1 use crate::coercion::CoerceMany;
2 use crate::fn_ctxt::arg_matrix::{ArgMatrix, Compatibility, Error, ExpectedIdx, ProvidedIdx};
3 use crate::gather_locals::Declaration;
4 use crate::method::MethodCallee;
5 use crate::Expectation::*;
6 use crate::TupleArgumentsFlag::*;
8 struct_span_err, BreakableCtxt, Diverges, Expectation, FnCtxt, LocalTy, Needs, RawTy,
12 use rustc_data_structures::fx::FxIndexSet;
13 use rustc_errors::{pluralize, Applicability, Diagnostic, DiagnosticId, MultiSpan};
15 use rustc_hir::def::{CtorOf, DefKind, Res};
16 use rustc_hir::def_id::DefId;
17 use rustc_hir::{ExprKind, Node, QPath};
18 use rustc_hir_analysis::astconv::AstConv;
19 use rustc_hir_analysis::check::intrinsicck::InlineAsmCtxt;
20 use rustc_hir_analysis::check::potentially_plural_count;
21 use rustc_hir_analysis::structured_errors::StructuredDiagnostic;
22 use rustc_index::vec::IndexVec;
23 use rustc_infer::infer::error_reporting::{FailureCode, ObligationCauseExt};
24 use rustc_infer::infer::type_variable::{TypeVariableOrigin, TypeVariableOriginKind};
25 use rustc_infer::infer::InferOk;
26 use rustc_infer::infer::TypeTrace;
27 use rustc_middle::ty::adjustment::AllowTwoPhase;
28 use rustc_middle::ty::visit::TypeVisitable;
29 use rustc_middle::ty::{self, DefIdTree, IsSuggestable, Ty, TypeSuperVisitable, TypeVisitor};
30 use rustc_session::Session;
31 use rustc_span::symbol::{kw, Ident};
32 use rustc_span::{self, sym, Span};
33 use rustc_trait_selection::traits::{self, ObligationCauseCode, SelectionContext};
37 use std::ops::ControlFlow;
40 impl<'a, 'tcx> FnCtxt<'a, 'tcx> {
41 pub(in super::super) fn check_casts(&mut self) {
42 // don't hold the borrow to deferred_cast_checks while checking to avoid borrow checker errors
43 // when writing to `self.param_env`.
44 let mut deferred_cast_checks = mem::take(&mut *self.deferred_cast_checks.borrow_mut());
46 debug!("FnCtxt::check_casts: {} deferred checks", deferred_cast_checks.len());
47 for cast in deferred_cast_checks.drain(..) {
48 let prev_env = self.param_env;
49 self.param_env = self.param_env.with_constness(cast.constness);
53 self.param_env = prev_env;
56 *self.deferred_cast_checks.borrow_mut() = deferred_cast_checks;
59 pub(in super::super) fn check_transmutes(&self) {
60 let mut deferred_transmute_checks = self.deferred_transmute_checks.borrow_mut();
61 debug!("FnCtxt::check_transmutes: {} deferred checks", deferred_transmute_checks.len());
62 for (from, to, hir_id) in deferred_transmute_checks.drain(..) {
63 self.check_transmute(from, to, hir_id);
67 pub(in super::super) fn check_asms(&self) {
68 let mut deferred_asm_checks = self.deferred_asm_checks.borrow_mut();
69 debug!("FnCtxt::check_asm: {} deferred checks", deferred_asm_checks.len());
70 for (asm, hir_id) in deferred_asm_checks.drain(..) {
71 let enclosing_id = self.tcx.hir().enclosing_body_owner(hir_id);
72 let get_operand_ty = |expr| {
73 let ty = self.typeck_results.borrow().expr_ty_adjusted(expr);
74 let ty = self.resolve_vars_if_possible(ty);
75 if ty.has_non_region_infer() {
78 self.tcx.erase_regions(ty)
81 InlineAsmCtxt::new_in_fn(self.tcx, self.param_env, get_operand_ty)
82 .check_asm(asm, self.tcx.hir().local_def_id_to_hir_id(enclosing_id));
86 pub(in super::super) fn check_method_argument_types(
89 expr: &'tcx hir::Expr<'tcx>,
90 method: Result<MethodCallee<'tcx>, ()>,
91 args_no_rcvr: &'tcx [hir::Expr<'tcx>],
92 tuple_arguments: TupleArgumentsFlag,
93 expected: Expectation<'tcx>,
95 let has_error = match method {
96 Ok(method) => method.substs.references_error() || method.sig.references_error(),
100 let err_inputs = self.err_args(args_no_rcvr.len());
102 let err_inputs = match tuple_arguments {
103 DontTupleArguments => err_inputs,
104 TupleArguments => vec![self.tcx.intern_tup(&err_inputs)],
107 self.check_argument_types(
115 method.ok().map(|method| method.def_id),
117 return self.tcx.ty_error();
120 let method = method.unwrap();
121 // HACK(eddyb) ignore self in the definition (see above).
122 let expected_input_tys = self.expected_inputs_for_expected_output(
126 &method.sig.inputs()[1..],
128 self.check_argument_types(
131 &method.sig.inputs()[1..],
134 method.sig.c_variadic,
142 /// Generic function that factors out common logic from function calls,
143 /// method calls and overloaded operators.
144 pub(in super::super) fn check_argument_types(
146 // Span enclosing the call site
148 // Expression of the call site
149 call_expr: &'tcx hir::Expr<'tcx>,
150 // Types (as defined in the *signature* of the target function)
151 formal_input_tys: &[Ty<'tcx>],
152 // More specific expected types, after unifying with caller output types
153 expected_input_tys: Option<Vec<Ty<'tcx>>>,
154 // The expressions for each provided argument
155 provided_args: &'tcx [hir::Expr<'tcx>],
156 // Whether the function is variadic, for example when imported from C
158 // Whether the arguments have been bundled in a tuple (ex: closures)
159 tuple_arguments: TupleArgumentsFlag,
160 // The DefId for the function being called, for better error messages
161 fn_def_id: Option<DefId>,
165 // Conceptually, we've got some number of expected inputs, and some number of provided arguments
166 // and we can form a grid of whether each argument could satisfy a given input:
167 // in1 | in2 | in3 | ...
172 // Initially, we just check the diagonal, because in the case of correct code
173 // these are the only checks that matter
174 // However, in the unhappy path, we'll fill in this whole grid to attempt to provide
175 // better error messages about invalid method calls.
177 // All the input types from the fn signature must outlive the call
178 // so as to validate implied bounds.
179 for (&fn_input_ty, arg_expr) in iter::zip(formal_input_tys, provided_args) {
180 self.register_wf_obligation(fn_input_ty.into(), arg_expr.span, traits::MiscObligation);
183 let mut err_code = "E0061";
185 // If the arguments should be wrapped in a tuple (ex: closures), unwrap them here
186 let (formal_input_tys, expected_input_tys) = if tuple_arguments == TupleArguments {
187 let tuple_type = self.structurally_resolved_type(call_span, formal_input_tys[0]);
188 match tuple_type.kind() {
189 // We expected a tuple and got a tuple
190 ty::Tuple(arg_types) => {
191 // Argument length differs
192 if arg_types.len() != provided_args.len() {
195 let expected_input_tys = match expected_input_tys {
196 Some(expected_input_tys) => match expected_input_tys.get(0) {
197 Some(ty) => match ty.kind() {
198 ty::Tuple(tys) => Some(tys.iter().collect()),
205 (arg_types.iter().collect(), expected_input_tys)
208 // Otherwise, there's a mismatch, so clear out what we're expecting, and set
209 // our input types to err_args so we don't blow up the error messages
214 "cannot use call notation; the first type parameter \
215 for the function trait is neither a tuple nor unit"
218 (self.err_args(provided_args.len()), None)
222 (formal_input_tys.to_vec(), expected_input_tys)
225 // If there are no external expectations at the call site, just use the types from the function defn
226 let expected_input_tys = if let Some(expected_input_tys) = expected_input_tys {
227 assert_eq!(expected_input_tys.len(), formal_input_tys.len());
230 formal_input_tys.clone()
233 let minimum_input_count = expected_input_tys.len();
234 let provided_arg_count = provided_args.len();
236 let is_const_eval_select = matches!(fn_def_id, Some(def_id) if
237 self.tcx.def_kind(def_id) == hir::def::DefKind::Fn
238 && self.tcx.is_intrinsic(def_id)
239 && self.tcx.item_name(def_id) == sym::const_eval_select);
241 // We introduce a helper function to demand that a given argument satisfy a given input
242 // This is more complicated than just checking type equality, as arguments could be coerced
243 // This version writes those types back so further type checking uses the narrowed types
244 let demand_compatible = |idx| {
245 let formal_input_ty: Ty<'tcx> = formal_input_tys[idx];
246 let expected_input_ty: Ty<'tcx> = expected_input_tys[idx];
247 let provided_arg = &provided_args[idx];
249 debug!("checking argument {}: {:?} = {:?}", idx, provided_arg, formal_input_ty);
251 // We're on the happy path here, so we'll do a more involved check and write back types
252 // To check compatibility, we'll do 3 things:
253 // 1. Unify the provided argument with the expected type
254 let expectation = Expectation::rvalue_hint(self, expected_input_ty);
256 let checked_ty = self.check_expr_with_expectation(provided_arg, expectation);
258 // 2. Coerce to the most detailed type that could be coerced
259 // to, which is `expected_ty` if `rvalue_hint` returns an
260 // `ExpectHasType(expected_ty)`, or the `formal_ty` otherwise.
