1 use crate::coercion::{AsCoercionSite, CoerceMany};
2 use crate::{Diverges, Expectation, FnCtxt, Needs};
3 use rustc_errors::{Applicability, Diagnostic, MultiSpan};
4 use rustc_hir::{self as hir, ExprKind};
5 use rustc_infer::infer::type_variable::{TypeVariableOrigin, TypeVariableOriginKind};
6 use rustc_infer::traits::Obligation;
7 use rustc_middle::ty::{self, Ty};
9 use rustc_trait_selection::traits::query::evaluate_obligation::InferCtxtExt;
10 use rustc_trait_selection::traits::{
11 IfExpressionCause, MatchExpressionArmCause, ObligationCause, ObligationCauseCode,
14 impl<'a, 'tcx> FnCtxt<'a, 'tcx> {
15 #[instrument(skip(self), level = "debug", ret)]
18 expr: &'tcx hir::Expr<'tcx>,
19 scrut: &'tcx hir::Expr<'tcx>,
20 arms: &'tcx [hir::Arm<'tcx>],
21 orig_expected: Expectation<'tcx>,
22 match_src: hir::MatchSource,
26 let acrb = arms_contain_ref_bindings(arms);
27 let scrutinee_ty = self.demand_scrutinee_type(scrut, acrb, arms.is_empty());
28 debug!(?scrutinee_ty);
30 // If there are no arms, that is a diverging match; a special case.
32 self.diverges.set(self.diverges.get() | Diverges::always(expr.span));
33 return tcx.types.never;
36 self.warn_arms_when_scrutinee_diverges(arms);
38 // Otherwise, we have to union together the types that the arms produce and so forth.
39 let scrut_diverges = self.diverges.replace(Diverges::Maybe);
41 // #55810: Type check patterns first so we get types for all bindings.
42 let scrut_span = scrut.span.find_ancestor_inside(expr.span).unwrap_or(scrut.span);
44 self.check_pat_top(&arm.pat, scrutinee_ty, Some(scrut_span), true);
47 // Now typecheck the blocks.
49 // The result of the match is the common supertype of all the
50 // arms. Start out the value as bottom, since it's the, well,
51 // bottom the type lattice, and we'll be moving up the lattice as
52 // we process each arm. (Note that any match with 0 arms is matching
53 // on any empty type and is therefore unreachable; should the flow
54 // of execution reach it, we will panic, so bottom is an appropriate
56 let mut all_arms_diverge = Diverges::WarnedAlways;
58 let expected = orig_expected.adjust_for_branches(self);
62 let coerce_first = match expected {
63 // We don't coerce to `()` so that if the match expression is a
64 // statement it's branches can have any consistent type. That allows
65 // us to give better error messages (pointing to a usually better
66 // arm for inconsistent arms or to the whole match when a `()` type
68 Expectation::ExpectHasType(ety) if ety != self.tcx.mk_unit() => ety,
69 _ => self.next_ty_var(TypeVariableOrigin {
70 kind: TypeVariableOriginKind::MiscVariable,
74 CoerceMany::with_coercion_sites(coerce_first, arms)
77 let mut other_arms = vec![]; // Used only for diagnostics.
78 let mut prior_arm = None;
80 if let Some(g) = &arm.guard {
81 self.diverges.set(Diverges::Maybe);
83 hir::Guard::If(e) => {
84 self.check_expr_has_type_or_error(e, tcx.types.bool, |_| {});
86 hir::Guard::IfLet(l) => {
87 self.check_expr_let(l);
92 self.diverges.set(Diverges::Maybe);
94 let arm_ty = self.check_expr_with_expectation(&arm.body, expected);
95 all_arms_diverge &= self.diverges.get();
97 let opt_suggest_box_span = prior_arm.and_then(|(_, prior_arm_ty, _)| {
98 self.opt_suggest_box_span(prior_arm_ty, arm_ty, orig_expected)
101 let (arm_block_id, arm_span) = if let hir::ExprKind::Block(blk, _) = arm.body.kind {
102 (Some(blk.hir_id), self.find_block_span(blk))
104 (None, arm.body.span)
107 let (span, code) = match prior_arm {
108 // The reason for the first arm to fail is not that the match arms diverge,
109 // but rather that there's a prior obligation that doesn't hold.
