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Point at unit structs on foreign crates in type errors when they are the pattern of a binding #153894

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Merged
rust-bors merged 2 commits into from
Mar 17, 2026
+134 −115
Merged

Point at unit structs on foreign crates in type errors when they are the pattern of a binding #153894

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39631fd
Point at unit structs on foreign crates in type errors when they are …
estebank Mar 15, 2026
408066f
Do not suggest introducing new binding when other suggestions are pre…
estebank Mar 15, 2026
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8 changes: 8 additions & 0 deletions compiler/rustc_errors/src/lib.rs
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Original file line number Diff line number Diff line change
Expand Up @@ -138,6 +138,14 @@ impl Suggestions {
Suggestions::Disabled => Vec::new(),
}
}

pub fn len(&self) -> usize {
match self {
Suggestions::Enabled(suggestions) => suggestions.len(),
Suggestions::Sealed(suggestions) => suggestions.len(),
Suggestions::Disabled => 0,
}
}
}

impl Default for Suggestions {
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124 changes: 66 additions & 58 deletions compiler/rustc_hir_typeck/src/pat.rs
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Expand Up @@ -1636,69 +1636,77 @@ impl<'a, 'tcx> FnCtxt<'a, 'tcx> {
span_bug!(pat_span, "unexpected resolution for path pattern: {resolved_pat:?}");
};

if let Some(span) = self.tcx.hir_res_span(pat_res) {
let span = match (self.tcx.hir_res_span(pat_res), res.opt_def_id()) {
(Some(span), _) => span,
(None, Some(def_id)) => self.tcx.def_span(def_id),
(None, None) => {
e.emit();
return;
}
};
if let [hir::PathSegment { ident, args: None, .. }] = segments
&& e.suggestions.len() == 0
{
e.span_label(span, format!("{} defined here", res.descr()));
if let [hir::PathSegment { ident, .. }] = segments {
e.span_label(
pat_span,
format!(
"`{}` is interpreted as {} {}, not a new binding",
ident,
res.article(),
res.descr(),
),
);
match self.tcx.parent_hir_node(hir_id) {
hir::Node::PatField(..) => {
e.span_label(
pat_span,
format!(
"`{}` is interpreted as {} {}, not a new binding",
ident,
res.article(),
res.descr(),
),
);
match self.tcx.parent_hir_node(hir_id) {
hir::Node::PatField(..) => {
e.span_suggestion_verbose(
ident.span.shrink_to_hi(),
"bind the struct field to a different name instead",
format!(": other_{}", ident.as_str().to_lowercase()),
Applicability::HasPlaceholders,
);
}
_ => {
let (type_def_id, item_def_id) = match resolved_pat.ty.kind() {
ty::Adt(def, _) => match res {
Res::Def(DefKind::Const { .. }, def_id) => {
(Some(def.did()), Some(def_id))
}
_ => (None, None),
},
_ => (None, None),
};

let is_range = matches!(
type_def_id.and_then(|id| self.tcx.as_lang_item(id)),
Some(
LangItem::Range
| LangItem::RangeFrom
| LangItem::RangeTo
| LangItem::RangeFull
| LangItem::RangeInclusiveStruct
| LangItem::RangeToInclusive,
)
);
if is_range {
if !self.maybe_suggest_range_literal(&mut e, item_def_id, *ident) {
let msg = "constants only support matching by type, \
if you meant to match against a range of values, \
consider using a range pattern like `min ..= max` in the match block";
e.note(msg);
}
} else {
let msg = "introduce a new binding instead";
let sugg = format!("other_{}", ident.as_str().to_lowercase());
e.span_suggestion_verbose(
ident.span.shrink_to_hi(),
"bind the struct field to a different name instead",
format!(": other_{}", ident.as_str().to_lowercase()),
ident.span,
msg,
sugg,
Applicability::HasPlaceholders,
);
}
_ => {
let (type_def_id, item_def_id) = match resolved_pat.ty.kind() {
ty::Adt(def, _) => match res {
Res::Def(DefKind::Const { .. }, def_id) => {
(Some(def.did()), Some(def_id))
}
_ => (None, None),
},
_ => (None, None),
};

let is_range = matches!(
type_def_id.and_then(|id| self.tcx.as_lang_item(id)),
Some(
LangItem::Range
| LangItem::RangeFrom
| LangItem::RangeTo
| LangItem::RangeFull
| LangItem::RangeInclusiveStruct
| LangItem::RangeToInclusive,
)
);
if is_range {
if !self.maybe_suggest_range_literal(&mut e, item_def_id, *ident) {
let msg = "constants only support matching by type, \
if you meant to match against a range of values, \
consider using a range pattern like `min ..= max` in the match block";
e.note(msg);
}
} else {
let msg = "introduce a new binding instead";
let sugg = format!("other_{}", ident.as_str().to_lowercase());
e.span_suggestion(
ident.span,
msg,
sugg,
Applicability::HasPlaceholders,
);
}
}
};
}
}
};
}
e.emit();
}
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6 changes: 5 additions & 1 deletion tests/ui/blind/blind-item-block-middle.stderr
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Original file line number Diff line number Diff line change
Expand Up @@ -9,7 +9,11 @@ LL | let bar = 5;
| |
| expected integer, found `bar`
| `bar` is interpreted as a unit struct, not a new binding
| help: introduce a new binding instead: `other_bar`
|
help: introduce a new binding instead
|
LL | let other_bar = 5;
| ++++++