261 let coerced_ty = expectation.only_has_type(self).unwrap_or(formal_input_ty);
263 // Cause selection errors caused by resolving a single argument to point at the
264 // argument and not the call. This lets us customize the span pointed to in the
265 // fulfillment error to be more accurate.
266 let coerced_ty = self.resolve_vars_with_obligations(coerced_ty);
268 let coerce_error = self
269 .try_coerce(provided_arg, checked_ty, coerced_ty, AllowTwoPhase::Yes, None)
272 if coerce_error.is_some() {
273 return Compatibility::Incompatible(coerce_error);
276 // Check that second and third argument of `const_eval_select` must be `FnDef`, and additionally that
277 // the second argument must be `const fn`. The first argument must be a tuple, but this is already expressed
278 // in the function signature (`F: FnOnce<ARG>`), so I did not bother to add another check here.
280 // This check is here because there is currently no way to express a trait bound for `FnDef` types only.
281 if is_const_eval_select && (1..=2).contains(&idx) {
282 if let ty::FnDef(def_id, _) = checked_ty.kind() {
283 if idx == 1 && !self.tcx.is_const_fn_raw(*def_id) {
286 .struct_span_err(provided_arg.span, "this argument must be a `const fn`")
287 .help("consult the documentation on `const_eval_select` for more information")
293 .struct_span_err(provided_arg.span, "this argument must be a function item")
294 .note(format!("expected a function item, found {checked_ty}"))
296 "consult the documentation on `const_eval_select` for more information",
302 // 3. Check if the formal type is a supertype of the checked one
303 // and register any such obligations for future type checks
304 let supertype_error = self
305 .at(&self.misc(provided_arg.span), self.param_env)
306 .sup(formal_input_ty, coerced_ty);
307 let subtyping_error = match supertype_error {
308 Ok(InferOk { obligations, value: () }) => {
309 self.register_predicates(obligations);
312 Err(err) => Some(err),
315 // If neither check failed, the types are compatible
316 match subtyping_error {
317 None => Compatibility::Compatible,
318 Some(_) => Compatibility::Incompatible(subtyping_error),
322 // To start, we only care "along the diagonal", where we expect every
323 // provided arg to be in the right spot
324 let mut compatibility_diagonal =
325 vec![Compatibility::Incompatible(None); provided_args.len()];
327 // Keep track of whether we *could possibly* be satisfied, i.e. whether we're on the happy path
328 // if the wrong number of arguments were supplied, we CAN'T be satisfied,
329 // and if we're c_variadic, the supplied arguments must be >= the minimum count from the function
330 // otherwise, they need to be identical, because rust doesn't currently support variadic functions
331 let mut call_appears_satisfied = if c_variadic {
332 provided_arg_count >= minimum_input_count
334 provided_arg_count == minimum_input_count
337 // Check the arguments.
338 // We do this in a pretty awful way: first we type-check any arguments
339 // that are not closures, then we type-check the closures. This is so
340 // that we have more information about the types of arguments when we
341 // type-check the functions. This isn't really the right way to do this.
342 for check_closures in [false, true] {
343 // More awful hacks: before we check argument types, try to do
344 // an "opportunistic" trait resolution of any trait bounds on
345 // the call. This helps coercions.
347 self.select_obligations_where_possible(|_| {})
350 // Check each argument, to satisfy the input it was provided for
351 // Visually, we're traveling down the diagonal of the compatibility matrix
352 for (idx, arg) in provided_args.iter().enumerate() {
353 // Warn only for the first loop (the "no closures" one).
354 // Closure arguments themselves can't be diverging, but
355 // a previous argument can, e.g., `foo(panic!(), || {})`.
357 self.warn_if_unreachable(arg.hir_id, arg.span, "expression");
360 // For C-variadic functions, we don't have a declared type for all of
361 // the arguments hence we only do our usual type checking with
362 // the arguments who's types we do know. However, we *can* check
363 // for unreachable expressions (see above).
364 // FIXME: unreachable warning current isn't emitted
365 if idx >= minimum_input_count {
369 let is_closure = matches!(arg.kind, ExprKind::Closure { .. });
370 if is_closure != check_closures {
374 let compatible = demand_compatible(idx);
375 let is_compatible = matches!(compatible, Compatibility::Compatible);
376 compatibility_diagonal[idx] = compatible;
379 call_appears_satisfied = false;
384 if c_variadic && provided_arg_count < minimum_input_count {
388 for arg in provided_args.iter().skip(minimum_input_count) {
389 // Make sure we've checked this expr at least once.
390 let arg_ty = self.check_expr(&arg);
392 // If the function is c-style variadic, we skipped a bunch of arguments
393 // so we need to check those, and write out the types
394 // Ideally this would be folded into the above, for uniform style
395 // but c-variadic is already a corner case
397 fn variadic_error<'tcx>(
403 use rustc_hir_analysis::structured_errors::MissingCastForVariadicArg;
405 MissingCastForVariadicArg { sess, span, ty, cast_ty }.diagnostic().emit();
408 // There are a few types which get autopromoted when passed via varargs
409 // in C but we just error out instead and require explicit casts.
410 let arg_ty = self.structurally_resolved_type(arg.span, arg_ty);
411 match arg_ty.kind() {
412 ty::Float(ty::FloatTy::F32) => {
413 variadic_error(tcx.sess, arg.span, arg_ty, "c_double");
415 ty::Int(ty::IntTy::I8 | ty::IntTy::I16) | ty::Bool => {
416 variadic_error(tcx.sess, arg.span, arg_ty, "c_int");
418 ty::Uint(ty::UintTy::U8 | ty::UintTy::U16) => {
419 variadic_error(tcx.sess, arg.span, arg_ty, "c_uint");
422 let ptr_ty = self.tcx.mk_fn_ptr(arg_ty.fn_sig(self.tcx));
423 let ptr_ty = self.resolve_vars_if_possible(ptr_ty);
424 variadic_error(tcx.sess, arg.span, arg_ty, &ptr_ty.to_string());
431 if !call_appears_satisfied {
432 let compatibility_diagonal = IndexVec::from_raw(compatibility_diagonal);
433 let provided_args = IndexVec::from_iter(provided_args.iter().take(if c_variadic {
439 formal_input_tys.len(),
440 expected_input_tys.len(),
441 "expected formal_input_tys to be the same size as expected_input_tys"
443 let formal_and_expected_inputs = IndexVec::from_iter(
447 .zip(expected_input_tys.iter().copied())
448 .map(|vars| self.resolve_vars_if_possible(vars)),
451 self.report_arg_errors(
452 compatibility_diagonal,
453 formal_and_expected_inputs,
464 fn report_arg_errors(
466 compatibility_diagonal: IndexVec<ProvidedIdx, Compatibility<'tcx>>,
467 formal_and_expected_inputs: IndexVec<ExpectedIdx, (Ty<'tcx>, Ty<'tcx>)>,
468 provided_args: IndexVec<ProvidedIdx, &'tcx hir::Expr<'tcx>>,
471 fn_def_id: Option<DefId>,
473 call_expr: &hir::Expr<'tcx>,
475 // Next, let's construct the error
476 let (error_span, full_call_span, call_name, is_method) = match &call_expr.kind {
478 hir::Expr { hir_id, span, kind: hir::ExprKind::Path(qpath), .. },
481 if let Res::Def(DefKind::Ctor(of, _), _) =
482 self.typeck_results.borrow().qpath_res(qpath, *hir_id)
484 let name = match of {
485 CtorOf::Struct => "struct",
486 CtorOf::Variant => "enum variant",
488 (call_span, *span, name, false)
490 (call_span, *span, "function", false)
493 hir::ExprKind::Call(hir::Expr { span, .. }, _) => (call_span, *span, "function", false),
494 hir::ExprKind::MethodCall(path_segment, _, _, span) => {
495 let ident_span = path_segment.ident.span;
496 let ident_span = if let Some(args) = path_segment.args {
497 ident_span.with_hi(args.span_ext.hi())
501 (*span, ident_span, "method", true)
503 k => span_bug!(call_span, "checking argument types on a non-call: `{:?}`", k),
505 let args_span = error_span.trim_start(full_call_span).unwrap_or(error_span);
507 // Don't print if it has error types or is just plain `_`
508 fn has_error_or_infer<'tcx>(tys: impl IntoIterator<Item = Ty<'tcx>>) -> bool {
509 tys.into_iter().any(|ty| ty.references_error() || ty.is_ty_var())
513 // FIXME: taint after emitting errors and pass through an `ErrorGuaranteed`
514 self.set_tainted_by_errors(
515 tcx.sess.delay_span_bug(call_span, "no errors reported for args"),
518 // Get the argument span in the context of the call span so that
519 // suggestions and labels are (more) correct when an arg is a
521 let normalize_span = |span: Span| -> Span {
522 let normalized_span = span.find_ancestor_inside(error_span).unwrap_or(span);
523 // Sometimes macros mess up the spans, so do not normalize the
524 // arg span to equal the error span, because that's less useful
525 // than pointing out the arg expr in the wrong context.
