Support deduction of the types of struct fields (#4500)

Follow-on to #4492 .

---------

Co-authored-by: Josh L <[email protected]>

Author: josh11b

toolchain/check/deduce.cpp

@@ -80,6 +80,28 @@ class DeductionWorklist {
     AddAll(context_.inst_blocks().Get(params), args, needs_substitution);
   }
 
+  auto AddAll(SemIR::StructTypeFieldsId params, SemIR::StructTypeFieldsId args,
+              bool needs_substitution) -> void {
+    const auto& param_fields = context_.struct_type_fields().Get(params);
+    const auto& arg_fields = context_.struct_type_fields().Get(args);
+    if (param_fields.size() != arg_fields.size()) {
+      // TODO: Decide whether to error on this or just treat the parameter list
+      // as non-deduced. For now we treat it as non-deduced.
+      return;
+    }
+    // Don't do deduction unless the names match in order.
+    // TODO: Support reordering of names.
+    for (auto [param, arg] : llvm::zip_equal(param_fields, arg_fields)) {
+      if (param.name_id != arg.name_id) {
+        return;
+      }
+    }
+    for (auto [param, arg] :
+         llvm::reverse(llvm::zip_equal(param_fields, arg_fields))) {
+      Add(param.type_id, arg.type_id, needs_substitution);
+    }
+  }
+
   auto AddAll(SemIR::InstBlockId params, SemIR::InstBlockId args,
               bool needs_substitution) -> void {
     AddAll(context_.inst_blocks().Get(params), context_.inst_blocks().Get(args),
@@ -107,6 +129,10 @@ class DeductionWorklist {
       case SemIR::IdKind::For<SemIR::TypeId>:
         Add(SemIR::TypeId(param), SemIR::TypeId(arg), needs_substitution);
         break;
+      case SemIR::IdKind::For<SemIR::StructTypeFieldsId>:
+        AddAll(SemIR::StructTypeFieldsId(param), SemIR::StructTypeFieldsId(arg),
+               needs_substitution);
+        break;
       case SemIR::IdKind::For<SemIR::InstBlockId>:
         AddAll(SemIR::InstBlockId(param), SemIR::InstBlockId(arg),
                needs_substitution);
@@ -378,6 +404,7 @@ auto DeductionContext::Deduce() -> bool {
       case SemIR::InterfaceType::Kind:
       case SemIR::IntType::Kind:
       case SemIR::PointerType::Kind:
+      case SemIR::StructType::Kind:
       case SemIR::TupleType::Kind:
       case SemIR::TupleValue::Kind: {
         auto arg_inst = context().insts().Get(arg_id);
@@ -392,7 +419,6 @@ auto DeductionContext::Deduce() -> bool {
         continue;
       }
 
-      case SemIR::StructType::Kind:
       case SemIR::StructValue::Kind:
         // TODO: Match field name order between param and arg.
         break;