1 use crate::check::coercion::CoerceMany;
2 use crate::check::{Diverges, Expectation, FnCtxt, Needs};
3 use rustc_hir::{self as hir, ExprKind};
4 use rustc_infer::infer::type_variable::{TypeVariableOrigin, TypeVariableOriginKind};
5 use rustc_infer::traits::Obligation;
6 use rustc_middle::ty::{self, ToPredicate, Ty};
8 use rustc_trait_selection::opaque_types::InferCtxtExt as _;
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> {
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 use hir::MatchSource::*;
27 let (source_if, if_no_else, force_scrutinee_bool) = match match_src {
28 IfDesugar { contains_else_clause } => (true, !contains_else_clause, true),
29 IfLetDesugar { contains_else_clause, .. } => (true, !contains_else_clause, false),
30 WhileDesugar => (false, false, true),
31 _ => (false, false, false),
34 // Type check the descriminant and get its type.
35 let scrutinee_ty = if force_scrutinee_bool {
36 // Here we want to ensure:
38 // 1. That default match bindings are *not* accepted in the condition of an
39 // `if` expression. E.g. given `fn foo() -> &bool;` we reject `if foo() { .. }`.
41 // 2. By expecting `bool` for `expr` we get nice diagnostics for e.g. `if x = y { .. }`.
43 // FIXME(60707): Consider removing hack with principled solution.
44 self.check_expr_has_type_or_error(scrut, self.tcx.types.bool, |_| {})
46 self.demand_scrutinee_type(arms, scrut)
49 // If there are no arms, that is a diverging match; a special case.
51 self.diverges.set(self.diverges.get() | Diverges::always(expr.span));
52 return tcx.types.never;
55 self.warn_arms_when_scrutinee_diverges(arms, match_src);
57 // Otherwise, we have to union together the types that the arms produce and so forth.
58 let scrut_diverges = self.diverges.replace(Diverges::Maybe);
60 // #55810: Type check patterns first so we get types for all bindings.
62 self.check_pat_top(&arm.pat, scrutinee_ty, Some(scrut.span), true);
65 // Now typecheck the blocks.
67 // The result of the match is the common supertype of all the
68 // arms. Start out the value as bottom, since it's the, well,
69 // bottom the type lattice, and we'll be moving up the lattice as
70 // we process each arm. (Note that any match with 0 arms is matching
71 // on any empty type and is therefore unreachable; should the flow
72 // of execution reach it, we will panic, so bottom is an appropriate
74 let mut all_arms_diverge = Diverges::WarnedAlways;
76 let expected = orig_expected.adjust_for_branches(self);
79 let coerce_first = match expected {
80 // We don't coerce to `()` so that if the match expression is a
81 // statement it's branches can have any consistent type. That allows
82 // us to give better error messages (pointing to a usually better
83 // arm for inconsistent arms or to the whole match when a `()` type
85 Expectation::ExpectHasType(ety) if ety != self.tcx.mk_unit() => ety,
86 _ => self.next_ty_var(TypeVariableOrigin {
87 kind: TypeVariableOriginKind::MiscVariable,
91 CoerceMany::with_coercion_sites(coerce_first, arms)
94 let mut other_arms = vec![]; // Used only for diagnostics.
95 let mut prior_arm_ty = None;
96 for (i, arm) in arms.iter().enumerate() {
97 if let Some(g) = &arm.guard {
98 self.diverges.set(Diverges::Maybe);
100 hir::Guard::If(e) => {
101 self.check_expr_has_type_or_error(e, tcx.types.bool, |_| {})
106 self.diverges.set(Diverges::Maybe);
107 let arm_ty = if source_if
110 && self.if_fallback_coercion(expr.span, &arms[0].body, &mut coercion)
114 // Only call this if this is not an `if` expr with an expected type and no `else`
115 // clause to avoid duplicated type errors. (#60254)
116 self.check_expr_with_expectation(&arm.body, expected)
118 all_arms_diverge &= self.diverges.get();
120 // When we have a `match` as a tail expression in a `fn` with a returned `impl Trait`
121 // we check if the different arms would work with boxed trait objects instead and
122 // provide a structured suggestion in that case.
