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DispatchFromDyn: require additional checks #149068

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3 changes: 3 additions & 0 deletions compiler/rustc_hir_analysis/messages.ftl
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Original file line number Diff line number Diff line change
Expand Up @@ -149,6 +149,9 @@ hir_analysis_cross_crate_traits = cross-crate traits with a default impl, like `
hir_analysis_cross_crate_traits_defined = cross-crate traits with a default impl, like `{$traits}`, can only be implemented for a struct/enum type defined in the current crate
.label = can't implement cross-crate trait for type in another crate

hir_analysis_dispatch_from_dyn_multiple_refs = the trait `DispatchFromDyn` does not allow dispatch through references
.note = the trait `DispatchFromDyn` may only be implemented when dispatch goes through at most one reference to a generic
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@theemathas theemathas Nov 20, 2025

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Why does this text not show up in the test error output?

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@dingxiangfei2009 dingxiangfei2009 Nov 26, 2025

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I forgot about a #[note] attribute on the error type. Now it should work.


hir_analysis_dispatch_from_dyn_repr = structs implementing `DispatchFromDyn` may not have `#[repr(packed)]` or `#[repr(C)]`

hir_analysis_dispatch_from_dyn_zst = the trait `DispatchFromDyn` may only be implemented for structs containing the field being coerced, ZST fields with 1 byte alignment that don't mention type/const generics, and nothing else
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253 changes: 143 additions & 110 deletions compiler/rustc_hir_analysis/src/coherence/builtin.rs
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Expand Up @@ -203,17 +203,15 @@ fn visit_implementation_of_dispatch_from_dyn(checker: &Checker<'_>) -> Result<()
let tcx = checker.tcx;
let impl_did = checker.impl_def_id;
let trait_ref = checker.impl_header.trait_ref.instantiate_identity();
assert!(tcx.is_lang_item(trait_ref.def_id, LangItem::DispatchFromDyn));
let dispatch_from_dyn_trait_did = trait_ref.def_id;
debug!("visit_implementation_of_dispatch_from_dyn: impl_did={:?}", impl_did);

let span = tcx.def_span(impl_did);
let trait_name = "DispatchFromDyn";

let source = trait_ref.self_ty();
let target = {
assert!(tcx.is_lang_item(trait_ref.def_id, LangItem::DispatchFromDyn));

trait_ref.args.type_at(1)
};
let mut source = trait_ref.self_ty();
let mut target = trait_ref.args.type_at(1);

// Check `CoercePointee` impl is WF -- if not, then there's no reason to report
// redundant errors for `DispatchFromDyn`. This is best effort, though.
Expand Down Expand Up @@ -242,138 +240,173 @@ fn visit_implementation_of_dispatch_from_dyn(checker: &Checker<'_>) -> Result<()
// trait, they *do* satisfy the repr(transparent) rules, and then we assume that everything else
// in the compiler (in particular, all the call ABI logic) will treat them as repr(transparent)
// even if they do not carry that attribute.
match (source.kind(), target.kind()) {
(&ty::Pat(_, pat_a), &ty::Pat(_, pat_b)) => {
if pat_a != pat_b {
return Err(tcx.dcx().emit_err(errors::CoerceSamePatKind {
span,
trait_name,
pat_a: pat_a.to_string(),
pat_b: pat_b.to_string(),
}));
let mut behind_ref = false;
loop {
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@jackh726 jackh726 Mar 31, 2026

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I don't quite like implementing this as a loop. I think, change this match into a function is better.

I would also have arms like

(Ref(..), Ref(..)) if behind_ref => { error },
(Ref(..), Ref(..)) if ... => { ... },
...
``
instead of have them all "fallthrough" to the `_` case.

match (source.kind(), target.kind()) {
(&ty::Pat(ty_a, pat_a), &ty::Pat(ty_b, pat_b)) => {
if pat_a != pat_b {
return Err(tcx.dcx().emit_err(errors::CoerceSamePatKind {
span,
trait_name,
pat_a: pat_a.to_string(),
pat_b: pat_b.to_string(),
}));
}
source = ty_a;
target = ty_b;
}
Ok(())
}

