carbon-language/carbon-lang
1
0
Fork 0
mirror of https://github.com/carbon-language/carbon-lang.git synced 2026-10-05 22:02:55 +01:00
Code Issues Packages Projects Releases 10 Wiki Activity

Use GetConstantValueInSpecific to get the impl's specific interface after deduction (#7584)

Browse Source 
During impl lookup, for each (generic) impl candidate, we form a
specific for that impl by deducing its generic arguments. Then we
compare the query interface against the impl's specific interface. That
comparison needs the deduced arguments applied to the impl's specific
interface. Previously we were doing this by getting the impl's
constraint facet type with the impl's specific applied (via
`GetConstantValueInSpecific()`) and then identifying that facet type
with the impl's deduced self.

Identify is a fairly expensive operation. It runs subst, trying to
replace `.Self` references. It walks named constraints. It collects
require declarations. We're looking at making it do _more_ in the future
too, including rewrite constraint resolution and collecting rewrite and
same-type constraints. For this reason we have a cache to make it cheap
on the second run, but it's still a very heavyweight operation to
involve in impl lookup, when all we want is to apply the impl's specific
to its target interface.

We almost have all the information we need to avoid the identification
step. We have the impl's specific after deduction. And we have the
SpecificInterface that the impl is targeting in the `Impl` struct. When
we form the specific for the impl itself, we resolve the declaration
block and form new constant values for all instructions in there, but
that does not cover the SpecificInterface that we're storing in the
`Impl` struct. So we add a new instruction to the impl's eval block,
which will be symbolic when the impl is generic and the target interface
depends on a generic parameter. And we store the `InstId` in the `Impl`
struct. This allows us to gets its constant value later with the impl's
specific applied. From that constant value we can then pull out the
SpecificInterface that the impl is targeting.
This commit is contained in:
Dana Jansens
2026-07-30 19:59:10 +00:00
committed by GitHub
parent a683fb574b
commit a5ba0a0f45
154 changed files with 3001 additions and 2103 deletions
Download Patch File Download Diff File Expand all files Collapse all files
toolchain/check/handle_impl.cpp
+24 -1
View File
@@ -252,6 +252,28 @@ static auto BuildImplDecl(Context& context, Parse::AnyImplDeclId node_id,
full_constraint_type_inst_id = SemIR::ErrorInst::TypeInstId;
}
// Store an instruction in the decl's eval block that contains the target
// interface's specific, whose constant value will be updated when specifics
// are applied to the impl.
//
// We can use ImplSelfWitness for this because it contains a
// SpecificInterfaceId operand, and it has a constant_kind of `Always` so it
// never evaluates to some other type of inst.
//
// TODO: We could avoid the extra indirection through a SpecificInterfaceId if
// we introduced a new instruction with a SpecificId operand instead of
// reusing ImplSelfWitness for this.
auto interface_inst_id =
specific_interface.interface_id.has_value()
? AddInst<SemIR::ImplSelfWitness>(
context, node_id,
{.type_id =
GetSingletonType(context, SemIR::WitnessType::TypeInstId),
.period_self = self_type_inst_id,
.specific_interface_id =
context.specific_interfaces().Add(specific_interface)})
: SemIR::ErrorInst::InstId;
// Strip off anything on the RHS of `where`, as they are not part of the
// constraint being implemented, they just represent requirements that must be
// met when the impl is defined. This drops any `.Self` references from the
@@ -282,7 +304,8 @@ static auto BuildImplDecl(Context& context, Parse::AnyImplDeclId node_id,
.is_final = is_final,
.self_id = self_type_inst_id,
.constraint_id = extend_constraint_type_inst_id,
.interface = specific_interface}};
.interface = specific_interface,
.interface_inst_id = interface_inst_id}};
if (has_definition) {
impl.definition_id = impl_decl_id;
}