1 use crate::check::coercion::{AsCoercionSite, CoerceMany};
2 use crate::check::{Diverges, Expectation, FnCtxt, Needs};
3 use rustc_errors::{Applicability, DiagnosticBuilder};
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, ToPredicate, Ty};
8 use rustc_span::{MultiSpan, Span};
9 use rustc_trait_selection::traits::query::evaluate_obligation::InferCtxtExt;
10 use rustc_trait_selection::traits::{
11 IfExpressionCause, MatchExpressionArmCause, ObligationCause, ObligationCauseCode,
12 StatementAsExpression,
15 impl<'a, 'tcx> FnCtxt<'a, 'tcx> {
16 #[instrument(skip(self), level = "debug")]
19 expr: &'tcx hir::Expr<'tcx>,
20 scrut: &'tcx hir::Expr<'tcx>,
21 arms: &'tcx [hir::Arm<'tcx>],
22 orig_expected: Expectation<'tcx>,
23 match_src: hir::MatchSource,
27 let acrb = arms_contain_ref_bindings(arms);
28 let scrutinee_ty = self.demand_scrutinee_type(scrut, acrb, arms.is_empty());
29 debug!(?scrutinee_ty);
31 // If there are no arms, that is a diverging match; a special case.
33 self.diverges.set(self.diverges.get() | Diverges::always(expr.span));
34 return tcx.types.never;
37 self.warn_arms_when_scrutinee_diverges(arms);
39 // Otherwise, we have to union together the types that the arms produce and so forth.
40 let scrut_diverges = self.diverges.replace(Diverges::Maybe);
42 // #55810: Type check patterns first so we get types for all bindings.
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);
61 let coerce_first = match expected {
62 // We don't coerce to `()` so that if the match expression is a
63 // statement it's branches can have any consistent type. That allows
64 // us to give better error messages (pointing to a usually better
65 // arm for inconsistent arms or to the whole match when a `()` type
67 Expectation::ExpectHasType(ety) if ety != self.tcx.mk_unit() => ety,
68 _ => self.next_ty_var(TypeVariableOrigin {
69 kind: TypeVariableOriginKind::MiscVariable,
73 CoerceMany::with_coercion_sites(coerce_first, arms)
76 let mut other_arms = vec![]; // Used only for diagnostics.
77 let mut prior_arm_ty = None;
78 for (i, arm) in arms.iter().enumerate() {
79 if let Some(g) = &arm.guard {
80 self.diverges.set(Diverges::Maybe);
82 hir::Guard::If(e) => {
83 self.check_expr_has_type_or_error(e, tcx.types.bool, |_| {});
85 hir::Guard::IfLet(pat, e) => {
86 let scrutinee_ty = self.demand_scrutinee_type(
88 pat.contains_explicit_ref_binding(),
91 self.check_pat_top(&pat, scrutinee_ty, None, true);
96 self.diverges.set(Diverges::Maybe);
98 let arm_ty = self.check_expr_with_expectation(&arm.body, expected);
99 all_arms_diverge &= self.diverges.get();
101 let opt_suggest_box_span =
102 self.opt_suggest_box_span(arm.body.span, arm_ty, orig_expected);
104 let (arm_span, semi_span) =
105 self.get_appropriate_arm_semicolon_removal_span(&arms, i, prior_arm_ty, arm_ty);
106 let (span, code) = match i {
107 // The reason for the first arm to fail is not that the match arms diverge,
108 // but rather that there's a prior obligation that doesn't hold.
109 0 => (arm_span, ObligationCauseCode::BlockTailExpression(arm.body.hir_id)),
112 ObligationCauseCode::MatchExpressionArm(Box::new(MatchExpressionArmCause {
114 scrut_span: scrut.span,
117 prior_arms: other_arms.clone(),
118 last_ty: prior_arm_ty.unwrap(),
119 scrut_hir_id: scrut.hir_id,
120 opt_suggest_box_span,
124 let cause = self.cause(span, code);
126 // This is the moral equivalent of `coercion.coerce(self, cause, arm.body, arm_ty)`.
127 // We use it this way to be able to expand on the potential error and detect when a
128 // `match` tail statement could be a tail expression instead. If so, we suggest
129 // removing the stray semicolon.
