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Suggest using equality comparison instead of pattern matching on non-structural constant in pattern #154010

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Merged
rust-bors merged 2 commits into from
Mar 26, 2026
Merged

Suggest using equality comparison instead of pattern matching on non-structural constant in pattern #154010

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3f08f67
Suggest using equality comparison instead of pattern matching on non-…
estebank Mar 17, 2026
801eafd
Suggest `if let PAT == expr` on `let PAT = expr else` with non-struct…
estebank Mar 17, 2026
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102 changes: 100 additions & 2 deletions compiler/rustc_mir_build/src/errors.rs
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Original file line number Diff line number Diff line change
Expand Up @@ -1053,17 +1053,115 @@ pub(crate) struct TypeNotStructural<'tcx> {
#[primary_span]
#[label("constant of non-structural type")]
pub(crate) span: Span,
#[label("`{$ty}` must be annotated with `#[derive(PartialEq)]` to be usable in patterns")]
#[label(
"{$is_local ->
*[true] `{$ty}` must be annotated with `#[derive(PartialEq)]` to be usable in patterns
[false] `{$ty}` is not usable in patterns
}"
)]
pub(crate) ty_def_span: Span,
pub(crate) ty: Ty<'tcx>,
#[note(
"the `PartialEq` trait must be derived, manual `impl`s are not sufficient; see https://doc.rust-lang.org/stable/std/marker/trait.StructuralPartialEq.html for details"
"the `PartialEq` trait must be derived, manual `impl`s are not sufficient; see \
https://doc.rust-lang.org/stable/std/marker/trait.StructuralPartialEq.html for details"
)]
pub(crate) manual_partialeq_impl_span: Option<Span>,
#[note(
"see https://doc.rust-lang.org/stable/std/marker/trait.StructuralPartialEq.html for details"
)]
pub(crate) manual_partialeq_impl_note: bool,
#[subdiagnostic]
pub(crate) suggestion: Option<SuggestEq<'tcx>>,
pub(crate) is_local: bool,
}

#[derive(Subdiagnostic)]
pub(crate) enum SuggestEq<'tcx> {
#[multipart_suggestion(
"{$manual_partialeq_impl ->
[false] if `{$ty}` manually implemented `PartialEq`, you could add
*[true] add
} a condition to the match arm checking for equality",
applicability = "maybe-incorrect",
style = "verbose"
)]
AddIf {
#[suggestion_part(code = "binding")]
pat_span: Span,
#[suggestion_part(code = " if binding == {name}")]
if_span: Span,
name: String,
ty: Ty<'tcx>,
manual_partialeq_impl: bool,
},
#[multipart_suggestion(
"{$manual_partialeq_impl ->
[false] if `{$ty}` manually implemented `PartialEq`, you could add
*[true] add
} a check for equality to the condition of the match arm",
applicability = "maybe-incorrect",
style = "verbose"
)]
AddToIf {
#[suggestion_part(code = "binding")]
pat_span: Span,
#[suggestion_part(code = " && binding == {name}")]
span: Span,
name: String,
ty: Ty<'tcx>,
manual_partialeq_impl: bool,
},
#[multipart_suggestion(
"{$manual_partialeq_impl ->
[false] if `{$ty}` manually implemented `PartialEq`, you could check
*[true] check
} for equality instead of pattern matching",
applicability = "maybe-incorrect",
style = "verbose"
)]
AddToLetChain {
#[suggestion_part(code = "binding")]
pat_span: Span,
#[suggestion_part(code = " && binding == {name}")]
span: Span,
name: String,
ty: Ty<'tcx>,
manual_partialeq_impl: bool,
},
#[multipart_suggestion(
"{$manual_partialeq_impl ->
[false] if `{$ty}` manually implemented `PartialEq`, you could check
*[true] check
} for equality instead of pattern matching",
applicability = "maybe-incorrect",
style = "verbose"
)]
ReplaceWithEq {
#[suggestion_part(code = "")]
removal: Span,
#[suggestion_part(code = " == ")]
eq: Span,
ty: Ty<'tcx>,
manual_partialeq_impl: bool,
},
#[multipart_suggestion(
"{$manual_partialeq_impl ->
[false] if `{$ty}` manually implemented `PartialEq`, you could check
*[true] check
} for equality instead of pattern matching",
applicability = "maybe-incorrect",
style = "verbose"
)]
ReplaceLetElseWithIf {
#[suggestion_part(code = "if ")]
if_span: Span,
#[suggestion_part(code = " == ")]
eq: Span,
#[suggestion_part(code = " ")]
else_span: Span,
ty: Ty<'tcx>,
manual_partialeq_impl: bool,
},
}