110 None => (arm_span, ObligationCauseCode::BlockTailExpression(arm.body.hir_id)),
111 Some((prior_arm_block_id, prior_arm_ty, prior_arm_span)) => (
113 ObligationCauseCode::MatchExpressionArm(Box::new(MatchExpressionArmCause {
120 scrut_span: scrut.span,
122 prior_arms: other_arms.clone(),
123 scrut_hir_id: scrut.hir_id,
124 opt_suggest_box_span,
128 let cause = self.cause(span, code);
130 // This is the moral equivalent of `coercion.coerce(self, cause, arm.body, arm_ty)`.
131 // We use it this way to be able to expand on the potential error and detect when a
132 // `match` tail statement could be a tail expression instead. If so, we suggest
133 // removing the stray semicolon.
134 coercion.coerce_inner(
140 self.suggest_removing_semicolon_for_coerce(
151 other_arms.push(arm_span);
152 if other_arms.len() > 5 {
153 other_arms.remove(0);
156 prior_arm = Some((arm_block_id, arm_ty, arm_span));
159 // If all of the arms in the `match` diverge,
160 // and we're dealing with an actual `match` block
161 // (as opposed to a `match` desugared from something else'),
162 // we can emit a better note. Rather than pointing
163 // at a diverging expression in an arbitrary arm,
164 // we can point at the entire `match` expression
165 if let (Diverges::Always { .. }, hir::MatchSource::Normal) = (all_arms_diverge, match_src) {
166 all_arms_diverge = Diverges::Always {
169 "any code following this `match` expression is unreachable, as all arms diverge",
174 // We won't diverge unless the scrutinee or all arms diverge.
175 self.diverges.set(scrut_diverges | all_arms_diverge);
177 coercion.complete(self)
180 fn suggest_removing_semicolon_for_coerce(
182 diag: &mut Diagnostic,
183 expr: &hir::Expr<'tcx>,
184 expectation: Expectation<'tcx>,
186 prior_arm: Option<(Option<hir::HirId>, Ty<'tcx>, Span)>,
188 let hir = self.tcx.hir();
190 // First, check that we're actually in the tail of a function.
191 let hir::Node::Expr(hir::Expr { kind: hir::ExprKind::Block(block, _), .. }) =
192 hir.get(self.body_id) else { return; };
193 let Some(hir::Stmt { kind: hir::StmtKind::Semi(last_expr), .. })
194 = block.innermost_block().stmts.last() else { return; };
195 if last_expr.hir_id != expr.hir_id {
199 // Next, make sure that we have no type expectation.
201 .find_by_def_id(self.body_id.owner.def_id)
202 .and_then(|owner| owner.fn_decl())
203 .map(|decl| decl.output.span()) else { return; };
204 let Expectation::IsLast(stmt) = expectation else {
208 let can_coerce_to_return_ty = match self.ret_coercion.as_ref() {
209 Some(ret_coercion) => {
210 let ret_ty = ret_coercion.borrow().expected_ty();
211 let ret_ty = self.inh.infcx.shallow_resolve(ret_ty);
212 self.can_coerce(arm_ty, ret_ty)
213 && prior_arm.map_or(true, |(_, ty, _)| self.can_coerce(ty, ret_ty))
214 // The match arms need to unify for the case of `impl Trait`.
215 && !matches!(ret_ty.kind(), ty::Alias(ty::Opaque, ..))
219 if !can_coerce_to_return_ty {
223 let semi_span = expr.span.shrink_to_hi().with_hi(stmt.hi());
224 let mut ret_span: MultiSpan = semi_span.into();
225 ret_span.push_span_label(
227 "this could be implicitly returned but it is a statement, not a tail expression",
229 ret_span.push_span_label(ret, "the `match` arms can conform to this return type");
230 ret_span.push_span_label(
232 "the `match` is a statement because of this semicolon, consider removing it",
234 diag.span_note(ret_span, "you might have meant to return the `match` expression");
235 diag.tool_only_span_suggestion(
237 "remove this semicolon",
239 Applicability::MaybeIncorrect,
243 /// When the previously checked expression (the scrutinee) diverges,
244 /// warn the user about the match arms being unreachable.
245 fn warn_arms_when_scrutinee_diverges(&self, arms: &'tcx [hir::Arm<'tcx>]) {
247 self.warn_if_unreachable(arm.body.hir_id, arm.body.span, "arm");
251 /// Handle the fallback arm of a desugared if(-let) like a missing else.
253 /// Returns `true` if there was an error forcing the coercion to the `()` type.