526 if normalized_span.source_equal(error_span) { span } else { normalized_span }
529 // Precompute the provided types and spans, since that's all we typically need for below
530 let provided_arg_tys: IndexVec<ProvidedIdx, (Ty<'tcx>, Span)> = provided_args
536 .expr_ty_adjusted_opt(*expr)
537 .unwrap_or_else(|| tcx.ty_error());
538 (self.resolve_vars_if_possible(ty), normalize_span(expr.span))
541 let callee_expr = match &call_expr.peel_blocks().kind {
542 hir::ExprKind::Call(callee, _) => Some(*callee),
543 hir::ExprKind::MethodCall(_, receiver, ..) => {
544 if let Some((DefKind::AssocFn, def_id)) =
545 self.typeck_results.borrow().type_dependent_def(call_expr.hir_id)
546 && let Some(assoc) = tcx.opt_associated_item(def_id)
547 && assoc.fn_has_self_parameter
556 let callee_ty = callee_expr
557 .and_then(|callee_expr| self.typeck_results.borrow().expr_ty_adjusted_opt(callee_expr));
559 // A "softer" version of the `demand_compatible`, which checks types without persisting them,
560 // and treats error types differently
561 // This will allow us to "probe" for other argument orders that would likely have been correct
562 let check_compatible = |provided_idx: ProvidedIdx, expected_idx: ExpectedIdx| {
563 if provided_idx.as_usize() == expected_idx.as_usize() {
564 return compatibility_diagonal[provided_idx].clone();
567 let (formal_input_ty, expected_input_ty) = formal_and_expected_inputs[expected_idx];
568 // If either is an error type, we defy the usual convention and consider them to *not* be
569 // coercible. This prevents our error message heuristic from trying to pass errors into
571 if (formal_input_ty, expected_input_ty).references_error() {
572 return Compatibility::Incompatible(None);
575 let (arg_ty, arg_span) = provided_arg_tys[provided_idx];
577 let expectation = Expectation::rvalue_hint(self, expected_input_ty);
578 let coerced_ty = expectation.only_has_type(self).unwrap_or(formal_input_ty);
579 let can_coerce = self.can_coerce(arg_ty, coerced_ty);
581 return Compatibility::Incompatible(Some(ty::error::TypeError::Sorts(
582 ty::error::ExpectedFound::new(true, coerced_ty, arg_ty),
586 // Using probe here, since we don't want this subtyping to affect inference.
587 let subtyping_error = self.probe(|_| {
588 self.at(&self.misc(arg_span), self.param_env).sup(formal_input_ty, coerced_ty).err()
591 // Same as above: if either the coerce type or the checked type is an error type,
592 // consider them *not* compatible.
593 let references_error = (coerced_ty, arg_ty).references_error();
594 match (references_error, subtyping_error) {
595 (false, None) => Compatibility::Compatible,
596 (_, subtyping_error) => Compatibility::Incompatible(subtyping_error),
600 let mk_trace = |span, (formal_ty, expected_ty), provided_ty| {
601 let mismatched_ty = if expected_ty == provided_ty {
602 // If expected == provided, then we must have failed to sup
603 // the formal type. Avoid printing out "expected Ty, found Ty"
609 TypeTrace::types(&self.misc(span), true, mismatched_ty, provided_ty)
612 // The algorithm here is inspired by levenshtein distance and longest common subsequence.
613 // We'll try to detect 4 different types of mistakes:
614 // - An extra parameter has been provided that doesn't satisfy *any* of the other inputs
615 // - An input is missing, which isn't satisfied by *any* of the other arguments
616 // - Some number of arguments have been provided in the wrong order
617 // - A type is straight up invalid
619 // First, let's find the errors
620 let (mut errors, matched_inputs) =
621 ArgMatrix::new(provided_args.len(), formal_and_expected_inputs.len(), check_compatible)
624 // First, check if we just need to wrap some arguments in a tuple.
625 if let Some((mismatch_idx, terr)) =
626 compatibility_diagonal.iter().enumerate().find_map(|(i, c)| {
627 if let Compatibility::Incompatible(Some(terr)) = c {
634 // Is the first bad expected argument a tuple?
635 // Do we have as many extra provided arguments as the tuple's length?
636 // If so, we might have just forgotten to wrap some args in a tuple.
637 if let Some(ty::Tuple(tys)) =
638 formal_and_expected_inputs.get(mismatch_idx.into()).map(|tys| tys.1.kind())
639 // If the tuple is unit, we're not actually wrapping any arguments.
641 && provided_arg_tys.len() == formal_and_expected_inputs.len() - 1 + tys.len()
643 // Wrap up the N provided arguments starting at this position in a tuple.
644 let provided_as_tuple = tcx.mk_tup(
645 provided_arg_tys.iter().map(|(ty, _)| *ty).skip(mismatch_idx).take(tys.len()),
648 let mut satisfied = true;
649 // Check if the newly wrapped tuple + rest of the arguments are compatible.
650 for ((_, expected_ty), provided_ty) in std::iter::zip(
651 formal_and_expected_inputs.iter().skip(mismatch_idx),
652 [provided_as_tuple].into_iter().chain(
653 provided_arg_tys.iter().map(|(ty, _)| *ty).skip(mismatch_idx + tys.len()),
656 if !self.can_coerce(provided_ty, *expected_ty) {
662 // If they're compatible, suggest wrapping in an arg, and we're done!
663 // Take some care with spans, so we don't suggest wrapping a macro's
664 // innards in parenthesis, for example.
666 && let Some((_, lo)) =
667 provided_arg_tys.get(ProvidedIdx::from_usize(mismatch_idx))
668 && let Some((_, hi)) =
669 provided_arg_tys.get(ProvidedIdx::from_usize(mismatch_idx + tys.len() - 1))
673 // A tuple wrap suggestion actually occurs within,
674 // so don't do anything special here.
675 err = self.err_ctxt().report_and_explain_type_error(
678 formal_and_expected_inputs[mismatch_idx.into()],
679 provided_arg_tys[mismatch_idx.into()].0,
685 format!("arguments to this {} are incorrect", call_name),
688 err = tcx.sess.struct_span_err_with_code(
691 "{call_name} takes {}{} but {} {} supplied",
692 if c_variadic { "at least " } else { "" },
693 potentially_plural_count(
694 formal_and_expected_inputs.len(),
697 potentially_plural_count(provided_args.len(), "argument"),
698 pluralize!("was", provided_args.len())
700 DiagnosticId::Error(err_code.to_owned()),
702 err.multipart_suggestion_verbose(
703 "wrap these arguments in parentheses to construct a tuple",
705 (lo.shrink_to_lo(), "(".to_string()),
706 (hi.shrink_to_hi(), ")".to_string()),
708 Applicability::MachineApplicable,
724 // Okay, so here's where it gets complicated in regards to what errors
726 // There are 3 different "types" of errors we might encounter.
727 // 1) Missing/extra/swapped arguments
728 // 2) Valid but incorrect arguments
729 // 3) Invalid arguments
730 // - Currently I think this only comes up with `CyclicTy`
732 // We first need to go through, remove those from (3) and emit those
733 // as their own error, particularly since they're error code and
734 // message is special. From what I can tell, we *must* emit these
735 // here (vs somewhere prior to this function) since the arguments
736 // become invalid *because* of how they get used in the function.
739 if errors.is_empty() {
740 if cfg!(debug_assertions) {
741 span_bug!(error_span, "expected errors from argument matrix");
746 "argument type mismatch was detected, \
747 but rustc had trouble determining where",
750 "we would appreciate a bug report: \
751 https://github.com/rust-lang/rust/issues/new",
758 errors.drain_filter(|error| {
759 let Error::Invalid(provided_idx, expected_idx, Compatibility::Incompatible(Some(e))) = error else { return false };
760 let (provided_ty, provided_span) = provided_arg_tys[*provided_idx];
761 let trace = mk_trace(provided_span, formal_and_expected_inputs[*expected_idx], provided_ty);
762 if !matches!(trace.cause.as_failure_code(*e), FailureCode::Error0308(_)) {
763 self.err_ctxt().report_and_explain_type_error(trace, *e).emit();
769 // We're done if we found errors, but we already emitted them.
770 if errors.is_empty() {
774 // Okay, now that we've emitted the special errors separately, we
775 // are only left missing/extra/swapped and mismatched arguments, both
776 // can be collated pretty easily if needed.
778 // Next special case: if there is only one "Incompatible" error, just emit that
780 Error::Invalid(provided_idx, expected_idx, Compatibility::Incompatible(Some(err))),
783 let (formal_ty, expected_ty) = formal_and_expected_inputs[*expected_idx];
784 let (provided_ty, provided_arg_span) = provided_arg_tys[*provided_idx];
785 let trace = mk_trace(provided_arg_span, (formal_ty, expected_ty), provided_ty);
786 let mut err = self.err_ctxt().report_and_explain_type_error(trace, *err);
787 self.emit_coerce_suggestions(
789 &provided_args[*provided_idx],
791 Expectation::rvalue_hint(self, expected_ty)
793 .unwrap_or(formal_ty),
799 format!("arguments to this {} are incorrect", call_name),
801 if let (Some(callee_ty), hir::ExprKind::MethodCall(_, rcvr, _, _)) =
802 (callee_ty, &call_expr.kind)
804 // Type that would have accepted this argument if it hadn't been inferred earlier.
805 // FIXME: We leave an inference variable for now, but it'd be nice to get a more
806 // specific type to increase the accuracy of the diagnostic.