#[derive(Diagnostic)]
Expand Down
82 changes: 81 additions & 1 deletion compiler/rustc_mir_build/src/thir/pattern/const_to_pat.rs
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Original file line number Diff line number Diff line change
Expand Up @@ -22,7 +22,7 @@ use tracing::{debug, instrument, trace};
use super::PatCtxt;
use crate::errors::{
ConstPatternDependsOnGenericParameter, CouldNotEvalConstPattern, InvalidPattern, NaNPattern,
PointerPattern, TypeNotPartialEq, TypeNotStructural, UnionPattern, UnsizedPattern,
PointerPattern, SuggestEq, TypeNotPartialEq, TypeNotStructural, UnionPattern, UnsizedPattern,
};

impl<'tcx> PatCtxt<'tcx> {
Expand Down Expand Up @@ -224,13 +224,93 @@ impl<'tcx> ConstToPat<'tcx> {
}
_ => (None, true),
};
let manual_partialeq_impl =
manual_partialeq_impl_note || manual_partialeq_impl_span.is_some();
let is_local = adt_def.did().is_local();
let ty_def_span = tcx.def_span(adt_def.did());
let suggestion = if let Ok(name) = tcx.sess.source_map().span_to_snippet(self.span)
&& (is_local || manual_partialeq_impl)
{
let mut hir_id = self.id;
while let hir::Node::Pat(pat) = tcx.parent_hir_node(hir_id) {
hir_id = pat.hir_id;
}
match tcx.parent_hir_node(hir_id) {
hir::Node::Arm(hir::Arm { pat, guard: None, .. }) => {
// Add an if condition to the match arm.
Some(SuggestEq::AddIf {
if_span: pat.span.shrink_to_hi(),
pat_span: self.span,
name,
ty,
manual_partialeq_impl,
})
}
hir::Node::Arm(hir::Arm { guard: Some(guard), .. }) => {
// Modify the the match arm if condition and add a check for equality.
Some(SuggestEq::AddToIf {
span: guard.span.shrink_to_hi(),
pat_span: self.span,
name,
ty,
manual_partialeq_impl,
})
}
hir::Node::Expr(hir::Expr {
kind: hir::ExprKind::Let(let_expr),
span,
..
}) => {
if let_expr.pat.span == self.span {
// `if let CONST = expr` -> `if CONST == expr`.
Some(SuggestEq::ReplaceWithEq {
removal: span.until(self.span),
eq: self.span.between(let_expr.init.span),
ty,
manual_partialeq_impl,
})
} else if tcx.sess.edition().at_least_rust_2024() {
// `if let Some(CONST) = expr` ->
// `if let Some(binding) = expr && binding == CONST`.
Some(SuggestEq::AddToLetChain {
span: span.shrink_to_hi(),
pat_span: self.span,
name,
ty,
manual_partialeq_impl,
})
} else {
None
}
}
hir::Node::LetStmt(let_stmt)
if let Some(init) = let_stmt.init
&& let Some(els) = let_stmt.els
&& init.span.ctxt().is_root()
&& els.span.ctxt().is_root() =>
{
// `let PAT = expr else {` -> `if PAT == expr {`.
Some(SuggestEq::ReplaceLetElseWithIf {
if_span: let_stmt.span.until(let_stmt.pat.span),
eq: let_stmt.pat.span.between(init.span),
else_span: init.span.between(els.span),
ty,
manual_partialeq_impl,
})
}
_ => None,
}
} else {
None
};
let err = TypeNotStructural {
span,
ty,
ty_def_span,
manual_partialeq_impl_span,
manual_partialeq_impl_note,
is_local,
suggestion,
};
return self.mk_err(tcx.dcx().create_err(err), ty);
}
Expand Down
2 changes: 1 addition & 1 deletion tests/ui/consts/const_in_pattern/cross-crate-fail.rs
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Expand Up @@ -14,7 +14,7 @@ fn main() {
}

