Move impl lookup out into its own file. (#4435)

In preparation for adding more logic here. This code doesn't belong in
member access.

toolchain/check/BUILD

@@ -24,6 +24,7 @@ cc_library(
         "function.cpp",
         "generic.cpp",
         "global_init.cpp",
+        "impl_lookup.cpp",
         "import.cpp",
         "import_ref.cpp",
         "inst_block_stack.cpp",
@@ -48,6 +49,7 @@ cc_library(
         "function.h",
         "generic.h",
         "global_init.h",
+        "impl_lookup.h",
         "import.h",
         "import_ref.h",
         "inst_block_stack.h",

toolchain/check/impl_lookup.cpp

@@ -0,0 +1,60 @@
+// Part of the Carbon Language project, under the Apache License v2.0 with LLVM
+// Exceptions. See /LICENSE for license information.
+// SPDX-License-Identifier: Apache-2.0 WITH LLVM-exception
+
+#include "toolchain/check/impl_lookup.h"
+
+#include "toolchain/check/deduce.h"
+#include "toolchain/check/generic.h"
+#include "toolchain/check/import_ref.h"
+
+namespace Carbon::Check {
+
+auto LookupInterfaceWitness(Context& context, SemIR::LocId loc_id,
+                            SemIR::ConstantId type_const_id,
+                            SemIR::ConstantId interface_const_id)
+    -> SemIR::InstId {
+  // TODO: Add a better impl lookup system. At the very least, we should only be
+  // considering impls that are for the same interface we're querying. We can
+  // also skip impls that mention any types that aren't part of our impl query.
+  for (const auto& impl : context.impls().array_ref()) {
+    auto specific_id = SemIR::SpecificId::Invalid;
+    if (impl.generic_id.is_valid()) {
+      specific_id = DeduceImplArguments(context, loc_id, impl, type_const_id,
+                                        interface_const_id);
+      if (!specific_id.is_valid()) {
+        continue;
+      }
+    }
+    if (!context.constant_values().AreEqualAcrossDeclarations(
+            SemIR::GetConstantValueInSpecific(context.sem_ir(), specific_id,
+                                              impl.self_id),
+            type_const_id)) {
+      continue;
+    }
+    if (!context.constant_values().AreEqualAcrossDeclarations(
+            SemIR::GetConstantValueInSpecific(context.sem_ir(), specific_id,
+                                              impl.constraint_id),
+            interface_const_id)) {
+      // TODO: An impl of a constraint type should be treated as implementing
+      // the constraint's interfaces.
+      continue;
+    }
+    if (!impl.witness_id.is_valid()) {
+      // TODO: Diagnose if the impl isn't defined yet?
+      return SemIR::InstId::Invalid;
+    }
+    LoadImportRef(context, impl.witness_id);
+    if (specific_id.is_valid()) {
+      // We need a definition of the specific `impl` so we can access its
+      // witness.
+      ResolveSpecificDefinition(context, specific_id);
+    }
+    return context.constant_values().GetInstId(
+        SemIR::GetConstantValueInSpecific(context.sem_ir(), specific_id,
+                                          impl.witness_id));
+  }
+  return SemIR::InstId::Invalid;
+}
+
+}  // namespace Carbon::Check

toolchain/check/impl_lookup.h

@@ -0,0 +1,23 @@
+// Part of the Carbon Language project, under the Apache License v2.0 with LLVM
+// Exceptions. See /LICENSE for license information.
+// SPDX-License-Identifier: Apache-2.0 WITH LLVM-exception
+
+#ifndef CARBON_TOOLCHAIN_CHECK_IMPL_LOOKUP_H_
+#define CARBON_TOOLCHAIN_CHECK_IMPL_LOOKUP_H_
+
+#include "toolchain/check/context.h"
+#include "toolchain/sem_ir/ids.h"
+
+namespace Carbon::Check {
+
+// Looks up the witness to use for a particular type and interface. Returns the
+// witness, or `InstId::Invalid` if the type is not known to implement the
+// interface.
+auto LookupInterfaceWitness(Context& context, SemIR::LocId loc_id,
+                            SemIR::ConstantId type_const_id,
+                            SemIR::ConstantId interface_const_id)
+    -> SemIR::InstId;
+
+}  // namespace Carbon::Check
+
+#endif  // CARBON_TOOLCHAIN_CHECK_IMPL_LOOKUP_H_

toolchain/check/member_access.cpp

@@ -10,9 +10,7 @@
 #include "toolchain/base/kind_switch.h"
 #include "toolchain/check/context.h"
 #include "toolchain/check/convert.h"
-#include "toolchain/check/deduce.h"
-#include "toolchain/check/generic.h"
-#include "toolchain/check/import_ref.h"
+#include "toolchain/check/impl_lookup.h"
 #include "toolchain/diagnostics/diagnostic_emitter.h"
 #include "toolchain/sem_ir/generic.h"
 #include "toolchain/sem_ir/ids.h"
@@ -171,56 +169,6 @@ static auto ScopeNeedsImplLookup(Context& context, LookupScope scope) -> bool {
   return true;
 }
 
-// Given a type and an interface, searches for an impl that describes how that
-// type implements that interface, and returns the corresponding witness.
-// Returns an invalid InstId if no matching impl is found.
-static auto LookupInterfaceWitness(Context& context, SemIR::LocId loc_id,
-                                   SemIR::ConstantId type_const_id,
-                                   SemIR::ConstantId interface_const_id)
-    -> SemIR::InstId {
-  // TODO: Add a better impl lookup system. At the very least, we should only be
-  // considering impls that are for the same interface we're querying. We can
-  // also skip impls that mention any types that aren't part of our impl query.
-  for (const auto& impl : context.impls().array_ref()) {
-    auto specific_id = SemIR::SpecificId::Invalid;
-    if (impl.generic_id.is_valid()) {
-      specific_id = DeduceImplArguments(context, loc_id, impl, type_const_id,
-                                        interface_const_id);
-      if (!specific_id.is_valid()) {
-        continue;
-      }
-    }
-    if (!context.constant_values().AreEqualAcrossDeclarations(
-            SemIR::GetConstantValueInSpecific(context.sem_ir(), specific_id,
-                                              impl.self_id),
-            type_const_id)) {
-      continue;
-    }
-    if (!context.constant_values().AreEqualAcrossDeclarations(
-            SemIR::GetConstantValueInSpecific(context.sem_ir(), specific_id,
-                                              impl.constraint_id),
-            interface_const_id)) {
-      // TODO: An impl of a constraint type should be treated as implementing
-      // the constraint's interfaces.
-      continue;
-    }
-    if (!impl.witness_id.is_valid()) {
-      // TODO: Diagnose if the impl isn't defined yet?
-      return SemIR::InstId::Invalid;
-    }
-    LoadImportRef(context, impl.witness_id);
-    if (specific_id.is_valid()) {
-      // We need a definition of the specific `impl` so we can access its
-      // witness.
-      ResolveSpecificDefinition(context, specific_id);
-    }
-    return context.constant_values().GetInstId(
-        SemIR::GetConstantValueInSpecific(context.sem_ir(), specific_id,
-                                          impl.witness_id));
-  }
-  return SemIR::InstId::Invalid;
-}
-
 // Performs impl lookup for a member name expression. This finds the relevant
 // impl witness and extracts the corresponding impl member.
 static auto PerformImplLookup(