Assign locations to instructions in a generic eval block. (#4393)

The locations point to the first instruction in the generic that needed
the relevant constant value or type.

For now, this must makes the formatted SemIR a bit more useful, but in
the future it will also provide locations for diagnostics caused by
monomorphization failure.

toolchain/check/generic.cpp

@@ -9,6 +9,7 @@
 #include "toolchain/check/generic_region_stack.h"
 #include "toolchain/check/subst.h"
 #include "toolchain/sem_ir/ids.h"
+#include "toolchain/sem_ir/inst.h"
 
 namespace Carbon::Check {
 
@@ -59,11 +60,12 @@ class RebuildGenericConstantInEvalBlockCallbacks final
  public:
   RebuildGenericConstantInEvalBlockCallbacks(
       Context& context, SemIR::GenericId generic_id,
-      SemIR::GenericInstIndex::Region region,
+      SemIR::GenericInstIndex::Region region, SemIR::LocId loc_id,
       ConstantsInGenericMap& constants_in_generic)
       : context_(context),
         generic_id_(generic_id),
         region_(region),
+        loc_id_(loc_id),
         constants_in_generic_(constants_in_generic) {}
 
   // Check for instructions for which we already have a mapping into the eval
@@ -121,10 +123,10 @@ class RebuildGenericConstantInEvalBlockCallbacks final
       // TODO: Add a function on `Context` to add the instruction without
       // inserting it into the dependent instructions list or computing a
       // constant value for it.
-      // TODO: Provide a location based on the location of the instruction
-      // that uses the constant.
+      // TODO: Is the location we pick here always appropriate for the new
+      // instruction?
       auto inst_id = context_.sem_ir().insts().AddInNoBlock(
-          SemIR::LocIdAndInst::NoLoc(new_inst));
+          SemIR::LocIdAndInst::UncheckedLoc(loc_id_, new_inst));
       auto const_id = AddGenericConstantInstToEvalBlock(
           context_, generic_id_, region_, const_inst_id, inst_id);
       context_.constant_values().Set(inst_id, const_id);
@@ -137,6 +139,7 @@ class RebuildGenericConstantInEvalBlockCallbacks final
   Context& context_;
   SemIR::GenericId generic_id_;
   SemIR::GenericInstIndex::Region region_;
+  SemIR::LocId loc_id_;
   ConstantsInGenericMap& constants_in_generic_;
 };
 }  // namespace
@@ -147,7 +150,7 @@ class RebuildGenericConstantInEvalBlockCallbacks final
 // type in each specific.
 static auto AddGenericTypeToEvalBlock(
     Context& context, SemIR::GenericId generic_id,
-    SemIR::GenericInstIndex::Region region,
+    SemIR::GenericInstIndex::Region region, SemIR::LocId loc_id,
     ConstantsInGenericMap& constants_in_generic, SemIR::TypeId type_id)
     -> SemIR::TypeId {
   // Substitute into the type's constant instruction and rebuild it in the eval
@@ -155,7 +158,7 @@ static auto AddGenericTypeToEvalBlock(
   auto type_inst_id =
       SubstInst(context, context.types().GetInstId(type_id),
                 RebuildGenericConstantInEvalBlockCallbacks(
-                    context, generic_id, region, constants_in_generic));
+                    context, generic_id, region, loc_id, constants_in_generic));
   return context.GetTypeIdForTypeInst(type_inst_id);
 }
 
@@ -174,7 +177,8 @@ static auto AddGenericConstantToEvalBlock(
   auto new_inst_id =
       SubstInst(context, const_inst_id,
                 RebuildGenericConstantInEvalBlockCallbacks(
-                    context, generic_id, region, constants_in_generic));
+                    context, generic_id, region,
+                    context.insts().GetLocId(inst_id), constants_in_generic));
   CARBON_CHECK(new_inst_id != const_inst_id,
                "Did not apply any substitutions to symbolic constant {0}",
                context.insts().Get(const_inst_id));
@@ -227,7 +231,8 @@ static auto MakeGenericEvalBlock(Context& context, SemIR::GenericId generic_id,
         GenericRegionStack::DependencyKind::None) {
       auto inst = context.insts().Get(inst_id);
       auto type_id = AddGenericTypeToEvalBlock(
-          context, generic_id, region, constants_in_generic, inst.type_id());
+          context, generic_id, region, context.insts().GetLocId(inst_id),
+          constants_in_generic, inst.type_id());
       // TODO: Eventually, completeness requirements should be modeled as
       // constraints on the generic rather than properties of the type. For now,
       // require the transformed type to be complete if the original was.