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fast reject: limit recursion depth
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@lcnr
lcnr committed Nov 29, 2024
1 parent cb2bd2b commit 59afc76
Showing 1 changed file with 53 additions and 17 deletions.
70 changes: 53 additions & 17 deletions compiler/rustc_type_ir/src/fast_reject.rs
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Original file line number Diff line number Diff line change
Expand Up @@ -214,27 +214,47 @@ impl<I: Interner> DeepRejectCtxt<I, false, true> {
impl<I: Interner, const INSTANTIATE_LHS_WITH_INFER: bool, const INSTANTIATE_RHS_WITH_INFER: bool>
DeepRejectCtxt<I, INSTANTIATE_LHS_WITH_INFER, INSTANTIATE_RHS_WITH_INFER>
{
// Quite arbitrary. Large enough to only affect a very tiny amount of impls/crates
// and small enough to still prevent hangs or major slowdowns.
const STARTING_DEPTH: usize = 8;

pub fn args_may_unify(
self,
obligation_args: I::GenericArgs,
impl_args: I::GenericArgs,
) -> bool {
self.args_may_unify_inner(obligation_args, impl_args, Self::STARTING_DEPTH)
}

pub fn types_may_unify(self, lhs: I::Ty, rhs: I::Ty) -> bool {
self.types_may_unify_inner(lhs, rhs, Self::STARTING_DEPTH)
}

fn args_may_unify_inner(
self,
obligation_args: I::GenericArgs,
impl_args: I::GenericArgs,
depth: usize,
) -> bool {
// No need to decrement the depth here as this function is only
// recursively reachable via `types_may_unify_inner` which already
// increments the depth for us.
iter::zip(obligation_args.iter(), impl_args.iter()).all(|(obl, imp)| {
match (obl.kind(), imp.kind()) {
// We don't fast reject based on regions.
(ty::GenericArgKind::Lifetime(_), ty::GenericArgKind::Lifetime(_)) => true,
(ty::GenericArgKind::Type(obl), ty::GenericArgKind::Type(imp)) => {
self.types_may_unify(obl, imp)
self.types_may_unify_inner(obl, imp, depth)
}
(ty::GenericArgKind::Const(obl), ty::GenericArgKind::Const(imp)) => {
self.consts_may_unify(obl, imp)
self.consts_may_unify_inner(obl, imp)
}
_ => panic!("kind mismatch: {obl:?} {imp:?}"),
}
})
}

pub fn types_may_unify(self, lhs: I::Ty, rhs: I::Ty) -> bool {
fn types_may_unify_inner(self, lhs: I::Ty, rhs: I::Ty, depth: usize) -> bool {
match rhs.kind() {
// Start by checking whether the `rhs` type may unify with
// pretty much everything. Just return `true` in that case.
Expand Down Expand Up @@ -273,18 +293,31 @@ impl<I: Interner, const INSTANTIATE_LHS_WITH_INFER: bool, const INSTANTIATE_RHS_
| ty::Placeholder(_) => {}
};

// The type system needs to support exponentially large types
// as long as they are self-similar. While most other folders
// use caching to handle them, this folder exists purely as a
// perf optimization and is incredibly hot. In pretty much all
// uses checking the cache is slower than simply recursing, so
// we instead just add an arbitrary depth cutoff.
//
// We only decrement the depth here as the match on `rhs`
// does not recurse.
let Some(depth) = depth.checked_sub(1) else {
return true;
};

// For purely rigid types, use structural equivalence.
match lhs.kind() {
ty::Ref(_, lhs_ty, lhs_mutbl) => match rhs.kind() {
ty::Ref(_, rhs_ty, rhs_mutbl) => {
lhs_mutbl == rhs_mutbl && self.types_may_unify(lhs_ty, rhs_ty)
lhs_mutbl == rhs_mutbl && self.types_may_unify_inner(lhs_ty, rhs_ty, depth)
}
_ => false,
},

ty::Adt(lhs_def, lhs_args) => match rhs.kind() {
ty::Adt(rhs_def, rhs_args) => {
lhs_def == rhs_def && self.args_may_unify(lhs_args, rhs_args)
lhs_def == rhs_def && self.args_may_unify_inner(lhs_args, rhs_args, depth)
}
_ => false,
},
Expand Down Expand Up @@ -326,27 +359,28 @@ impl<I: Interner, const INSTANTIATE_LHS_WITH_INFER: bool, const INSTANTIATE_RHS_
ty::Tuple(rhs) => {
lhs.len() == rhs.len()
&& iter::zip(lhs.iter(), rhs.iter())
.all(|(lhs, rhs)| self.types_may_unify(lhs, rhs))
.all(|(lhs, rhs)| self.types_may_unify_inner(lhs, rhs, depth))
}
_ => false,
},