807 let expected = self.infcx.next_ty_var(TypeVariableOrigin {
808 kind: TypeVariableOriginKind::MiscVariable,
809 span: full_call_span,
811 self.point_at_expr_source_of_inferred_type(
819 // Call out where the function is defined
824 Some(expected_idx.as_usize()),
831 let mut err = if formal_and_expected_inputs.len() == provided_args.len() {
836 "arguments to this {} are incorrect",
840 tcx.sess.struct_span_err_with_code(
843 "this {} takes {}{} but {} {} supplied",
845 if c_variadic { "at least " } else { "" },
846 potentially_plural_count(formal_and_expected_inputs.len(), "argument"),
847 potentially_plural_count(provided_args.len(), "argument"),
848 pluralize!("was", provided_args.len())
850 DiagnosticId::Error(err_code.to_owned()),
854 // As we encounter issues, keep track of what we want to provide for the suggestion
855 let mut labels = vec![];
856 // If there is a single error, we give a specific suggestion; otherwise, we change to
857 // "did you mean" with the suggested function call
858 enum SuggestionText {
866 let mut suggestion_text = SuggestionText::None;
868 let mut errors = errors.into_iter().peekable();
869 while let Some(error) = errors.next() {
871 Error::Invalid(provided_idx, expected_idx, compatibility) => {
872 let (formal_ty, expected_ty) = formal_and_expected_inputs[expected_idx];
873 let (provided_ty, provided_span) = provided_arg_tys[provided_idx];
874 if let Compatibility::Incompatible(error) = compatibility {
875 let trace = mk_trace(provided_span, (formal_ty, expected_ty), provided_ty);
876 if let Some(e) = error {
877 self.err_ctxt().note_type_err(
889 self.emit_coerce_suggestions(
891 &provided_args[provided_idx],
893 Expectation::rvalue_hint(self, expected_ty)
895 .unwrap_or(formal_ty),
900 Error::Extra(arg_idx) => {
901 let (provided_ty, provided_span) = provided_arg_tys[arg_idx];
902 let provided_ty_name = if !has_error_or_infer([provided_ty]) {
903 // FIXME: not suggestable, use something else
904 format!(" of type `{}`", provided_ty)
909 .push((provided_span, format!("argument{} unexpected", provided_ty_name)));
910 suggestion_text = match suggestion_text {
911 SuggestionText::None => SuggestionText::Remove(false),
912 SuggestionText::Remove(_) => SuggestionText::Remove(true),
913 _ => SuggestionText::DidYouMean,
916 Error::Missing(expected_idx) => {
917 // If there are multiple missing arguments adjacent to each other,
918 // then we can provide a single error.
920 let mut missing_idxs = vec![expected_idx];
921 while let Some(e) = errors.next_if(|e| {
922 matches!(e, Error::Missing(next_expected_idx)
923 if *next_expected_idx == *missing_idxs.last().unwrap() + 1)
926 Error::Missing(expected_idx) => missing_idxs.push(expected_idx),
931 // NOTE: Because we might be re-arranging arguments, might have extra
932 // arguments, etc. it's hard to *really* know where we should provide
933 // this error label, so as a heuristic, we point to the provided arg, or
934 // to the call if the missing inputs pass the provided args.
935 match &missing_idxs[..] {
937 let (_, input_ty) = formal_and_expected_inputs[expected_idx];
938 let span = if let Some((_, arg_span)) =
939 provided_arg_tys.get(expected_idx.to_provided_idx())
945 let rendered = if !has_error_or_infer([input_ty]) {
946 format!(" of type `{}`", input_ty)
950 labels.push((span, format!("an argument{} is missing", rendered)));
951 suggestion_text = match suggestion_text {
952 SuggestionText::None => SuggestionText::Provide(false),
953 SuggestionText::Provide(_) => SuggestionText::Provide(true),
954 _ => SuggestionText::DidYouMean,
957 &[first_idx, second_idx] => {
958 let (_, first_expected_ty) = formal_and_expected_inputs[first_idx];
959 let (_, second_expected_ty) = formal_and_expected_inputs[second_idx];
960 let span = if let (Some((_, first_span)), Some((_, second_span))) = (
961 provided_arg_tys.get(first_idx.to_provided_idx()),
962 provided_arg_tys.get(second_idx.to_provided_idx()),
964 first_span.to(*second_span)
969 if !has_error_or_infer([first_expected_ty, second_expected_ty]) {
971 " of type `{}` and `{}`",
972 first_expected_ty, second_expected_ty
977 labels.push((span, format!("two arguments{} are missing", rendered)));
978 suggestion_text = match suggestion_text {
979 SuggestionText::None | SuggestionText::Provide(_) => {
980 SuggestionText::Provide(true)
982 _ => SuggestionText::DidYouMean,
985 &[first_idx, second_idx, third_idx] => {
986 let (_, first_expected_ty) = formal_and_expected_inputs[first_idx];
987 let (_, second_expected_ty) = formal_and_expected_inputs[second_idx];
988 let (_, third_expected_ty) = formal_and_expected_inputs[third_idx];
989 let span = if let (Some((_, first_span)), Some((_, third_span))) = (
990 provided_arg_tys.get(first_idx.to_provided_idx()),
991 provided_arg_tys.get(third_idx.to_provided_idx()),
993 first_span.to(*third_span)
997 let rendered = if !has_error_or_infer([
1003 " of type `{}`, `{}`, and `{}`",
1004 first_expected_ty, second_expected_ty, third_expected_ty
1009 labels.push((span, format!("three arguments{} are missing", rendered)));
1010 suggestion_text = match suggestion_text {
1011 SuggestionText::None | SuggestionText::Provide(_) => {
1012 SuggestionText::Provide(true)
1014 _ => SuggestionText::DidYouMean,
1018 let first_idx = *missing_idxs.first().unwrap();
1019 let last_idx = *missing_idxs.last().unwrap();
1020 // NOTE: Because we might be re-arranging arguments, might have extra arguments, etc.
1021 // It's hard to *really* know where we should provide this error label, so this is a
1023 let span = if let (Some((_, first_span)), Some((_, last_span))) = (
1024 provided_arg_tys.get(first_idx.to_provided_idx()),
1025 provided_arg_tys.get(last_idx.to_provided_idx()),
1027 first_span.to(*last_span)
1031 labels.push((span, "multiple arguments are missing".to_string()));
1032 suggestion_text = match suggestion_text {
1033 SuggestionText::None | SuggestionText::Provide(_) => {
1034 SuggestionText::Provide(true)
1036 _ => SuggestionText::DidYouMean,
1043 second_provided_idx,
1045 second_expected_idx,
1047 let (first_provided_ty, first_span) = provided_arg_tys[first_provided_idx];
1048 let (_, first_expected_ty) = formal_and_expected_inputs[first_expected_idx];
1049 let first_provided_ty_name = if !has_error_or_infer([first_provided_ty]) {
1050 format!(", found `{}`", first_provided_ty)
1056 format!("expected `{}`{}", first_expected_ty, first_provided_ty_name),
1059 let (second_provided_ty, second_span) = provided_arg_tys[second_provided_idx];
1060 let (_, second_expected_ty) = formal_and_expected_inputs[second_expected_idx];
1061 let second_provided_ty_name = if !has_error_or_infer([second_provided_ty]) {
1062 format!(", found `{}`", second_provided_ty)
1068 format!("expected `{}`{}", second_expected_ty, second_provided_ty_name),
1071 suggestion_text = match suggestion_text {
1072 SuggestionText::None => SuggestionText::Swap,
1073 _ => SuggestionText::DidYouMean,
1076 Error::Permutation(args) => {
1077 for (dst_arg, dest_input) in args {
1078 let (_, expected_ty) = formal_and_expected_inputs[dst_arg];
1079 let (provided_ty, provided_span) = provided_arg_tys[dest_input];
1080 let provided_ty_name = if !has_error_or_infer([provided_ty]) {
1081 format!(", found `{}`", provided_ty)
1087 format!("expected `{}`{}", expected_ty, provided_ty_name),
1091 suggestion_text = match suggestion_text {
1092 SuggestionText::None => SuggestionText::Reorder,
1093 _ => SuggestionText::DidYouMean,
1099 // If we have less than 5 things to say, it would be useful to call out exactly what's wrong
1100 if labels.len() <= 5 {
1101 for (span, label) in labels {
1102 err.span_label(span, label);
1106 // Call out where the function is defined
1107 self.label_fn_like(&mut err, fn_def_id, callee_ty, None, is_method);
1109 // And add a suggestion block for all of the parameters
1110 let suggestion_text = match suggestion_text {
1111 SuggestionText::None => None,
1112 SuggestionText::Provide(plural) => {
1113 Some(format!("provide the argument{}", if plural { "s" } else { "" }))
1115 SuggestionText::Remove(plural) => {
1116 Some(format!("remove the extra argument{}", if plural { "s" } else { "" }))
1118 SuggestionText::Swap => Some("swap these arguments".to_string()),
1119 SuggestionText::Reorder => Some("reorder these arguments".to_string()),
1120 SuggestionText::DidYouMean => Some("did you mean".to_string()),
1122 if let Some(suggestion_text) = suggestion_text {