error: aborting due to 1 previous error

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24 changes: 20 additions & 4 deletions tests/ui/consts/const_in_pattern/arrays-and-slices.stderr
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Original file line number Diff line number Diff line change
Expand Up @@ -11,10 +11,14 @@ LL | BSTR_SIZED => {}
| |
| expected `&[u8]`, found `&[u8; 3]`
| `BSTR_SIZED` is interpreted as a constant, not a new binding
| help: introduce a new binding instead: `other_bstr_sized`
|
= note: expected reference `&[u8]`
found reference `&'static [u8; 3]`
help: introduce a new binding instead
|
LL - BSTR_SIZED => {}
LL + other_bstr_sized => {}
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@nnethercote nnethercote Mar 17, 2026

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This one feels marginal, and likewise the other ones in this test. I can imagine a user thinking this would match against b"012". But it's sufficiently broken thinking that it's hard to say for sure, so I can live with it.

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@estebank estebank Mar 17, 2026 •
edited
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I'm currently looking at a completely different case, and maybe the suggestion I'm adding might also be relevant here?

error: constant of non-structural type `S` in a pattern
  --> $DIR/match_ice.rs:11:9
   |
LL | struct S;
   | -------- `S` must be annotated with `#[derive(PartialEq)]` to be usable in patterns
...
LL |     const C: &S = &S;
   |     ----------- constant defined here
LL |     match C {
LL |         C => {}
   |         ^ constant of non-structural type
   |
   = note: see https://doc.rust-lang.org/stable/std/marker/trait.StructuralPartialEq.html for details
help: if `S` manually implemented `PartialEq`, you could check for equality
   |
LL -         C => {}
LL +         binding if binding == C => {}
   |

Note that the binding suggestion is pre-existing, it's just being made more obvious now.

|

error[E0308]: mismatched types
--> $DIR/arrays-and-slices.rs:23:9
Expand All @@ -29,10 +33,14 @@ LL | STRUCT_SIZED => {}
| |
| expected `&SomeStruct<[u8]>`, found `&SomeStruct<[u8; 3]>`
| `STRUCT_SIZED` is interpreted as a constant, not a new binding
| help: introduce a new binding instead: `other_struct_sized`
|
= note: expected reference `&SomeStruct<[u8]>`
found reference `&'static SomeStruct<[u8; 3]>`
help: introduce a new binding instead
|
LL - STRUCT_SIZED => {}
LL + other_struct_sized => {}
|

error[E0308]: mismatched types
--> $DIR/arrays-and-slices.rs:30:9
Expand All @@ -47,10 +55,14 @@ LL | BSTR_SIZED => {}
| |
| expected `&[u8]`, found `&[u8; 3]`
| `BSTR_SIZED` is interpreted as a constant, not a new binding
| help: introduce a new binding instead: `other_bstr_sized`
|
= note: expected reference `&[u8]`
found reference `&'static [u8; 3]`
help: introduce a new binding instead
|
LL - BSTR_SIZED => {}
LL + other_bstr_sized => {}
|

error[E0308]: mismatched types
--> $DIR/arrays-and-slices.rs:37:9
Expand All @@ -65,10 +77,14 @@ LL | STRUCT_SIZED => {}
| |
| expected `&SomeStruct<[u8]>`, found `&SomeStruct<[u8; 3]>`
| `STRUCT_SIZED` is interpreted as a constant, not a new binding
| help: introduce a new binding instead: `other_struct_sized`
|
= note: expected reference `&SomeStruct<[u8]>`
found reference `&'static SomeStruct<[u8; 3]>`
help: introduce a new binding instead
|
LL - STRUCT_SIZED => {}
LL + other_struct_sized => {}
|