ty::Array(lhs_ty, lhs_len) => match rhs.kind() {
ty::Array(rhs_ty, rhs_len) => {
self.types_may_unify(lhs_ty, rhs_ty) && self.consts_may_unify(lhs_len, rhs_len)
self.types_may_unify_inner(lhs_ty, rhs_ty, depth)
&& self.consts_may_unify_inner(lhs_len, rhs_len)
}
_ => false,
},

ty::RawPtr(lhs_ty, lhs_mutbl) => match rhs.kind() {
ty::RawPtr(rhs_ty, rhs_mutbl) => {
lhs_mutbl == rhs_mutbl && self.types_may_unify(lhs_ty, rhs_ty)
lhs_mutbl == rhs_mutbl && self.types_may_unify_inner(lhs_ty, rhs_ty, depth)
}
_ => false,
},

ty::Slice(lhs_ty) => {
matches!(rhs.kind(), ty::Slice(rhs_ty) if self.types_may_unify(lhs_ty, rhs_ty))
matches!(rhs.kind(), ty::Slice(rhs_ty) if self.types_may_unify_inner(lhs_ty, rhs_ty, depth))
}

ty::Dynamic(lhs_preds, ..) => {
Expand All @@ -366,7 +400,7 @@ impl<I: Interner, const INSTANTIATE_LHS_WITH_INFER: bool, const INSTANTIATE_RHS_
lhs_hdr == rhs_hdr
&& lhs_sig_tys.len() == rhs_sig_tys.len()
&& iter::zip(lhs_sig_tys.iter(), rhs_sig_tys.iter())
.all(|(lhs, rhs)| self.types_may_unify(lhs, rhs))
.all(|(lhs, rhs)| self.types_may_unify_inner(lhs, rhs, depth))
}
_ => false,
},
Expand All @@ -375,49 +409,51 @@ impl<I: Interner, const INSTANTIATE_LHS_WITH_INFER: bool, const INSTANTIATE_RHS_

ty::FnDef(lhs_def_id, lhs_args) => match rhs.kind() {
ty::FnDef(rhs_def_id, rhs_args) => {
lhs_def_id == rhs_def_id && self.args_may_unify(lhs_args, rhs_args)
lhs_def_id == rhs_def_id && self.args_may_unify_inner(lhs_args, rhs_args, depth)
}
_ => false,
},

ty::Closure(lhs_def_id, lhs_args) => match rhs.kind() {
ty::Closure(rhs_def_id, rhs_args) => {
lhs_def_id == rhs_def_id && self.args_may_unify(lhs_args, rhs_args)
lhs_def_id == rhs_def_id && self.args_may_unify_inner(lhs_args, rhs_args, depth)
}
_ => false,
},

ty::CoroutineClosure(lhs_def_id, lhs_args) => match rhs.kind() {
ty::CoroutineClosure(rhs_def_id, rhs_args) => {
lhs_def_id == rhs_def_id && self.args_may_unify(lhs_args, rhs_args)
lhs_def_id == rhs_def_id && self.args_may_unify_inner(lhs_args, rhs_args, depth)
}
_ => false,
},

ty::Coroutine(lhs_def_id, lhs_args) => match rhs.kind() {
ty::Coroutine(rhs_def_id, rhs_args) => {
lhs_def_id == rhs_def_id && self.args_may_unify(lhs_args, rhs_args)
lhs_def_id == rhs_def_id && self.args_may_unify_inner(lhs_args, rhs_args, depth)
}
_ => false,
},

ty::CoroutineWitness(lhs_def_id, lhs_args) => match rhs.kind() {
ty::CoroutineWitness(rhs_def_id, rhs_args) => {
lhs_def_id == rhs_def_id && self.args_may_unify(lhs_args, rhs_args)
lhs_def_id == rhs_def_id && self.args_may_unify_inner(lhs_args, rhs_args, depth)
}
_ => false,
},

ty::Pat(lhs_ty, _) => {
// FIXME(pattern_types): take pattern into account
matches!(rhs.kind(), ty::Pat(rhs_ty, _) if self.types_may_unify(lhs_ty, rhs_ty))
matches!(rhs.kind(), ty::Pat(rhs_ty, _) if self.types_may_unify_inner(lhs_ty, rhs_ty, depth))
}

ty::Error(..) => true,
}
}

pub fn consts_may_unify(self, lhs: I::Const, rhs: I::Const) -> bool {
// Unlike `types_may_unify_inner`, this does not take a depth as
// we never recurse from this function.
fn consts_may_unify_inner(self, lhs: I::Const, rhs: I::Const) -> bool {
match rhs.kind() {
ty::ConstKind::Param(_) => {
if INSTANTIATE_RHS_WITH_INFER {
Expand Down

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