1123 let source_map = self.sess().source_map();
1124 let (mut suggestion, suggestion_span) =
1125 if let Some(call_span) = full_call_span.find_ancestor_inside(error_span) {
1126 ("(".to_string(), call_span.shrink_to_hi().to(error_span.shrink_to_hi()))
1131 source_map.span_to_snippet(full_call_span).unwrap_or_else(|_| {
1132 fn_def_id.map_or("".to_string(), |fn_def_id| {
1133 tcx.item_name(fn_def_id).to_string()
1140 let mut needs_comma = false;
1141 for (expected_idx, provided_idx) in matched_inputs.iter_enumerated() {
1147 let suggestion_text = if let Some(provided_idx) = provided_idx
1148 && let (_, provided_span) = provided_arg_tys[*provided_idx]
1149 && let Ok(arg_text) = source_map.span_to_snippet(provided_span)
1153 // Propose a placeholder of the correct type
1154 let (_, expected_ty) = formal_and_expected_inputs[expected_idx];
1155 if expected_ty.is_unit() {
1157 } else if expected_ty.is_suggestable(tcx, false) {
1158 format!("/* {} */", expected_ty)
1159 } else if let Some(fn_def_id) = fn_def_id
1160 && self.tcx.def_kind(fn_def_id).is_fn_like()
1161 && let self_implicit = matches!(call_expr.kind, hir::ExprKind::MethodCall(..)) as usize
1162 && let Some(arg) = self.tcx.fn_arg_names(fn_def_id).get(expected_idx.as_usize() + self_implicit)
1163 && arg.name != kw::SelfLower
1165 format!("/* {} */", arg.name)
1167 "/* value */".to_string()
1170 suggestion += &suggestion_text;
1173 err.span_suggestion_verbose(
1177 Applicability::HasPlaceholders,
1184 // AST fragment checking
1185 pub(in super::super) fn check_lit(
1188 expected: Expectation<'tcx>,
1193 ast::LitKind::Str(..) => tcx.mk_static_str(),
1194 ast::LitKind::ByteStr(ref v, _) => {
1195 tcx.mk_imm_ref(tcx.lifetimes.re_static, tcx.mk_array(tcx.types.u8, v.len() as u64))
1197 ast::LitKind::Byte(_) => tcx.types.u8,
1198 ast::LitKind::Char(_) => tcx.types.char,
1199 ast::LitKind::Int(_, ast::LitIntType::Signed(t)) => tcx.mk_mach_int(ty::int_ty(t)),
1200 ast::LitKind::Int(_, ast::LitIntType::Unsigned(t)) => tcx.mk_mach_uint(ty::uint_ty(t)),
1201 ast::LitKind::Int(_, ast::LitIntType::Unsuffixed) => {
1202 let opt_ty = expected.to_option(self).and_then(|ty| match ty.kind() {
1203 ty::Int(_) | ty::Uint(_) => Some(ty),
1204 ty::Char => Some(tcx.types.u8),
1205 ty::RawPtr(..) => Some(tcx.types.usize),
1206 ty::FnDef(..) | ty::FnPtr(_) => Some(tcx.types.usize),
1209 opt_ty.unwrap_or_else(|| self.next_int_var())
1211 ast::LitKind::Float(_, ast::LitFloatType::Suffixed(t)) => {
1212 tcx.mk_mach_float(ty::float_ty(t))
1214 ast::LitKind::Float(_, ast::LitFloatType::Unsuffixed) => {
1215 let opt_ty = expected.to_option(self).and_then(|ty| match ty.kind() {
1216 ty::Float(_) => Some(ty),
1219 opt_ty.unwrap_or_else(|| self.next_float_var())
1221 ast::LitKind::Bool(_) => tcx.types.bool,
1222 ast::LitKind::Err => tcx.ty_error(),
1226 pub fn check_struct_path(
1230 ) -> Option<(&'tcx ty::VariantDef, Ty<'tcx>)> {
1231 let path_span = qpath.span();
1232 let (def, ty) = self.finish_resolving_struct_path(qpath, path_span, hir_id);
1233 let variant = match def {
1235 self.set_tainted_by_errors(
1236 self.tcx.sess.delay_span_bug(path_span, "`Res::Err` but no error emitted"),
1240 Res::Def(DefKind::Variant, _) => match ty.normalized.ty_adt_def() {
1242 Some((adt.variant_of_res(def), adt.did(), Self::user_substs_for_adt(ty)))
1244 _ => bug!("unexpected type: {:?}", ty.normalized),
1246 Res::Def(DefKind::Struct | DefKind::Union | DefKind::TyAlias | DefKind::AssocTy, _)
1247 | Res::SelfTyParam { .. }
1248 | Res::SelfTyAlias { .. } => match ty.normalized.ty_adt_def() {
1249 Some(adt) if !adt.is_enum() => {
1250 Some((adt.non_enum_variant(), adt.did(), Self::user_substs_for_adt(ty)))
1254 _ => bug!("unexpected definition: {:?}", def),
1257 if let Some((variant, did, ty::UserSubsts { substs, user_self_ty })) = variant {
1258 debug!("check_struct_path: did={:?} substs={:?}", did, substs);
1260 // Register type annotation.
1261 self.write_user_type_annotation_from_substs(hir_id, did, substs, user_self_ty);
1263 // Check bounds on type arguments used in the path.
1264 self.add_required_obligations_for_hir(path_span, did, substs, hir_id);
1266 Some((variant, ty.normalized))
1268 match ty.normalized.kind() {
1270 // E0071 might be caused by a spelling error, which will have
1271 // already caused an error message and probably a suggestion
1272 // elsewhere. Refrain from emitting more unhelpful errors here
1280 "expected struct, variant or union type, found {}",
1281 ty.normalized.sort_string(self.tcx)
1283 .span_label(path_span, "not a struct")
1291 pub fn check_decl_initializer(
1294 pat: &'tcx hir::Pat<'tcx>,
1295 init: &'tcx hir::Expr<'tcx>,
1297 // FIXME(tschottdorf): `contains_explicit_ref_binding()` must be removed
1298 // for #42640 (default match binding modes).
1301 let ref_bindings = pat.contains_explicit_ref_binding();
1303 let local_ty = self.local_ty(init.span, hir_id).revealed_ty;
1304 if let Some(m) = ref_bindings {
1305 // Somewhat subtle: if we have a `ref` binding in the pattern,
1306 // we want to avoid introducing coercions for the RHS. This is
1307 // both because it helps preserve sanity and, in the case of
1308 // ref mut, for soundness (issue #23116). In particular, in
1309 // the latter case, we need to be clear that the type of the
1310 // referent for the reference that results is *equal to* the
1311 // type of the place it is referencing, and not some
1312 // supertype thereof.
1313 let init_ty = self.check_expr_with_needs(init, Needs::maybe_mut_place(m));
1314 self.demand_eqtype(init.span, local_ty, init_ty);
1317 self.check_expr_coercable_to_type(init, local_ty, None)
1321 pub(in super::super) fn check_decl(&self, decl: Declaration<'tcx>) {
1322 // Determine and write the type which we'll check the pattern against.
1323 let decl_ty = self.local_ty(decl.span, decl.hir_id).decl_ty;
1324 self.write_ty(decl.hir_id, decl_ty);
1326 // Type check the initializer.
1327 if let Some(ref init) = decl.init {
1328 let init_ty = self.check_decl_initializer(decl.hir_id, decl.pat, &init);
1329 self.overwrite_local_ty_if_err(decl.hir_id, decl.pat, init_ty);
1332 // Does the expected pattern type originate from an expression and what is the span?
1333 let (origin_expr, ty_span) = match (decl.ty, decl.init) {
1334 (Some(ty), _) => (false, Some(ty.span)), // Bias towards the explicit user type.
1335 (_, Some(init)) => {
1336 (true, Some(init.span.find_ancestor_inside(decl.span).unwrap_or(init.span)))
1337 } // No explicit type; so use the scrutinee.
1338 _ => (false, None), // We have `let $pat;`, so the expected type is unconstrained.
1341 // Type check the pattern. Override if necessary to avoid knock-on errors.
1342 self.check_pat_top(&decl.pat, decl_ty, ty_span, origin_expr);
1343 let pat_ty = self.node_ty(decl.pat.hir_id);
1344 self.overwrite_local_ty_if_err(decl.hir_id, decl.pat, pat_ty);
1346 if let Some(blk) = decl.els {
1347 let previous_diverges = self.diverges.get();
1348 let else_ty = self.check_block_with_expected(blk, NoExpectation);
1349 let cause = self.cause(blk.span, ObligationCauseCode::LetElse);
1350 if let Some(mut err) =
1351 self.demand_eqtype_with_origin(&cause, self.tcx.types.never, else_ty)
1355 self.diverges.set(previous_diverges);
1359 /// Type check a `let` statement.
1360 pub fn check_decl_local(&self, local: &'tcx hir::Local<'tcx>) {
1361 self.check_decl(local.into());
1364 pub fn check_stmt(&self, stmt: &'tcx hir::Stmt<'tcx>, is_last: bool) {
1365 // Don't do all the complex logic below for `DeclItem`.
1367 hir::StmtKind::Item(..) => return,
1368 hir::StmtKind::Local(..) | hir::StmtKind::Expr(..) | hir::StmtKind::Semi(..) => {}
1371 self.warn_if_unreachable(stmt.hir_id, stmt.span, "statement");
1373 // Hide the outer diverging and `has_errors` flags.
1374 let old_diverges = self.diverges.replace(Diverges::Maybe);
1377 hir::StmtKind::Local(l) => {
1378 self.check_decl_local(l);
1381 hir::StmtKind::Item(_) => {}
1382 hir::StmtKind::Expr(ref expr) => {
1383 // Check with expected type of `()`.
1384 self.check_expr_has_type_or_error(&expr, self.tcx.mk_unit(), |err| {
1385 if expr.can_have_side_effects() {
1386 self.suggest_semicolon_at_end(expr.span, err);
1390 hir::StmtKind::Semi(ref expr) => {
1391 // All of this is equivalent to calling `check_expr`, but it is inlined out here
1392 // in order to capture the fact that this `match` is the last statement in its
1393 // function. This is done for better suggestions to remove the `;`.