(&ty::Ref(r_a, _, mutbl_a), ty::Ref(r_b, _, mutbl_b))
if r_a == *r_b && mutbl_a == *mutbl_b =>
{
Ok(())
}
(&ty::RawPtr(_, a_mutbl), &ty::RawPtr(_, b_mutbl)) if a_mutbl == b_mutbl => Ok(()),
(&ty::Adt(def_a, args_a), &ty::Adt(def_b, args_b))
if def_a.is_struct() && def_b.is_struct() =>
{
if def_a != def_b {
let source_path = tcx.def_path_str(def_a.did());
let target_path = tcx.def_path_str(def_b.did());
return Err(tcx.dcx().emit_err(errors::CoerceSameStruct {
span,
trait_name,
note: true,
source_path,
target_path,
}));
(&ty::Ref(r_a, ty_a, mutbl_a), &ty::Ref(r_b, ty_b, mutbl_b))
if !behind_ref && r_a == r_b && mutbl_a == mutbl_b =>
{
source = ty_a;
target = ty_b;
behind_ref = true;
}

if def_a.repr().c() || def_a.repr().packed() {
return Err(tcx.dcx().emit_err(errors::DispatchFromDynRepr { span }));
(&ty::RawPtr(ty_a, a_mutbl), &ty::RawPtr(ty_b, b_mutbl))
if !behind_ref && a_mutbl == b_mutbl =>
{
source = ty_a;
target = ty_b;
behind_ref = true;
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@jackh726 jackh726 Mar 31, 2026

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Similarly, test for this.

}
(&ty::Adt(def_a, args_a), &ty::Adt(def_b, args_b))
if !behind_ref && def_a.is_struct() && def_b.is_struct() =>
{
if def_a != def_b {
let source_path = tcx.def_path_str(def_a.did());
let target_path = tcx.def_path_str(def_b.did());
return Err(tcx.dcx().emit_err(errors::CoerceSameStruct {
span,
trait_name,
note: true,
source_path,
target_path,
}));
}

let fields = &def_a.non_enum_variant().fields;

let mut res = Ok(());
let coerced_fields = fields
.iter_enumerated()
.filter_map(|(i, field)| {
// Ignore PhantomData fields
let unnormalized_ty = tcx.type_of(field.did).instantiate_identity();
if tcx
.try_normalize_erasing_regions(
ty::TypingEnv::non_body_analysis(tcx, def_a.did()),
unnormalized_ty,
)
.unwrap_or(unnormalized_ty)
.is_phantom_data()
{
return None;
}
if def_a.repr().c() || def_a.repr().packed() {
return Err(tcx.dcx().emit_err(errors::DispatchFromDynRepr { span }));
}

let ty_a = field.ty(tcx, args_a);
let ty_b = field.ty(tcx, args_b);

// FIXME: We could do normalization here, but is it really worth it?
if ty_a == ty_b {
// Allow 1-ZSTs that don't mention type params.
//
// Allowing type params here would allow us to possibly transmute
// between ZSTs, which may be used to create library unsoundness.
if let Ok(layout) =
tcx.layout_of(infcx.typing_env(param_env).as_query_input(ty_a))
&& layout.is_1zst()
&& !ty_a.has_non_region_param()
let fields = &def_a.non_enum_variant().fields;

let coerced_fields: Vec<_> = fields
.iter_enumerated()
.filter_map(|(i, field)| {
// Ignore PhantomData fields
let unnormalized_ty = tcx.type_of(field.did).instantiate_identity();
if tcx
.try_normalize_erasing_regions(
ty::TypingEnv::non_body_analysis(tcx, def_a.did()),
unnormalized_ty,
)
.unwrap_or(unnormalized_ty)
.is_phantom_data()
{
// ignore 1-ZST fields
return None;
}

res = Err(tcx.dcx().emit_err(errors::DispatchFromDynZST {
span,
name: field.ident(tcx),
ty: ty_a,
}));
let ty_a = field.ty(tcx, args_a);
let ty_b = field.ty(tcx, args_b);

// FIXME: We could do normalization here, but is it really worth it?
if ty_a == ty_b {
// Allow 1-ZSTs that don't mention type params.
//
// Allowing type params here would allow us to possibly transmute
// between ZSTs, which may be used to create library unsoundness.
if let Ok(layout) =
tcx.layout_of(infcx.typing_env(param_env).as_query_input(ty_a))
&& layout.is_1zst()
&& !ty_a.has_non_region_param()
{
// ignore 1-ZST fields
return None;
}