error: cannot use unsized non-slice type `SomeStruct<[u8]>` in constant patterns
--> $DIR/arrays-and-slices.rs:47:9
Expand Down
2 changes: 0 additions & 2 deletions tests/ui/did_you_mean/compatible-variants-in-pat.rs
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Expand Up @@ -21,7 +21,6 @@ fn b(s: Option<S>) {
S => {
//~^ ERROR mismatched types
//~| HELP try wrapping
//~| HELP introduce a new binding instead
}
_ => {}
}
Expand All @@ -32,7 +31,6 @@ fn c(s: Result<S, S>) {
S => {
//~^ ERROR mismatched types
//~| HELP try wrapping
//~| HELP introduce a new binding instead
}
_ => {}
}
Expand Down
28 changes: 3 additions & 25 deletions tests/ui/did_you_mean/compatible-variants-in-pat.stderr
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Expand Up @@ -14,42 +14,25 @@ LL | Foo::Bar(Bar { x }) => {
error[E0308]: mismatched types
--> $DIR/compatible-variants-in-pat.rs:21:9
|
LL | struct S;
| -------- unit struct defined here
...
LL | match s {
| - this expression has type `Option<S>`
LL | S => {
| ^
| |
| expected `Option<S>`, found `S`
| `S` is interpreted as a unit struct, not a new binding
| ^ expected `Option<S>`, found `S`
|
= note: expected enum `Option<S>`
found struct `S`
help: try wrapping the pattern in `Some`
|
LL | Some(S) => {
| +++++ +
help: introduce a new binding instead
|
LL - S => {
LL + other_s => {
|

error[E0308]: mismatched types
--> $DIR/compatible-variants-in-pat.rs:32:9
--> $DIR/compatible-variants-in-pat.rs:31:9
|
LL | struct S;
| -------- unit struct defined here
...
LL | match s {
| - this expression has type `Result<S, S>`
LL | S => {
| ^
| |
| expected `Result<S, S>`, found `S`
| `S` is interpreted as a unit struct, not a new binding
| ^ expected `Result<S, S>`, found `S`
|
= note: expected enum `Result<S, S>`
found struct `S`
Expand All @@ -59,11 +42,6 @@ LL | Ok(S) => {
| +++ +
LL | Err(S) => {
| ++++ +
help: introduce a new binding instead
|
LL - S => {
LL + other_s => {
|

error: aborting due to 3 previous errors

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7 changes: 6 additions & 1 deletion tests/ui/issues/issue-33504.stderr
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Expand Up @@ -9,7 +9,12 @@ LL | let Test = 1;
| |
| expected integer, found `Test`
| `Test` is interpreted as a unit struct, not a new binding
| help: introduce a new binding instead: `other_test`
|
help: introduce a new binding instead
|
LL - let Test = 1;
LL + let other_test = 1;
|

error: aborting due to 1 previous error

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3 changes: 0 additions & 3 deletions tests/ui/issues/issue-5100.stderr
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@@ -1,9 +1,6 @@
error[E0308]: mismatched types
--> $DIR/issue-5100.rs:9:9
|
LL | enum A { B, C }
| - unit variant defined here
...
LL | match (true, false) {
| ------------- this expression has type `(bool, bool)`
LL | A::B => (),
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6 changes: 5 additions & 1 deletion tests/ui/match/match-const-tuple-type-mismatch.stderr
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Expand Up @@ -11,10 +11,14 @@ LL | A => (),
| |
| expected integer, found `(isize, isize)`
| `A` is interpreted as a constant, not a new binding
| help: introduce a new binding instead: `other_a`
|
= note: expected type `{integer}`
found tuple `(isize, isize)`
help: introduce a new binding instead
|
LL - A => (),
LL + other_a => (),
|

error: aborting due to 1 previous error

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3 changes: 0 additions & 3 deletions tests/ui/match/match-tag-nullary.stderr
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@@ -1,9 +1,6 @@
error[E0308]: mismatched types
--> $DIR/match-tag-nullary.rs:4:40
|
LL | enum B { B }
| - unit variant defined here
LL |
LL | fn main() { let x: A = A::A; match x { B::B => { } } }
| - ^^^^ expected `A`, found `B`
| |
Expand Down
3 changes: 0 additions & 3 deletions tests/ui/match/mismatched-types-in-match-7867.stderr
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@@ -1,9 +1,6 @@
error[E0308]: mismatched types
--> $DIR/mismatched-types-in-match-7867.rs:10:9
|
LL | enum A { B, C }
| - unit variant defined here
...
LL | match (true, false) {
| ------------- this expression has type `(bool, bool)`
LL | A::B => (),
Expand Down
3 changes: 0 additions & 3 deletions tests/ui/privacy/private-unit-struct-assignment.stderr
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Expand Up @@ -13,9 +13,6 @@ LL | struct C;
error[E0308]: mismatched types
--> $DIR/private-unit-struct-assignment.rs:8:5
|
LL | struct C;
| -------- unit struct defined here
...
LL | A::C = 1;
| ^^^^ - this expression has type `{integer}`
| |
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10 changes: 10 additions & 0 deletions tests/ui/resolve/name-clash-nullary.stderr
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Expand Up @@ -5,9 +5,19 @@ LL | let None: isize = 42;
| ^^^^ ----- expected due to this
| |
| expected `isize`, found `Option<_>`
| `None` is interpreted as a unit variant, not a new binding
|
--> $SRC_DIR/core/src/option.rs:LL:COL
|
= note: unit variant defined here
|
= note: expected type `isize`
found enum `Option<_>`
help: introduce a new binding instead
|
LL - let None: isize = 42;
LL + let other_none: isize = 42;
|

error: aborting due to 1 previous error

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