1394 let expectation = match expr.kind {
1395 hir::ExprKind::Match(..) if is_last => IsLast(stmt.span),
1398 self.check_expr_with_expectation(expr, expectation);
1402 // Combine the diverging and `has_error` flags.
1403 self.diverges.set(self.diverges.get() | old_diverges);
1406 pub fn check_block_no_value(&self, blk: &'tcx hir::Block<'tcx>) {
1407 let unit = self.tcx.mk_unit();
1408 let ty = self.check_block_with_expected(blk, ExpectHasType(unit));
1410 // if the block produces a `!` value, that can always be
1411 // (effectively) coerced to unit.
1413 self.demand_suptype(blk.span, unit, ty);
1417 pub(in super::super) fn check_block_with_expected(
1419 blk: &'tcx hir::Block<'tcx>,
1420 expected: Expectation<'tcx>,
1422 // In some cases, blocks have just one exit, but other blocks
1423 // can be targeted by multiple breaks. This can happen both
1424 // with labeled blocks as well as when we desugar
1425 // a `try { ... }` expression.
1429 // 'a: { if true { break 'a Err(()); } Ok(()) }
1431 // Here we would wind up with two coercions, one from
1432 // `Err(())` and the other from the tail expression
1433 // `Ok(())`. If the tail expression is omitted, that's a
1434 // "forced unit" -- unless the block diverges, in which
1435 // case we can ignore the tail expression (e.g., `'a: {
1436 // break 'a 22; }` would not force the type of the block
1438 let tail_expr = blk.expr.as_ref();
1439 let coerce_to_ty = expected.coercion_target_type(self, blk.span);
1440 let coerce = if blk.targeted_by_break {
1441 CoerceMany::new(coerce_to_ty)
1443 let tail_expr: &[&hir::Expr<'_>] = match tail_expr {
1444 Some(e) => slice::from_ref(e),
1447 CoerceMany::with_coercion_sites(coerce_to_ty, tail_expr)
1450 let prev_diverges = self.diverges.get();
1451 let ctxt = BreakableCtxt { coerce: Some(coerce), may_break: false };
1453 let (ctxt, ()) = self.with_breakable_ctxt(blk.hir_id, ctxt, || {
1454 for (pos, s) in blk.stmts.iter().enumerate() {
1455 self.check_stmt(s, blk.stmts.len() - 1 == pos);
1458 // check the tail expression **without** holding the
1459 // `enclosing_breakables` lock below.
1460 let tail_expr_ty = tail_expr.map(|t| self.check_expr_with_expectation(t, expected));
1462 let mut enclosing_breakables = self.enclosing_breakables.borrow_mut();
1463 let ctxt = enclosing_breakables.find_breakable(blk.hir_id);
1464 let coerce = ctxt.coerce.as_mut().unwrap();
1465 if let Some(tail_expr_ty) = tail_expr_ty {
1466 let tail_expr = tail_expr.unwrap();
1467 let span = self.get_expr_coercion_span(tail_expr);
1468 let cause = self.cause(span, ObligationCauseCode::BlockTailExpression(blk.hir_id));
1469 let ty_for_diagnostic = coerce.merged_ty();
1470 // We use coerce_inner here because we want to augment the error
1471 // suggesting to wrap the block in square brackets if it might've
1472 // been mistaken array syntax
1473 coerce.coerce_inner(
1478 Some(&mut |diag: &mut Diagnostic| {
1479 self.suggest_block_to_brackets(diag, blk, tail_expr_ty, ty_for_diagnostic);
1484 // Subtle: if there is no explicit tail expression,
1485 // that is typically equivalent to a tail expression
1486 // of `()` -- except if the block diverges. In that
1487 // case, there is no value supplied from the tail
1488 // expression (assuming there are no other breaks,
1489 // this implies that the type of the block will be
1492 // #41425 -- label the implicit `()` as being the
1493 // "found type" here, rather than the "expected type".
1494 if !self.diverges.get().is_always() {
1495 // #50009 -- Do not point at the entire fn block span, point at the return type
1496 // span, as it is the cause of the requirement, and
1497 // `consider_hint_about_removing_semicolon` will point at the last expression
1498 // if it were a relevant part of the error. This improves usability in editors
1499 // that highlight errors inline.
1500 let mut sp = blk.span;
1501 let mut fn_span = None;
1502 if let Some((decl, ident)) = self.get_parent_fn_decl(blk.hir_id) {
1503 let ret_sp = decl.output.span();
1504 if let Some(block_sp) = self.parent_item_span(blk.hir_id) {
1505 // HACK: on some cases (`ui/liveness/liveness-issue-2163.rs`) the
1506 // output would otherwise be incorrect and even misleading. Make sure
1507 // the span we're aiming at correspond to a `fn` body.
1508 if block_sp == blk.span {
1510 fn_span = Some(ident.span);
1514 coerce.coerce_forced_unit(
1518 if let Some(expected_ty) = expected.only_has_type(self) {
1519 if !self.consider_removing_semicolon(blk, expected_ty, err) {
1520 self.err_ctxt().consider_returning_binding(
1526 if expected_ty == self.tcx.types.bool {
1527 // If this is caused by a missing `let` in a `while let`,
1528 // silence this redundant error, as we already emit E0070.
1530 // Our block must be a `assign desugar local; assignment`
1531 if let Some(hir::Node::Block(hir::Block {
1536 hir::StmtKind::Local(hir::Local {
1538 hir::LocalSource::AssignDesugar(_),
1545 hir::StmtKind::Expr(hir::Expr {
1546 kind: hir::ExprKind::Assign(..),
1553 })) = self.tcx.hir().find(blk.hir_id)
1555 self.comes_from_while_condition(blk.hir_id, |_| {
1556 err.downgrade_to_delayed_bug();
1561 if let Some(fn_span) = fn_span {
1564 "implicitly returns `()` as its body has no tail or `return` \
1576 // If we can break from the block, then the block's exit is always reachable
1577 // (... as long as the entry is reachable) - regardless of the tail of the block.
1578 self.diverges.set(prev_diverges);
1581 let ty = ctxt.coerce.unwrap().complete(self);
1583 self.write_ty(blk.hir_id, ty);
1588 fn parent_item_span(&self, id: hir::HirId) -> Option<Span> {
1589 let node = self.tcx.hir().get_by_def_id(self.tcx.hir().get_parent_item(id).def_id);
1591 Node::Item(&hir::Item { kind: hir::ItemKind::Fn(_, _, body_id), .. })
1592 | Node::ImplItem(&hir::ImplItem { kind: hir::ImplItemKind::Fn(_, body_id), .. }) => {
1593 let body = self.tcx.hir().body(body_id);
1594 if let ExprKind::Block(block, _) = &body.value.kind {
1595 return Some(block.span);
1603 /// Given a function block's `HirId`, returns its `FnDecl` if it exists, or `None` otherwise.
1604 fn get_parent_fn_decl(&self, blk_id: hir::HirId) -> Option<(&'tcx hir::FnDecl<'tcx>, Ident)> {
1605 let parent = self.tcx.hir().get_by_def_id(self.tcx.hir().get_parent_item(blk_id).def_id);
1606 self.get_node_fn_decl(parent).map(|(fn_decl, ident, _)| (fn_decl, ident))
1609 /// If `expr` is a `match` expression that has only one non-`!` arm, use that arm's tail
1610 /// expression's `Span`, otherwise return `expr.span`. This is done to give better errors
1611 /// when given code like the following:
1613 /// if false { return 0i32; } else { 1u32 }
1614 /// // ^^^^ point at this instead of the whole `if` expression
1616 fn get_expr_coercion_span(&self, expr: &hir::Expr<'_>) -> rustc_span::Span {
1617 let check_in_progress = |elem: &hir::Expr<'_>| {
1618 self.typeck_results.borrow().node_type_opt(elem.hir_id).filter(|ty| !ty.is_never()).map(
1619 |_| match elem.kind {
1620 // Point at the tail expression when possible.
1621 hir::ExprKind::Block(block, _) => block.expr.map_or(block.span, |e| e.span),
1627 if let hir::ExprKind::If(_, _, Some(el)) = expr.kind {
1628 if let Some(rslt) = check_in_progress(el) {
1633 if let hir::ExprKind::Match(_, arms, _) = expr.kind {
1634 let mut iter = arms.iter().filter_map(|arm| check_in_progress(arm.body));
1635 if let Some(span) = iter.next() {
1636 if iter.next().is_none() {
1645 fn overwrite_local_ty_if_err(
1648 pat: &'tcx hir::Pat<'tcx>,
1651 if ty.references_error() {
1652 // Override the types everywhere with `err()` to avoid knock on errors.
1653 let err = self.tcx.ty_error();
1654 self.write_ty(hir_id, err);
1655 self.write_ty(pat.hir_id, err);
1656 let local_ty = LocalTy { decl_ty: err, revealed_ty: err };
1657 self.locals.borrow_mut().insert(hir_id, local_ty);
1658 self.locals.borrow_mut().insert(pat.hir_id, local_ty);
1662 // Finish resolving a path in a struct expression or pattern `S::A { .. }` if necessary.
1663 // The newly resolved definition is written into `type_dependent_defs`.