130 coercion.coerce_inner(
135 Some(&mut |err: &mut DiagnosticBuilder<'_>| {
136 let can_coerce_to_return_ty = match self.ret_coercion.as_ref() {
137 Some(ret_coercion) if self.in_tail_expr => {
138 let ret_ty = ret_coercion.borrow().expected_ty();
139 let ret_ty = self.inh.infcx.shallow_resolve(ret_ty);
140 self.can_coerce(arm_ty, ret_ty)
141 && prior_arm_ty.map_or(true, |t| self.can_coerce(t, ret_ty))
142 // The match arms need to unify for the case of `impl Trait`.
143 && !matches!(ret_ty.kind(), ty::Opaque(..))
147 if let (Expectation::IsLast(stmt), Some(ret), true) =
148 (orig_expected, self.ret_type_span, can_coerce_to_return_ty)
150 let semi_span = expr.span.shrink_to_hi().with_hi(stmt.hi());
151 let mut ret_span: MultiSpan = semi_span.into();
152 ret_span.push_span_label(
154 "this could be implicitly returned but it is a statement, not a \
158 ret_span.push_span_label(
160 "the `match` arms can conform to this return type".to_owned(),
162 ret_span.push_span_label(
164 "the `match` is a statement because of this semicolon, consider \
170 "you might have meant to return the `match` expression",
172 err.tool_only_span_suggestion(
174 "remove this semicolon",
176 Applicability::MaybeIncorrect,
183 other_arms.push(arm_span);
184 if other_arms.len() > 5 {
185 other_arms.remove(0);
187 prior_arm_ty = Some(arm_ty);
190 // If all of the arms in the `match` diverge,
191 // and we're dealing with an actual `match` block
192 // (as opposed to a `match` desugared from something else'),
193 // we can emit a better note. Rather than pointing
194 // at a diverging expression in an arbitrary arm,
195 // we can point at the entire `match` expression
196 if let (Diverges::Always { .. }, hir::MatchSource::Normal) = (all_arms_diverge, match_src) {
197 all_arms_diverge = Diverges::Always {
200 "any code following this `match` expression is unreachable, as all arms diverge",
205 // We won't diverge unless the scrutinee or all arms diverge.
206 self.diverges.set(scrut_diverges | all_arms_diverge);
208 coercion.complete(self)
211 fn get_appropriate_arm_semicolon_removal_span(
213 arms: &'tcx [hir::Arm<'tcx>],
215 prior_arm_ty: Option<Ty<'tcx>>,
217 ) -> (Span, Option<(Span, StatementAsExpression)>) {
219 let (arm_span, mut semi_span) = if let hir::ExprKind::Block(blk, _) = &arm.body.kind {
220 self.find_block_span(blk, prior_arm_ty)
222 (arm.body.span, None)
224 if semi_span.is_none() && i > 0 {
225 if let hir::ExprKind::Block(blk, _) = &arms[i - 1].body.kind {
226 let (_, semi_span_prev) = self.find_block_span(blk, Some(arm_ty));
227 semi_span = semi_span_prev;
230 (arm_span, semi_span)
233 /// When the previously checked expression (the scrutinee) diverges,
234 /// warn the user about the match arms being unreachable.
235 fn warn_arms_when_scrutinee_diverges(&self, arms: &'tcx [hir::Arm<'tcx>]) {
237 self.warn_if_unreachable(arm.body.hir_id, arm.body.span, "arm");
241 /// Handle the fallback arm of a desugared if(-let) like a missing else.
243 /// Returns `true` if there was an error forcing the coercion to the `()` type.