match None {
<Defaulted as consts::AssocConst>::SOME => panic!(),
<Defaulted as consts::AssocConst>::SOME => panic!(),
//~^ ERROR constant of non-structural type `CustomEq` in a pattern
_ => {}
}
Expand Down
16 changes: 13 additions & 3 deletions tests/ui/consts/const_in_pattern/cross-crate-fail.stderr
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Expand Up @@ -7,28 +7,38 @@ LL | consts::SOME => panic!(),
::: $DIR/auxiliary/consts.rs:1:1
|
LL | pub struct CustomEq;
| ------------------- `CustomEq` must be annotated with `#[derive(PartialEq)]` to be usable in patterns
| ------------------- `CustomEq` is not usable in patterns
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@nnethercote nnethercote Mar 24, 2026

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This non-local case is tricky, because the other crate might be under the author's control and able to be changed. I guess it's impossible to know for sure.

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@estebank estebank Mar 24, 2026

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I was going back and forth on this one. I settled on "we're giving them a link to the docs". I was concerned with giving the impression to the user that they had to somehow fork their dep. We could instead say something like "CustomEq could be used in patterns if it was annotated with #[derive(PartialEq)]", but I fear that has the same problem.

...
LL | pub const SOME: Option<CustomEq> = Some(CustomEq);
| -------------------------------- constant defined here
|
= note: see https://doc.rust-lang.org/stable/std/marker/trait.StructuralPartialEq.html for details
help: add a condition to the match arm checking for equality
|
LL - consts::SOME => panic!(),
LL + binding if binding == consts::SOME => panic!(),
|

error: constant of non-structural type `CustomEq` in a pattern
--> $DIR/cross-crate-fail.rs:17:9
|
LL | <Defaulted as consts::AssocConst>::SOME => panic!(),
LL | <Defaulted as consts::AssocConst>::SOME => panic!(),
| ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^ constant of non-structural type
|
::: $DIR/auxiliary/consts.rs:1:1
|
LL | pub struct CustomEq;
| ------------------- `CustomEq` must be annotated with `#[derive(PartialEq)]` to be usable in patterns
| ------------------- `CustomEq` is not usable in patterns
...
LL | const SOME: Option<CustomEq> = Some(CustomEq);
| ---------------------------- constant defined here
|
= note: see https://doc.rust-lang.org/stable/std/marker/trait.StructuralPartialEq.html for details
help: add a condition to the match arm checking for equality
|
LL - <Defaulted as consts::AssocConst>::SOME => panic!(),
LL + binding if binding == <Defaulted as consts::AssocConst>::SOME => panic!(),
|

error: aborting due to 2 previous errors

5 changes: 5 additions & 0 deletions tests/ui/consts/const_in_pattern/no-eq-branch-fail.stderr
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Expand Up @@ -11,6 +11,11 @@ LL | BAR_BAZ => panic!(),
| ^^^^^^^ constant of non-structural type
|
= note: see https://doc.rust-lang.org/stable/std/marker/trait.StructuralPartialEq.html for details
help: add a condition to the match arm checking for equality
|
LL - BAR_BAZ => panic!(),
LL + binding if binding == BAR_BAZ => panic!(),
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@nnethercote nnethercote Mar 24, 2026 •
edited
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It's a bit confusing that the error message suggests two separate ways of fixing this -- add derive(PartialEq) and use a match arm condition. Could the one-or-the-other nature be made clearer? Maybe by inserting "alternatively, " at the start of the help message?

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@estebank estebank Mar 26, 2026

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Ah, I was looking on how to address this. I'll follow up.

|

error: aborting due to 1 previous error

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