254 pub(super) fn if_fallback_coercion<T>(
257 then_expr: &'tcx hir::Expr<'tcx>,
258 coercion: &mut CoerceMany<'tcx, '_, T>,
263 // If this `if` expr is the parent's function return expr,
264 // the cause of the type coercion is the return type, point at it. (#25228)
265 let ret_reason = self.maybe_get_coercion_reason(then_expr.hir_id, span);
266 let cause = self.cause(span, ObligationCauseCode::IfExpressionWithNoElse);
267 let mut error = false;
268 coercion.coerce_forced_unit(
272 if let Some((span, msg)) = &ret_reason {
273 err.span_label(*span, msg);
274 } else if let ExprKind::Block(block, _) = &then_expr.kind
275 && let Some(expr) = &block.expr
277 err.span_label(expr.span, "found here");
279 err.note("`if` expressions without `else` evaluate to `()`");
280 err.help("consider adding an `else` block that evaluates to the expected type");
288 fn maybe_get_coercion_reason(&self, hir_id: hir::HirId, sp: Span) -> Option<(Span, String)> {
290 let rslt = self.tcx.hir().parent_id(self.tcx.hir().parent_id(hir_id));
291 self.tcx.hir().get(rslt)
293 if let hir::Node::Block(block) = node {
294 // check that the body's parent is an fn
295 let parent = self.tcx.hir().get_parent(self.tcx.hir().parent_id(block.hir_id));
296 if let (Some(expr), hir::Node::Item(hir::Item { kind: hir::ItemKind::Fn(..), .. })) =
297 (&block.expr, parent)
299 // check that the `if` expr without `else` is the fn body's expr
301 return self.get_fn_decl(hir_id).and_then(|(fn_decl, _)| {
302 let span = fn_decl.output.span();
303 let snippet = self.tcx.sess.source_map().span_to_snippet(span).ok()?;
304 Some((span, format!("expected `{snippet}` because of this return type")))
309 if let hir::Node::Local(hir::Local { ty: Some(_), pat, .. }) = node {
310 return Some((pat.span, "expected because of this assignment".to_string()));
315 pub(crate) fn if_cause(
319 then_expr: &'tcx hir::Expr<'tcx>,
320 else_expr: &'tcx hir::Expr<'tcx>,
323 opt_suggest_box_span: Option<Span>,
324 ) -> ObligationCause<'tcx> {
325 let mut outer_span = if self.tcx.sess.source_map().is_multiline(span) {
326 // The `if`/`else` isn't in one line in the output, include some context to make it
327 // clear it is an if/else expression:
329 // LL | let x = if true {
332 // || ----- expected because of this
335 // || ^^^^^ expected `i32`, found `u32`
337 // ||_____- `if` and `else` have incompatible types
341 // The entire expression is in one line, only point at the arms
343 // LL | let x = if true { 10i32 } else { 10u32 };
344 // | ----- ^^^^^ expected `i32`, found `u32`
346 // | expected because of this
351 let (error_sp, else_id) = if let ExprKind::Block(block, _) = &else_expr.kind {
352 let block = block.innermost_block();
354 // Avoid overlapping spans that aren't as readable:
356 // 2 | let x = if true {
359 // | | - expected because of this
366 // | |______if and else have incompatible types
367 // | expected integer, found `()`
369 // by not pointing at the entire expression:
371 // 2 | let x = if true {
372 // | ------- `if` and `else` have incompatible types
374 // | - expected because of this
379 // | |_____^ expected integer, found `()`
381 if block.expr.is_none() && block.stmts.is_empty()
382 && let Some(outer_span) = &mut outer_span
383 && let Some(cond_span) = cond_span.find_ancestor_inside(*outer_span)
385 *outer_span = outer_span.with_hi(cond_span.hi())
388 (self.find_block_span(block), block.hir_id)
390 (else_expr.span, else_expr.hir_id)
393 let then_id = if let ExprKind::Block(block, _) = &then_expr.kind {
394 let block = block.innermost_block();
395 // Exclude overlapping spans
396 if block.expr.is_none() && block.stmts.is_empty() {
404 // Finally construct the cause:
407 ObligationCauseCode::IfExpression(Box::new(IfExpressionCause {
413 opt_suggest_box_span,
418 pub(super) fn demand_scrutinee_type(
420 scrut: &'tcx hir::Expr<'tcx>,
421 contains_ref_bindings: Option<hir::Mutability>,
424 // Not entirely obvious: if matches may create ref bindings, we want to
425 // use the *precise* type of the scrutinee, *not* some supertype, as
426 // the "scrutinee type" (issue #23116).