None
} else {
Some((i, ty_a, ty_b, tcx.def_span(field.did)))
Some(Err(tcx.dcx().emit_err(errors::DispatchFromDynZST {
span,
name: field.ident(tcx),
ty: ty_a,
})))
} else {
Some(Ok((i, ty_a, ty_b, tcx.def_span(field.did))))
}
})
.collect::<Result<_, _>>()?;

if coerced_fields.is_empty() {
return Err(tcx.dcx().emit_err(errors::CoerceNoField {
span,
trait_name,
note: true,
}));
}
if let &[(_, ty_a, ty_b, field_span)] = &coerced_fields[..] {
let ocx = ObligationCtxt::new_with_diagnostics(&infcx);
ocx.register_obligation(Obligation::new(
tcx,
cause.clone(),
param_env,
ty::TraitRef::new(tcx, dispatch_from_dyn_trait_did, [ty_a, ty_b]),
));
let errors = ocx.evaluate_obligations_error_on_ambiguity();
if !errors.is_empty() {
if is_from_coerce_pointee_derive(tcx, span) {
return Err(tcx.dcx().emit_err(errors::CoerceFieldValidity {
span,
trait_name,
ty: trait_ref.self_ty(),
field_span,
field_ty: ty_a,
}));
} else {
return Err(infcx.err_ctxt().report_fulfillment_errors(errors));
}
}
})
.collect::<Vec<_>>();
res?;

if coerced_fields.is_empty() {
return Err(tcx.dcx().emit_err(errors::CoerceNoField {
// Finally, resolve all regions.
ocx.resolve_regions_and_report_errors(impl_did, param_env, [])?;

return Ok(());
}
return Err(tcx.dcx().emit_err(errors::CoerceMulti {
span,
trait_name,
note: true,
number: coerced_fields.len(),
fields: coerced_fields.iter().map(|(_, _, _, s)| *s).collect::<Vec<_>>().into(),
}));
} else if let &[(_, ty_a, ty_b, field_span)] = &coerced_fields[..] {
}
(ty::Param(_), ty::Param(_)) => {
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@jackh726 jackh726 Mar 31, 2026

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I guess, the test here is just the std impl?

I guess, this technically allows impl<T: Unsize<U>, U> DispatchFromDyn<U> for T {} (though, this obviously this conflicts via coherence).

let ocx = ObligationCtxt::new_with_diagnostics(&infcx);
ocx.register_obligation(Obligation::new(
tcx,
cause.clone(),
param_env,
ty::TraitRef::new(tcx, trait_ref.def_id, [ty_a, ty_b]),
ty::TraitRef::new(
tcx,
tcx.require_lang_item(LangItem::Unsize, span),
[source, target],
),
));
let errors = ocx.evaluate_obligations_error_on_ambiguity();
if !errors.is_empty() {
if is_from_coerce_pointee_derive(tcx, span) {
return Err(tcx.dcx().emit_err(errors::CoerceFieldValidity {
span,
trait_name,
ty: trait_ref.self_ty(),
field_span,
field_ty: ty_a,
}));
} else {
return Err(infcx.err_ctxt().report_fulfillment_errors(errors));
}
return Err(infcx.err_ctxt().report_fulfillment_errors(errors));
}

// Finally, resolve all regions.
ocx.resolve_regions_and_report_errors(impl_did, param_env, [])?;

Ok(())
} else {
return Err(tcx.dcx().emit_err(errors::CoerceMulti {
span,
trait_name,
number: coerced_fields.len(),
fields: coerced_fields.iter().map(|(_, _, _, s)| *s).collect::<Vec<_>>().into(),
}));
return Ok(());
}
_ => {
if behind_ref {
return Err(tcx.dcx().emit_err(errors::DispatchFromDynMultiRefs { span }));
} else {
Comment on lines +401 to +403
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@jackh726 jackh726 Mar 31, 2026

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Hmm, not sure I like this implementation. But let me comment above.

return Err(tcx
.dcx()
.emit_err(errors::CoerceUnsizedNonStruct { span, trait_name }));
}
}
}
_ => Err(tcx.dcx().emit_err(errors::CoerceUnsizedNonStruct { span, trait_name })),
}
}

Expand Down
8 changes: 8 additions & 0 deletions compiler/rustc_hir_analysis/src/errors.rs
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Expand Up @@ -1247,6 +1247,14 @@ pub(crate) struct CoerceSamePatKind {
pub pat_b: String,
}

#[derive(Diagnostic)]
#[diag(hir_analysis_dispatch_from_dyn_multiple_refs)]
#[note]
pub(crate) struct DispatchFromDynMultiRefs {
#[primary_span]
pub span: Span,
}

#[derive(Diagnostic)]
#[diag(hir_analysis_coerce_unsized_may, code = E0377)]
pub(crate) struct CoerceSameStruct {
Expand Down
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