244 pub(super) fn if_fallback_coercion<T>(
247 then_expr: &'tcx hir::Expr<'tcx>,
248 coercion: &mut CoerceMany<'tcx, '_, T>,
253 // If this `if` expr is the parent's function return expr,
254 // the cause of the type coercion is the return type, point at it. (#25228)
255 let ret_reason = self.maybe_get_coercion_reason(then_expr.hir_id, span);
256 let cause = self.cause(span, ObligationCauseCode::IfExpressionWithNoElse);
257 let mut error = false;
258 coercion.coerce_forced_unit(
262 if let Some((span, msg)) = &ret_reason {
263 err.span_label(*span, msg.as_str());
264 } else if let ExprKind::Block(block, _) = &then_expr.kind {
265 if let Some(expr) = &block.expr {
266 err.span_label(expr.span, "found here".to_string());
269 err.note("`if` expressions without `else` evaluate to `()`");
270 err.help("consider adding an `else` block that evaluates to the expected type");
273 ret_reason.is_none(),
278 fn maybe_get_coercion_reason(&self, hir_id: hir::HirId, sp: Span) -> Option<(Span, String)> {
280 let rslt = self.tcx.hir().get_parent_node(self.tcx.hir().get_parent_node(hir_id));
281 self.tcx.hir().get(rslt)
283 if let hir::Node::Block(block) = node {
284 // check that the body's parent is an fn
288 .get(self.tcx.hir().get_parent_node(self.tcx.hir().get_parent_node(block.hir_id)));
289 if let (Some(expr), hir::Node::Item(hir::Item { kind: hir::ItemKind::Fn(..), .. })) =
290 (&block.expr, parent)
292 // check that the `if` expr without `else` is the fn body's expr
294 return self.get_fn_decl(hir_id).and_then(|(fn_decl, _)| {
295 let span = fn_decl.output.span();
296 let snippet = self.tcx.sess.source_map().span_to_snippet(span).ok()?;
297 Some((span, format!("expected `{}` because of this return type", snippet)))
302 if let hir::Node::Local(hir::Local { ty: Some(_), pat, .. }) = node {
303 return Some((pat.span, "expected because of this assignment".to_string()));
308 pub(crate) fn if_cause(
311 then_expr: &'tcx hir::Expr<'tcx>,
312 else_expr: &'tcx hir::Expr<'tcx>,
315 opt_suggest_box_span: Option<Span>,
316 ) -> ObligationCause<'tcx> {
317 let mut outer_sp = if self.tcx.sess.source_map().is_multiline(span) {
318 // The `if`/`else` isn't in one line in the output, include some context to make it
319 // clear it is an if/else expression:
321 // LL | let x = if true {
324 // || ----- expected because of this
327 // || ^^^^^ expected `i32`, found `u32`
329 // ||_____- `if` and `else` have incompatible types
333 // The entire expression is in one line, only point at the arms
335 // LL | let x = if true { 10i32 } else { 10u32 };
336 // | ----- ^^^^^ expected `i32`, found `u32`
338 // | expected because of this
343 let mut remove_semicolon = None;
344 let error_sp = if let ExprKind::Block(block, _) = &else_expr.kind {
345 let (error_sp, semi_sp) = self.find_block_span(block, Some(then_ty));
346 remove_semicolon = semi_sp;
347 if block.expr.is_none() && block.stmts.is_empty() {
348 // Avoid overlapping spans that aren't as readable:
350 // 2 | let x = if true {
353 // | | - expected because of this
360 // | |______if and else have incompatible types
361 // | expected integer, found `()`
363 // by not pointing at the entire expression:
365 // 2 | let x = if true {
366 // | ------- `if` and `else` have incompatible types
368 // | - expected because of this
373 // | |_____^ expected integer, found `()`
375 if outer_sp.is_some() {
376 outer_sp = Some(self.tcx.sess.source_map().guess_head_span(span));
381 // shouldn't happen unless the parser has done something weird
385 // Compute `Span` of `then` part of `if`-expression.
386 let then_sp = if let ExprKind::Block(block, _) = &then_expr.kind {
387 let (then_sp, semi_sp) = self.find_block_span(block, Some(else_ty));
388 remove_semicolon = remove_semicolon.or(semi_sp);
389 if block.expr.is_none() && block.stmts.is_empty() {
390 outer_sp = None; // same as in `error_sp`; cleanup output
394 // shouldn't happen unless the parser has done something weird
398 // Finally construct the cause:
401 ObligationCauseCode::IfExpression(Box::new(IfExpressionCause {
405 semicolon: remove_semicolon,
406 opt_suggest_box_span,
411 pub(super) fn demand_scrutinee_type(
413 scrut: &'tcx hir::Expr<'tcx>,
414 contains_ref_bindings: Option<hir::Mutability>,
417 // Not entirely obvious: if matches may create ref bindings, we want to
418 // use the *precise* type of the scrutinee, *not* some supertype, as
419 // the "scrutinee type" (issue #23116).
