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more interesting fix for coercion hack perf regression #139172

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more interesting fix for coercion hack perf regression #139172

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e57808d
rarw
lcnr Mar 31, 2025
363d339
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lcnr Mar 31, 2025
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104 changes: 58 additions & 46 deletions compiler/rustc_hir_typeck/src/coercion.rs
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Original file line number Diff line number Diff line change
Expand Up @@ -58,6 +58,7 @@ use rustc_middle::ty::error::TypeError;
use rustc_middle::ty::{self, AliasTy, GenericArgsRef, Ty, TyCtxt, TypeVisitableExt};
use rustc_span::{BytePos, DUMMY_SP, DesugaringKind, Span};
use rustc_trait_selection::infer::InferCtxtExt as _;
use rustc_trait_selection::traits::EvaluationResult::*;
use rustc_trait_selection::traits::query::evaluate_obligation::InferCtxtExt;
use rustc_trait_selection::traits::{
self, NormalizeExt, ObligationCause, ObligationCauseCode, ObligationCtxt,
Expand Down Expand Up @@ -599,54 +600,65 @@ impl<'f, 'tcx> Coerce<'f, 'tcx> {
ty::TraitRef::new(self.tcx, coerce_unsized_did, [coerce_source, coerce_target]),
);

// If the root `Source: CoerceUnsized<Target>` obligation can't possibly hold,
// we don't have to assume that this is unsizing coercion (it will always lead to an error)
//
// However, we don't want to bail early all the time, since the unholdable obligations
// may be interesting for diagnostics (such as trying to coerce `&T` to `&dyn Id<This = U>`),
// so we only bail if there (likely) is another way to convert the types.
if !self.infcx.predicate_may_hold(&root_obligation) {
if let Some(dyn_metadata_adt_def_id) = self.tcx.lang_items().get(LangItem::DynMetadata)
&& let Some(metadata_type_def_id) = self.tcx.lang_items().get(LangItem::Metadata)
{
self.probe(|_| {
let ocx = ObligationCtxt::new(&self.infcx);

// returns `true` if `<ty as Pointee>::Metadata` is `DynMetadata<_>`
let has_dyn_trait_metadata = |ty| {
let metadata_ty: Result<_, _> = ocx.structurally_normalize_ty(
&ObligationCause::dummy(),
self.fcx.param_env,
Ty::new_alias(
self.tcx,
ty::AliasTyKind::Projection,
AliasTy::new(self.tcx, metadata_type_def_id, [ty]),
),
);
match self.infcx.evaluate_obligation_no_overflow(&root_obligation) {
// Fast path if we're definitely able to coerce. This allows us to use the
// cache of the `FulfillmentContext` and avoids manual calls to select.
EvaluatedToOk | EvaluatedToOkModuloRegions | EvaluatedToOkModuloOpaqueTypes => {
coercion.obligations.push(root_obligation);
return Ok(coercion);
}
EvaluatedToAmbig | EvaluatedToAmbigStackDependent => {}
EvaluatedToErr => {
// If the root `Source: CoerceUnsized<Target>` obligation can't possibly hold,
// we don't have to assume that this is unsizing coercion (it will always lead to an error)
//
// However, we don't want to bail early all the time, since the unholdable obligations
// may be interesting for diagnostics (such as trying to coerce `&T` to `&dyn Id<This = U>`),
// so we only bail if there (likely) is another way to convert the types.
if let &ty::RawPtr(source_pointee, _) = coerce_source.kind()
&& let &ty::RawPtr(target_pointee, _) = target.kind()
&& let Some(dyn_metadata_adt_def_id) =
self.tcx.lang_items().get(LangItem::DynMetadata)
&& let Some(metadata_type_def_id) =
self.tcx.lang_items().get(LangItem::Metadata)
{
self.probe(|_| {
let ocx = ObligationCtxt::new(&self.infcx);

// returns `true` if `<ty as Pointee>::Metadata` is `DynMetadata<_>`
let has_dyn_trait_metadata = |ty| {
let metadata_ty: Result<_, _> = ocx.structurally_normalize_ty(
&ObligationCause::dummy(),
self.fcx.param_env,
Ty::new_alias(
self.tcx,
ty::AliasTyKind::Projection,
AliasTy::new(self.tcx, metadata_type_def_id, [ty]),
),
);

metadata_ty.is_ok_and(|metadata_ty| {
metadata_ty
.ty_adt_def()
.is_some_and(|d| d.did() == dyn_metadata_adt_def_id)
})
};

// If both types are raw pointers to a (wrapper over a) trait object,
// this might be a cast like `*const W<dyn Trait> -> *const dyn Trait`.
// So it's better to bail and try that. (even if the cast is not possible, for
// example due to vtables not matching, cast diagnostic will likely still be better)
//
// N.B. use `target`, not `coerce_target` (the latter is a var)
if let &ty::RawPtr(source_pointee, _) = coerce_source.kind()
&& let &ty::RawPtr(target_pointee, _) = target.kind()
&& has_dyn_trait_metadata(source_pointee)
&& has_dyn_trait_metadata(target_pointee)
{
return Err(TypeError::Mismatch);
}
metadata_ty.is_ok_and(|metadata_ty| {
metadata_ty
.ty_adt_def()
.is_some_and(|d| d.did() == dyn_metadata_adt_def_id)
})
};

Ok(())
})?;
// If both types are raw pointers to a (wrapper over a) trait object,
// this might be a cast like `*const W<dyn Trait> -> *const dyn Trait`.
// So it's better to bail and try that. (even if the cast is not possible, for
// example due to vtables not matching, cast diagnostic will likely still be better)
//
// N.B. use `target`, not `coerce_target` (the latter is a var)
if has_dyn_trait_metadata(source_pointee)
&& has_dyn_trait_metadata(target_pointee)
{
return Err(TypeError::Mismatch);
}

Ok(())
})?;
}
}
}

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