1664 fn finish_resolving_struct_path(
1669 ) -> (Res, RawTy<'tcx>) {
1671 QPath::Resolved(ref maybe_qself, ref path) => {
1672 let self_ty = maybe_qself.as_ref().map(|qself| self.to_ty(qself).raw);
1673 let ty = self.astconv().res_to_ty(self_ty, path, true);
1674 (path.res, self.handle_raw_ty(path_span, ty))
1676 QPath::TypeRelative(ref qself, ref segment) => {
1677 let ty = self.to_ty(qself);
1681 .associated_path_to_ty(hir_id, path_span, ty.raw, qself, segment, true);
1682 let ty = result.map(|(ty, _, _)| ty).unwrap_or_else(|_| self.tcx().ty_error());
1683 let ty = self.handle_raw_ty(path_span, ty);
1684 let result = result.map(|(_, kind, def_id)| (kind, def_id));
1686 // Write back the new resolution.
1687 self.write_resolution(hir_id, result);
1689 (result.map_or(Res::Err, |(kind, def_id)| Res::Def(kind, def_id)), ty)
1691 QPath::LangItem(lang_item, span, id) => {
1692 let (res, ty) = self.resolve_lang_item_path(lang_item, span, hir_id, id);
1693 (res, self.handle_raw_ty(path_span, ty))
1698 /// Given a vector of fulfillment errors, try to adjust the spans of the
1699 /// errors to more accurately point at the cause of the failure.
1701 /// This applies to calls, methods, and struct expressions. This will also
1702 /// try to deduplicate errors that are due to the same cause but might
1703 /// have been created with different [`ObligationCause`][traits::ObligationCause]s.
1704 pub(super) fn adjust_fulfillment_errors_for_expr_obligation(
1706 errors: &mut Vec<traits::FulfillmentError<'tcx>>,
1708 // Store a mapping from `(Span, Predicate) -> ObligationCause`, so that
1709 // other errors that have the same span and predicate can also get fixed,
1710 // even if their `ObligationCauseCode` isn't an `Expr*Obligation` kind.
1711 // This is important since if we adjust one span but not the other, then
1712 // we will have "duplicated" the error on the UI side.
1713 let mut remap_cause = FxIndexSet::default();
1714 let mut not_adjusted = vec![];
1716 for error in errors {
1717 let before_span = error.obligation.cause.span;
1718 if self.adjust_fulfillment_error_for_expr_obligation(error)
1719 || before_span != error.obligation.cause.span
1721 // Store both the predicate and the predicate *without constness*
1722 // since sometimes we instantiate and check both of these in a
1723 // method call, for example.
1724 remap_cause.insert((
1726 error.obligation.predicate,
1727 error.obligation.cause.clone(),
1729 remap_cause.insert((
1731 error.obligation.predicate.without_const(self.tcx),
1732 error.obligation.cause.clone(),
1735 // If it failed to be adjusted once around, it may be adjusted
1736 // via the "remap cause" mapping the second time...
1737 not_adjusted.push(error);
1741 // Adjust any other errors that come from other cause codes, when these
1742 // errors are of the same predicate as one we successfully adjusted, and
1743 // when their spans overlap (suggesting they're due to the same root cause).
1745 // This is because due to normalization, we often register duplicate
1746 // obligations with misc obligations that are basically impossible to
1747 // line back up with a useful ExprBindingObligation.
1748 for error in not_adjusted {
1749 for (span, predicate, cause) in &remap_cause {
1750 if *predicate == error.obligation.predicate
1751 && span.contains(error.obligation.cause.span)
1753 error.obligation.cause = cause.clone();
1760 fn adjust_fulfillment_error_for_expr_obligation(
1762 error: &mut traits::FulfillmentError<'tcx>,
1764 let (traits::ExprItemObligation(def_id, hir_id, idx) | traits::ExprBindingObligation(def_id, _, hir_id, idx))
1765 = *error.obligation.cause.code().peel_derives() else { return false; };
1766 let hir = self.tcx.hir();
1767 let hir::Node::Expr(expr) = hir.get(hir_id) else { return false; };
1769 let Some(unsubstituted_pred) =
1770 self.tcx.predicates_of(def_id).instantiate_identity(self.tcx).predicates.into_iter().nth(idx)
1771 else { return false; };
1773 let generics = self.tcx.generics_of(def_id);
1774 let predicate_substs = match unsubstituted_pred.kind().skip_binder() {
1775 ty::PredicateKind::Clause(ty::Clause::Trait(pred)) => pred.trait_ref.substs,
1776 ty::PredicateKind::Clause(ty::Clause::Projection(pred)) => pred.projection_ty.substs,
1777 _ => ty::List::empty(),
1780 let find_param_matching = |matches: &dyn Fn(&ty::ParamTy) -> bool| {
1781 predicate_substs.types().find_map(|ty| {
1782 ty.walk().find_map(|arg| {
1783 if let ty::GenericArgKind::Type(ty) = arg.unpack()
1784 && let ty::Param(param_ty) = ty.kind()
1785 && matches(param_ty)
1795 // Prefer generics that are local to the fn item, since these are likely
1796 // to be the cause of the unsatisfied predicate.
1797 let mut param_to_point_at = find_param_matching(&|param_ty| {
1798 self.tcx.parent(generics.type_param(param_ty, self.tcx).def_id) == def_id
1800 // Fall back to generic that isn't local to the fn item. This will come
1801 // from a trait or impl, for example.
1802 let mut fallback_param_to_point_at = find_param_matching(&|param_ty| {
1803 self.tcx.parent(generics.type_param(param_ty, self.tcx).def_id) != def_id
1804 && param_ty.name != rustc_span::symbol::kw::SelfUpper
1806 // Finally, the `Self` parameter is possibly the reason that the predicate
1807 // is unsatisfied. This is less likely to be true for methods, because
1808 // method probe means that we already kinda check that the predicates due
1809 // to the `Self` type are true.
1810 let mut self_param_to_point_at =
1811 find_param_matching(&|param_ty| param_ty.name == rustc_span::symbol::kw::SelfUpper);
1813 // Finally, for ambiguity-related errors, we actually want to look
1814 // for a parameter that is the source of the inference type left
1815 // over in this predicate.
1816 if let traits::FulfillmentErrorCode::CodeAmbiguity = error.code {
1817 fallback_param_to_point_at = None;
1818 self_param_to_point_at = None;
1820 self.find_ambiguous_parameter_in(def_id, error.root_obligation.predicate);
1823 if self.closure_span_overlaps_error(error, expr.span) {
1828 hir::ExprKind::Path(qpath) => {
1829 if let hir::Node::Expr(hir::Expr {
1830 kind: hir::ExprKind::Call(callee, args),
1831 hir_id: call_hir_id,
1834 }) = hir.get_parent(expr.hir_id)
1835 && callee.hir_id == expr.hir_id
1837 if self.closure_span_overlaps_error(error, *call_span) {
1842 [param_to_point_at, fallback_param_to_point_at, self_param_to_point_at]
1846 if self.point_at_arg_if_possible(
1860 // Notably, we only point to params that are local to the
1861 // item we're checking, since those are the ones we are able
1862 // to look in the final `hir::PathSegment` for. Everything else
1863 // would require a deeper search into the `qpath` than I think
1865 if let Some(param_to_point_at) = param_to_point_at
1866 && self.point_at_path_if_possible(error, def_id, param_to_point_at, qpath)
1871 hir::ExprKind::MethodCall(segment, receiver, args, ..) => {
1872 for param in [param_to_point_at, fallback_param_to_point_at, self_param_to_point_at]
1876 if self.point_at_arg_if_possible(
1888 if let Some(param_to_point_at) = param_to_point_at
1889 && self.point_at_generic_if_possible(error, def_id, param_to_point_at, segment)
1894 hir::ExprKind::Struct(qpath, fields, ..) => {
1895 if let Res::Def(DefKind::Struct | DefKind::Variant, variant_def_id) =
1896 self.typeck_results.borrow().qpath_res(qpath, hir_id)
1899 [param_to_point_at, fallback_param_to_point_at, self_param_to_point_at]
1901 if let Some(param) = param
1902 && self.point_at_field_if_possible(
1914 if let Some(param_to_point_at) = param_to_point_at
1915 && self.point_at_path_if_possible(error, def_id, param_to_point_at, qpath)
1926 fn closure_span_overlaps_error(
1928 error: &traits::FulfillmentError<'tcx>,
1931 if let traits::FulfillmentErrorCode::CodeSelectionError(
1932 traits::SelectionError::OutputTypeParameterMismatch(_, expected, _),
1934 && let ty::Closure(def_id, _) | ty::Generator(def_id, ..) = expected.skip_binder().self_ty().kind()
1935 && span.overlaps(self.tcx.def_span(*def_id))
1943 fn point_at_arg_if_possible(
1945 error: &mut traits::FulfillmentError<'tcx>,
1947 param_to_point_at: ty::GenericArg<'tcx>,
1948 call_hir_id: hir::HirId,
1950 receiver: Option<&'tcx hir::Expr<'tcx>>,
1951 args: &'tcx [hir::Expr<'tcx>],
1953 let ty = self.tcx.type_of(def_id);
1957 let sig = ty.fn_sig(self.tcx).skip_binder();
1958 let args_referencing_param: Vec<_> = sig
1962 .filter(|(_, ty)| find_param_in_ty(**ty, param_to_point_at))
1964 // If there's one field that references the given generic, great!
1965 if let [(idx, _)] = args_referencing_param.as_slice()
1966 && let Some(arg) = receiver
1967 .map_or(args.get(*idx), |rcvr| if *idx == 0 { Some(rcvr) } else { args.get(*idx - 1) }) {
1968 error.obligation.cause.span = arg.span.find_ancestor_in_same_ctxt(error.obligation.cause.span).unwrap_or(arg.span);
1969 error.obligation.cause.map_code(|parent_code| {
1970 ObligationCauseCode::FunctionArgumentObligation {
1971 arg_hir_id: arg.hir_id,
1977 } else if args_referencing_param.len() > 0 {
1978 // If more than one argument applies, then point to the callee span at least...
