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Key-type customization in CanonicalValueStore and ClangDecl cleanups (#5743)

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The `ClangDecl` struct caused some confusion here -- it is embedding
extra data into a `CanonicalValueStore` that isn't used for lookups or
canonicalization, but is useful to store along side. This changes the
`CanonicalValueStore` to support customized key type for `Lookup` so
that we can provide the more direct API that only takes the relevant
key.

This in turn takes advantage of the support for heterogenous keys in the
underlying `Set` as long as hashing and equality are consistent. We do
need to add support for heterogenous equality comparison with
`clang::Decl*`, but that is fairly easily done now that the
argument-reversed form isn't needed as well.

Lastly, this cleans up the `ClangDecl` customization points to be more
idiomatic by using `operator==` and `CarbonHashValue`. While there, I've
added comments to make it unambiguous why we can use the pointer value
for the underlying `clang::Decl` due to the Clang AST's
address-as-identity model.

Resolves the immediate TODOs around this type.

Future work might involve changing from the current `Add` API to one
more like `Map` and `Set`'s API where a callback is used to create the
object, but that level of API complexity isn't necessarily motivated yet
and can easily be a follow-on if and when its worth doing. The `Add`
code paths *are* working with the `inst_id` in order to create an
instruction if we are importing the Clang declaration. It is the
`Lookup` code paths that never needed to know about the `inst_id` and
became more confusing for having to stub it out in the API.

---------

Co-authored-by: Geoff Romer <gromer@google.com>
This commit is contained in:
Chandler Carruth
2025-06-27 20:30:05 +00:00
committed by GitHub
co-authored by Geoff Romer
parent 124313269a
commit bba037738d
3 changed files with 61 additions and 20 deletions
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toolchain/check/import_cpp.cpp
+4 -7
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@@ -350,9 +350,8 @@ static auto AsCarbonNamespace(Context& context,
auto& clang_decls = context.sem_ir().clang_decls();
// Check if the decl context is already mapped to a Carbon namespace.
if (auto context_clang_decl_id = clang_decls.Lookup(
{.decl = clang::dyn_cast<clang::Decl>(decl_context),
.inst_id = SemIR::InstId::None});
if (auto context_clang_decl_id =
clang_decls.Lookup(clang::dyn_cast<clang::Decl>(decl_context));
context_clang_decl_id.has_value()) {
return clang_decls.Get(context_clang_decl_id).inst_id;
}
@@ -365,8 +364,7 @@ static auto AsCarbonNamespace(Context& context,
decl_contexts.push_back(decl_context);
decl_context = decl_context->getParent();
parent_decl_id =
clang_decls.Lookup({.decl = clang::dyn_cast<clang::Decl>(decl_context),
.inst_id = SemIR::InstId::None});
clang_decls.Lookup(clang::dyn_cast<clang::Decl>(decl_context));
} while (!parent_decl_id.has_value());
// We know the parent of the last decl context is mapped, map the rest.
@@ -532,8 +530,7 @@ static auto MapRecordType(Context& context, SemIR::LocId loc_id,
if (record_decl && !record_decl->isUnion()) {
auto& clang_decls = context.sem_ir().clang_decls();
SemIR::InstId record_inst_id = SemIR::InstId::None;
if (auto record_clang_decl_id = clang_decls.Lookup(
{.decl = record_decl, .inst_id = SemIR::InstId::None});
if (auto record_clang_decl_id = clang_decls.Lookup(record_decl);
record_clang_decl_id.has_value()) {
record_inst_id = clang_decls.Get(record_clang_decl_id).inst_id;
} else {