL | impl Foo for i32 {
| ^^^^^^^^^^^^^^^^

error[E0277]: the trait bound `u32: Foo` is not satisfied
--> $DIR/associated-types-path-2.rs:37:8
Expand All @@ -48,7 +56,11 @@ LL | f1(2u32, 4i32);
| |
| required by a bound introduced by this call
|
= help: the trait `Foo` is implemented for `i32`
help: the trait `Foo` is implemented for `i32`
--> $DIR/associated-types-path-2.rs:11:1
|
LL | impl Foo for i32 {
| ^^^^^^^^^^^^^^^^
note: required by a bound in `f1`
--> $DIR/associated-types-path-2.rs:15:14
|
Expand All @@ -61,7 +73,11 @@ error[E0277]: the trait bound `u32: Foo` is not satisfied
LL | f1(2u32, 4i32);
| ^^^^ the trait `Foo` is not implemented for `u32`
|
= help: the trait `Foo` is implemented for `i32`
help: the trait `Foo` is implemented for `i32`
--> $DIR/associated-types-path-2.rs:11:1
|
LL | impl Foo for i32 {
| ^^^^^^^^^^^^^^^^

error[E0308]: mismatched types
--> $DIR/associated-types-path-2.rs:43:18
Expand Down
6 changes: 4 additions & 2 deletions tests/ui/associated-types/hr-associated-type-bound-1.stderr
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Expand Up @@ -4,7 +4,8 @@ error[E0277]: the trait bound `str: Clone` is not satisfied
LL | type U = str;
| ^^^ the trait `Clone` is not implemented for `str`
|
= help: the trait `Clone` is implemented for `String`
help: the trait `Clone` is implemented for `String`
--> $SRC_DIR/alloc/src/string.rs:LL:COL
note: required by a bound in `X`
--> $DIR/hr-associated-type-bound-1.rs:3:33
|
Expand All @@ -20,7 +21,8 @@ error[E0277]: the trait bound `str: Clone` is not satisfied
LL | 1i32.f("abc");
| ^ the trait `Clone` is not implemented for `str`
|
= help: the trait `Clone` is implemented for `String`
help: the trait `Clone` is implemented for `String`
--> $SRC_DIR/alloc/src/string.rs:LL:COL
note: required by a bound in `X::f`
--> $DIR/hr-associated-type-bound-1.rs:3:33
|
Expand Down
6 changes: 4 additions & 2 deletions tests/ui/associated-types/hr-associated-type-bound-param-1.stderr
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Original file line number Diff line number Diff line change
Expand Up @@ -4,7 +4,8 @@ error[E0277]: the trait bound `str: Clone` is not satisfied
LL | type V = str;
| ^^^ the trait `Clone` is not implemented for `str`
|
= help: the trait `Clone` is implemented for `String`
help: the trait `Clone` is implemented for `String`
--> $SRC_DIR/alloc/src/string.rs:LL:COL
note: required by a bound in `Y`
--> $DIR/hr-associated-type-bound-param-1.rs:4:36
|
Expand All @@ -20,7 +21,8 @@ error[E0277]: the trait bound `str: Clone` is not satisfied
LL | 1u8.g("abc");
| ^ the trait `Clone` is not implemented for `str`
|
= help: the trait `Clone` is implemented for `String`
help: the trait `Clone` is implemented for `String`
--> $SRC_DIR/alloc/src/string.rs:LL:COL
note: required by a bound in `Y::g`
--> $DIR/hr-associated-type-bound-param-1.rs:4:36
|
Expand Down
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