1979 // We have chance to fix this up further in `point_at_generics_if_possible`
1980 error.obligation.cause.span = callee_span;
1986 fn point_at_field_if_possible(
1988 error: &mut traits::FulfillmentError<'tcx>,
1990 param_to_point_at: ty::GenericArg<'tcx>,
1991 variant_def_id: DefId,
1992 expr_fields: &[hir::ExprField<'tcx>],
1994 let def = self.tcx.adt_def(def_id);
1996 let identity_substs = ty::InternalSubsts::identity_for_item(self.tcx, def_id);
1997 let fields_referencing_param: Vec<_> = def
1998 .variant_with_id(variant_def_id)
2002 let field_ty = field.ty(self.tcx, identity_substs);
2003 find_param_in_ty(field_ty, param_to_point_at)
2007 if let [field] = fields_referencing_param.as_slice() {
2008 for expr_field in expr_fields {
2009 // Look for the ExprField that matches the field, using the
2010 // same rules that check_expr_struct uses for macro hygiene.
2011 if self.tcx.adjust_ident(expr_field.ident, variant_def_id) == field.ident(self.tcx)
2013 error.obligation.cause.span = expr_field
2016 .find_ancestor_in_same_ctxt(error.obligation.cause.span)
2017 .unwrap_or(expr_field.span);
2026 fn point_at_path_if_possible(
2028 error: &mut traits::FulfillmentError<'tcx>,
2030 param: ty::GenericArg<'tcx>,
2031 qpath: &QPath<'tcx>,
2034 hir::QPath::Resolved(_, path) => {
2035 if let Some(segment) = path.segments.last()
2036 && self.point_at_generic_if_possible(error, def_id, param, segment)
2041 hir::QPath::TypeRelative(_, segment) => {
2042 if self.point_at_generic_if_possible(error, def_id, param, segment) {
2052 fn point_at_generic_if_possible(
2054 error: &mut traits::FulfillmentError<'tcx>,
2056 param_to_point_at: ty::GenericArg<'tcx>,
2057 segment: &hir::PathSegment<'tcx>,
2059 let own_substs = self
2061 .generics_of(def_id)
2062 .own_substs(ty::InternalSubsts::identity_for_item(self.tcx, def_id));
2063 let Some((index, _)) = own_substs
2065 .filter(|arg| matches!(arg.unpack(), ty::GenericArgKind::Type(_)))
2067 .find(|(_, arg)| **arg == param_to_point_at) else { return false };
2068 let Some(arg) = segment
2072 .filter(|arg| matches!(arg, hir::GenericArg::Type(_)))
2073 .nth(index) else { return false; };
2074 error.obligation.cause.span = arg
2076 .find_ancestor_in_same_ctxt(error.obligation.cause.span)
2077 .unwrap_or(arg.span());
2081 fn find_ambiguous_parameter_in<T: TypeVisitable<'tcx>>(
2085 ) -> Option<ty::GenericArg<'tcx>> {
2086 struct FindAmbiguousParameter<'a, 'tcx>(&'a FnCtxt<'a, 'tcx>, DefId);
2087 impl<'tcx> TypeVisitor<'tcx> for FindAmbiguousParameter<'_, 'tcx> {
2088 type BreakTy = ty::GenericArg<'tcx>;
2089 fn visit_ty(&mut self, ty: Ty<'tcx>) -> std::ops::ControlFlow<Self::BreakTy> {
2090 if let Some(origin) = self.0.type_var_origin(ty)
2091 && let TypeVariableOriginKind::TypeParameterDefinition(_, Some(def_id)) =
2093 && let generics = self.0.tcx.generics_of(self.1)
2094 && let Some(index) = generics.param_def_id_to_index(self.0.tcx, def_id)
2095 && let Some(subst) = ty::InternalSubsts::identity_for_item(self.0.tcx, self.1)
2096 .get(index as usize)
2098 ControlFlow::Break(*subst)
2100 ty.super_visit_with(self)
2104 t.visit_with(&mut FindAmbiguousParameter(self, item_def_id)).break_value()
2109 err: &mut Diagnostic,
2110 callable_def_id: Option<DefId>,
2111 callee_ty: Option<Ty<'tcx>>,
2112 // A specific argument should be labeled, instead of all of them
2113 expected_idx: Option<usize>,
2116 let Some(mut def_id) = callable_def_id else {
2120 if let Some(assoc_item) = self.tcx.opt_associated_item(def_id)
2121 // Possibly points at either impl or trait item, so try to get it
2122 // to point to trait item, then get the parent.
2123 // This parent might be an impl in the case of an inherent function,
2124 // but the next check will fail.
2125 && let maybe_trait_item_def_id = assoc_item.trait_item_def_id.unwrap_or(def_id)
2126 && let maybe_trait_def_id = self.tcx.parent(maybe_trait_item_def_id)
2127 // Just an easy way to check "trait_def_id == Fn/FnMut/FnOnce"
2128 && let Some(call_kind) = self.tcx.fn_trait_kind_from_def_id(maybe_trait_def_id)
2129 && let Some(callee_ty) = callee_ty
2131 let callee_ty = callee_ty.peel_refs();
2132 match *callee_ty.kind() {
2133 ty::Param(param) => {
2135 self.tcx.generics_of(self.body_id).type_param(¶m, self.tcx);
2136 if param.kind.is_synthetic() {
2137 // if it's `impl Fn() -> ..` then just fall down to the def-id based logic
2138 def_id = param.def_id;
2140 // Otherwise, find the predicate that makes this generic callable,
2141 // and point at that.
2142 let instantiated = self
2144 .explicit_predicates_of(self.body_id)
2145 .instantiate_identity(self.tcx);
2146 // FIXME(compiler-errors): This could be problematic if something has two
2147 // fn-like predicates with different args, but callable types really never
2148 // do that, so it's OK.
2149 for (predicate, span) in instantiated
2151 if let ty::PredicateKind::Clause(ty::Clause::Trait(pred)) = predicate.kind().skip_binder()
2152 && pred.self_ty().peel_refs() == callee_ty
2153 && self.tcx.is_fn_trait(pred.def_id())
2155 err.span_note(span, "callable defined here");
2161 ty::Alias(ty::Opaque, ty::AliasTy { def_id: new_def_id, .. })
2162 | ty::Closure(new_def_id, _)
2163 | ty::FnDef(new_def_id, _) => {
2164 def_id = new_def_id;
2167 // Look for a user-provided impl of a `Fn` trait, and point to it.
2168 let new_def_id = self.probe(|_| {
2169 let trait_ref = self.tcx.mk_trait_ref(
2170 call_kind.to_def_id(self.tcx),
2173 self.next_ty_var(TypeVariableOrigin {
2174 kind: TypeVariableOriginKind::MiscVariable,
2175 span: rustc_span::DUMMY_SP,
2179 let obligation = traits::Obligation::new(
2181 traits::ObligationCause::dummy(),
2183 ty::Binder::dummy(trait_ref),
2185 match SelectionContext::new(&self).select(&obligation) {
2186 Ok(Some(traits::ImplSource::UserDefined(impl_source))) => {
2187 Some(impl_source.impl_def_id)
2192 if let Some(new_def_id) = new_def_id {
2193 def_id = new_def_id;
2201 if let Some(def_span) = self.tcx.def_ident_span(def_id) && !def_span.is_dummy() {
2202 let mut spans: MultiSpan = def_span.into();
2207 .get_if_local(def_id)
2208 .and_then(|node| node.body_id())
2210 .flat_map(|id| self.tcx.hir().body(id).params)
2211 .skip(if is_method { 1 } else { 0 });
2213 for (_, param) in params
2216 .filter(|(idx, _)| expected_idx.map_or(true, |expected_idx| expected_idx == *idx))
2218 spans.push_span_label(param.span, "");
2221 let def_kind = self.tcx.def_kind(def_id);
2222 err.span_note(spans, &format!("{} defined here", def_kind.descr(def_id)));
2223 } else if let Some(hir::Node::Expr(e)) = self.tcx.hir().get_if_local(def_id)
2224 && let hir::ExprKind::Closure(hir::Closure { body, .. }) = &e.kind
2226 let param = expected_idx
2227 .and_then(|expected_idx| self.tcx.hir().body(*body).params.get(expected_idx));
2228 let (kind, span) = if let Some(param) = param {
2229 ("closure parameter", param.span)
2231 ("closure", self.tcx.def_span(def_id))
2233 err.span_note(span, &format!("{} defined here", kind));
2235 let def_kind = self.tcx.def_kind(def_id);
2237 self.tcx.def_span(def_id),
2238 &format!("{} defined here", def_kind.descr(def_id)),
2244 fn find_param_in_ty<'tcx>(ty: Ty<'tcx>, param_to_point_at: ty::GenericArg<'tcx>) -> bool {
2245 let mut walk = ty.walk();
2246 while let Some(arg) = walk.next() {
2247 if arg == param_to_point_at {
2249 } else if let ty::GenericArgKind::Type(ty) = arg.unpack()
2250 && let ty::Alias(ty::Projection, ..) = ty.kind()
2252 // This logic may seem a bit strange, but typically when
2253 // we have a projection type in a function signature, the
2254 // argument that's being passed into that signature is
2255 // not actually constraining that projection's substs in
2256 // a meaningful way. So we skip it, and see improvements
2257 // in some UI tests.
2258 walk.skip_current_subtree();