Make use of new `ArrayRef::consume_*` functions. (#5975)

Minor code simplifications.

common/command_line.cpp

@@ -1218,8 +1218,7 @@ auto Parser::ParsePositionalSuffix(
   // arguments.
   bool empty_positional = false;
   while (!unparsed_args.empty()) {
-    llvm::StringRef unparsed_arg = unparsed_args.front();
-    unparsed_args = unparsed_args.drop_front();
+    llvm::StringRef unparsed_arg = unparsed_args.consume_front();
 
     if (unparsed_arg != "--") {
       CARBON_RETURN_IF_ERROR(ParsePositionalArg(unparsed_arg));
@@ -1260,11 +1259,9 @@ auto Parser::Parse(llvm::ArrayRef<llvm::StringRef> unparsed_args)
   PopulateMaps(*command_);
 
   while (!unparsed_args.empty()) {
-    llvm::StringRef unparsed_arg = unparsed_args.front();
-
     // Peak at the front for an exact `--` argument that switches to a
     // positional suffix parsing without dropping this argument.
-    if (unparsed_arg == "--") {
+    if (unparsed_args.front() == "--") {
       if (command_->positional_args.empty()) {
         return Error(
             "cannot meaningfully end option and subcommand arguments with a "
@@ -1284,7 +1281,7 @@ auto Parser::Parse(llvm::ArrayRef<llvm::StringRef> unparsed_args)
 
     // Now that we're not switching parse modes, drop the current unparsed
     // argument and parse it.
-    unparsed_args = unparsed_args.drop_front();
+    llvm::StringRef unparsed_arg = unparsed_args.consume_front();
 
     if (unparsed_arg.starts_with("--")) {
       // Note that the exact argument "--" has been handled above already.

testing/file_test/file_test_base.cpp

@@ -179,7 +179,7 @@ static auto RunAutoupdater(FileTestBase* test_base, const TestFile& test_file,
 
   llvm::ArrayRef expected_filenames = filenames;
   if (filenames.size() > 1) {
-    expected_filenames = expected_filenames.drop_front();
+    expected_filenames.consume_front();
   }
 
   return FileTestAutoupdater(

testing/file_test/file_test_base_test.cpp

@@ -76,7 +76,7 @@ static auto GetFilesFromArgs(llvm::ArrayRef<llvm::StringRef> args,
   if (args.empty() || args.front() != "default_args") {
     return ErrorBuilder() << "missing `default_args` argument";
   }
-  args = args.drop_front();
+  args.consume_front();
 
   for (auto arg : args) {
     if (!fs.exists(arg)) {

toolchain/check/function.cpp

@@ -140,7 +140,7 @@ auto CheckFunctionDefinitionSignature(Context& context,
                             SemIR::LocId(function.return_slot_pattern_id),
                             function, SemIR::SpecificId::None);
     // Don't re-check the return type below.
-    params_to_complete = params_to_complete.drop_back();
+    params_to_complete.consume_back();
   }
 
   // Check the parameter types are complete.

toolchain/check/handle_function.cpp

@@ -694,7 +694,7 @@ static auto IsValidBuiltinDeclaration(Context& context,
   // Find the list of call parameters other than the implicit return slot.
   auto call_params = context.inst_blocks().Get(function.call_params_id);
   if (function.return_slot_pattern_id.has_value()) {
-    call_params = call_params.drop_back();
+    call_params.consume_back();
   }
 
   // Form the list of parameter types for the declaration.

toolchain/lower/handle_call.cpp

@@ -516,8 +516,7 @@ auto HandleInst(FunctionContext& context, SemIR::InstId inst_id,
 
   auto inst_type = context.GetTypeIdOfInst(inst_id);
   if (context.GetReturnTypeInfo(inst_type).info.has_return_slot()) {
-    args.push_back(context.GetValue(arg_ids.back()));
-    arg_ids = arg_ids.drop_back();
+    args.push_back(context.GetValue(arg_ids.consume_back()));
   }
 
   for (auto arg_id : arg_ids) {

toolchain/sem_ir/formatter.cpp

@@ -1189,8 +1189,7 @@ auto Formatter::FormatCallRhs(Call inst) -> void {
   bool has_return_slot = return_info.has_return_slot();
   InstId return_slot_arg_id = InstId::None;
   if (has_return_slot) {
-    return_slot_arg_id = args.back();
-    args = args.drop_back();
+    return_slot_arg_id = args.consume_back();
   }
 
   llvm::ListSeparator sep;