421 // arielb1 [writes here in this comment thread][c] that there
422 // is certainly *some* potential danger, e.g., for an example
425 // [c]: https://github.com/rust-lang/rust/pull/43399#discussion_r130223956
428 // let Foo(x) = f()[0];
431 // Then if the pattern matches by reference, we want to match
432 // `f()[0]` as a lexpr, so we can't allow it to be
433 // coerced. But if the pattern matches by value, `f()[0]` is
434 // still syntactically a lexpr, but we *do* want to allow
437 // However, *likely* we are ok with allowing coercions to
438 // happen if there are no explicit ref mut patterns - all
439 // implicit ref mut patterns must occur behind a reference, so
440 // they will have the "correct" variance and lifetime.
442 // This does mean that the following pattern would be legal:
447 // impl Deref for Foo {
448 // type Target = Bar;
449 // fn deref(&self) -> &Bar { &self.0 }
451 // impl DerefMut for Foo {
452 // fn deref_mut(&mut self) -> &mut Bar { &mut self.0 }
454 // fn foo(x: &mut Foo) {
456 // let Bar(z): &mut Bar = x;
459 // assert_eq!(foo.0.0, 42);
463 // FIXME(tschottdorf): don't call contains_explicit_ref_binding, which
464 // is problematic as the HIR is being scraped, but ref bindings may be
465 // implicit after #42640. We need to make sure that pat_adjustments
466 // (once introduced) is populated by the time we get here.
469 if let Some(m) = contains_ref_bindings {
470 self.check_expr_with_needs(scrut, Needs::maybe_mut_place(m))
472 self.check_expr(scrut)
474 // ...but otherwise we want to use any supertype of the
475 // scrutinee. This is sort of a workaround, see note (*) in
476 // `check_pat` for some details.
477 let scrut_ty = self.next_ty_var(TypeVariableOrigin {
478 kind: TypeVariableOriginKind::TypeInference,
481 self.check_expr_has_type_or_error(scrut, scrut_ty, |_| {});
488 block: &'tcx hir::Block<'tcx>,
489 expected_ty: Option<Ty<'tcx>>,
490 ) -> (Span, Option<(Span, StatementAsExpression)>) {
491 if let Some(expr) = &block.expr {
493 } else if let Some(stmt) = block.stmts.last() {
494 // possibly incorrect trailing `;` in the else arm
495 (stmt.span, expected_ty.and_then(|ty| self.could_remove_semicolon(block, ty)))
497 // empty block; point at its entirety
502 // When we have a `match` as a tail expression in a `fn` with a returned `impl Trait`
503 // we check if the different arms would work with boxed trait objects instead and
504 // provide a structured suggestion in that case.
505 pub(crate) fn opt_suggest_box_span(
509 orig_expected: Expectation<'tcx>,
511 match (orig_expected, self.ret_coercion_impl_trait.map(|ty| (self.body_id.owner, ty))) {
512 (Expectation::ExpectHasType(expected), Some((_id, ty)))
513 if self.in_tail_expr && self.can_coerce(outer_ty, expected) =>
515 let impl_trait_ret_ty =
516 self.infcx.instantiate_opaque_types(self.body_id, self.param_env, ty, span);
518 impl_trait_ret_ty.obligations.is_empty(),
519 "we should never get new obligations here"
521 let obligations = self.fulfillment_cx.borrow().pending_obligations();
522 let mut suggest_box = !obligations.is_empty();
523 for o in obligations {
524 match o.predicate.kind().skip_binder() {
525 ty::PredicateKind::Trait(t) => {
527 ty::Binder::dummy(ty::PredicateKind::Trait(ty::TraitPredicate {
528 trait_ref: ty::TraitRef {
530 substs: self.infcx.tcx.mk_substs_trait(outer_ty, &[]),
532 constness: t.constness,
533 polarity: t.polarity,
535 let obl = Obligation::new(
538 pred.to_predicate(self.infcx.tcx),
540 suggest_box &= self.infcx.predicate_must_hold_modulo_regions(&obl);
542 // We've encountered some obligation that didn't hold, so the
543 // return expression can't just be boxed. We don't need to
544 // evaluate the rest of the obligations.
551 // If all the obligations hold (or there are no obligations) the tail expression
552 // we can suggest to return a boxed trait object instead of an opaque type.
553 if suggest_box { self.ret_type_span } else { None }
560 fn arms_contain_ref_bindings<'tcx>(arms: &'tcx [hir::Arm<'tcx>]) -> Option<hir::Mutability> {
561 arms.iter().filter_map(|a| a.pat.contains_explicit_ref_binding()).max_by_key(|m| match *m {
562 hir::Mutability::Mut => 1,
563 hir::Mutability::Not => 0,