428 // arielb1 [writes here in this comment thread][c] that there
429 // is certainly *some* potential danger, e.g., for an example
432 // [c]: https://github.com/rust-lang/rust/pull/43399#discussion_r130223956
435 // let Foo(x) = f()[0];
438 // Then if the pattern matches by reference, we want to match
439 // `f()[0]` as a lexpr, so we can't allow it to be
440 // coerced. But if the pattern matches by value, `f()[0]` is
441 // still syntactically a lexpr, but we *do* want to allow
444 // However, *likely* we are ok with allowing coercions to
445 // happen if there are no explicit ref mut patterns - all
446 // implicit ref mut patterns must occur behind a reference, so
447 // they will have the "correct" variance and lifetime.
449 // This does mean that the following pattern would be legal:
454 // impl Deref for Foo {
455 // type Target = Bar;
456 // fn deref(&self) -> &Bar { &self.0 }
458 // impl DerefMut for Foo {
459 // fn deref_mut(&mut self) -> &mut Bar { &mut self.0 }
461 // fn foo(x: &mut Foo) {
463 // let Bar(z): &mut Bar = x;
466 // assert_eq!(foo.0.0, 42);
470 // FIXME(tschottdorf): don't call contains_explicit_ref_binding, which
471 // is problematic as the HIR is being scraped, but ref bindings may be
472 // implicit after #42640. We need to make sure that pat_adjustments
473 // (once introduced) is populated by the time we get here.
476 if let Some(m) = contains_ref_bindings {
477 self.check_expr_with_needs(scrut, Needs::maybe_mut_place(m))
479 self.check_expr(scrut)
481 // ...but otherwise we want to use any supertype of the
482 // scrutinee. This is sort of a workaround, see note (*) in
483 // `check_pat` for some details.
484 let scrut_ty = self.next_ty_var(TypeVariableOrigin {
485 kind: TypeVariableOriginKind::TypeInference,
488 self.check_expr_has_type_or_error(scrut, scrut_ty, |_| {});
493 /// When we have a `match` as a tail expression in a `fn` with a returned `impl Trait`
494 /// we check if the different arms would work with boxed trait objects instead and
495 /// provide a structured suggestion in that case.
496 pub(crate) fn opt_suggest_box_span(
500 orig_expected: Expectation<'tcx>,
502 // FIXME(compiler-errors): This really shouldn't need to be done during the
503 // "good" path of typeck, but here we are.
504 match orig_expected {
505 Expectation::ExpectHasType(expected) => {
506 let TypeVariableOrigin {
508 kind: TypeVariableOriginKind::OpaqueTypeInference(rpit_def_id),
510 } = self.type_var_origin(expected)? else { return None; };
512 let sig = self.body_fn_sig()?;
514 let substs = sig.output().walk().find_map(|arg| {
515 if let ty::GenericArgKind::Type(ty) = arg.unpack()
516 && let ty::Alias(ty::Opaque, ty::AliasTy { def_id, substs, .. }) = *ty.kind()
517 && def_id == rpit_def_id
525 if !self.can_coerce(first_ty, expected) || !self.can_coerce(second_ty, expected) {
529 for ty in [first_ty, second_ty] {
530 for (pred, _) in self
532 .bound_explicit_item_bounds(rpit_def_id)
533 .subst_iter_copied(self.tcx, substs)
535 let pred = pred.kind().rebind(match pred.kind().skip_binder() {
536 ty::PredicateKind::Clause(ty::Clause::Trait(trait_pred)) => {
537 // FIXME(rpitit): This will need to be fixed when we move to associated types
539 *trait_pred.trait_ref.self_ty().kind(),
540 ty::Alias(_, ty::AliasTy { def_id, substs, .. })
541 if def_id == rpit_def_id && substs == substs
543 ty::PredicateKind::Clause(ty::Clause::Trait(
544 trait_pred.with_self_ty(self.tcx, ty),
547 ty::PredicateKind::Clause(ty::Clause::Projection(mut proj_pred)) => {
549 *proj_pred.projection_ty.self_ty().kind(),
550 ty::Alias(_, ty::AliasTy { def_id, substs, .. })
551 if def_id == rpit_def_id && substs == substs
553 proj_pred = proj_pred.with_self_ty(self.tcx, ty);
554 ty::PredicateKind::Clause(ty::Clause::Projection(proj_pred))
558 if !self.predicate_must_hold_modulo_regions(&Obligation::new(
560 ObligationCause::misc(span, self.body_id),
576 fn arms_contain_ref_bindings<'tcx>(arms: &'tcx [hir::Arm<'tcx>]) -> Option<hir::Mutability> {
577 arms.iter().filter_map(|a| a.pat.contains_explicit_ref_binding()).max()