123 let opt_suggest_box_span = match (
125 self.ret_coercion_impl_trait.map(|ty| (self.body_id.owner, ty)),
127 (Expectation::ExpectHasType(expected), Some((id, ty)))
128 if self.in_tail_expr && self.can_coerce(arm_ty, expected) =>
130 let impl_trait_ret_ty = self.infcx.instantiate_opaque_types(
137 let mut suggest_box = !impl_trait_ret_ty.obligations.is_empty();
138 for o in impl_trait_ret_ty.obligations {
139 match o.predicate.skip_binders_unchecked() {
140 ty::PredicateAtom::Trait(t, constness) => {
141 let pred = ty::PredicateAtom::Trait(
143 trait_ref: ty::TraitRef {
145 substs: self.infcx.tcx.mk_substs_trait(arm_ty, &[]),
150 let obl = Obligation::new(
153 pred.to_predicate(self.infcx.tcx),
155 suggest_box &= self.infcx.predicate_must_hold_modulo_regions(&obl);
157 // We've encountered some obligation that didn't hold, so the
158 // return expression can't just be boxed. We don't need to
159 // evaluate the rest of the obligations.
166 // If all the obligations hold (or there are no obligations) the tail expression
167 // we can suggest to return a boxed trait object instead of an opaque type.
168 if suggest_box { self.ret_type_span } else { None }
174 let then_expr = &arms[0].body;
175 match (i, if_no_else) {
176 (0, _) => coercion.coerce(self, &self.misc(expr.span), &arm.body, arm_ty),
177 (_, true) => {} // Handled above to avoid duplicated type errors (#60254).
179 let then_ty = prior_arm_ty.unwrap();
180 let cause = self.if_cause(
186 opt_suggest_box_span,
188 coercion.coerce(self, &cause, &arm.body, arm_ty);
192 let (arm_span, semi_span) =
193 self.get_appropriate_arm_semicolon_removal_span(&arms, i, prior_arm_ty, arm_ty);
194 let (span, code) = match i {
195 // The reason for the first arm to fail is not that the match arms diverge,
196 // but rather that there's a prior obligation that doesn't hold.
197 0 => (arm_span, ObligationCauseCode::BlockTailExpression(arm.body.hir_id)),
200 ObligationCauseCode::MatchExpressionArm(box MatchExpressionArmCause {
202 scrut_span: scrut.span,
205 prior_arms: other_arms.clone(),
206 last_ty: prior_arm_ty.unwrap(),
207 scrut_hir_id: scrut.hir_id,
208 opt_suggest_box_span,
212 let cause = self.cause(span, code);
213 coercion.coerce(self, &cause, &arm.body, arm_ty);
214 other_arms.push(arm_span);
215 if other_arms.len() > 5 {
216 other_arms.remove(0);
219 prior_arm_ty = Some(arm_ty);
222 // If all of the arms in the `match` diverge,
223 // and we're dealing with an actual `match` block
224 // (as opposed to a `match` desugared from something else'),
225 // we can emit a better note. Rather than pointing
226 // at a diverging expression in an arbitrary arm,
227 // we can point at the entire `match` expression
228 if let (Diverges::Always { .. }, hir::MatchSource::Normal) = (all_arms_diverge, match_src) {
229 all_arms_diverge = Diverges::Always {
232 "any code following this `match` expression is unreachable, as all arms diverge",
237 // We won't diverge unless the scrutinee or all arms diverge.
238 self.diverges.set(scrut_diverges | all_arms_diverge);
240 coercion.complete(self)
243 fn get_appropriate_arm_semicolon_removal_span(
245 arms: &'tcx [hir::Arm<'tcx>],
247 prior_arm_ty: Option<Ty<'tcx>>,
249 ) -> (Span, Option<(Span, StatementAsExpression)>) {
251 let (arm_span, mut semi_span) = if let hir::ExprKind::Block(blk, _) = &arm.body.kind {
252 self.find_block_span(blk, prior_arm_ty)
254 (arm.body.span, None)
256 if semi_span.is_none() && i > 0 {
257 if let hir::ExprKind::Block(blk, _) = &arms[i - 1].body.kind {
258 let (_, semi_span_prev) = self.find_block_span(blk, Some(arm_ty));
259 semi_span = semi_span_prev;
262 (arm_span, semi_span)
265 /// When the previously checked expression (the scrutinee) diverges,
266 /// warn the user about the match arms being unreachable.
267 fn warn_arms_when_scrutinee_diverges(
269 arms: &'tcx [hir::Arm<'tcx>],
270 source: hir::MatchSource,
272 use hir::MatchSource::*;
273 let msg = match source {
274 IfDesugar { .. } | IfLetDesugar { .. } => "block in `if` expression",
275 WhileDesugar { .. } | WhileLetDesugar { .. } => "block in `while` expression",
279 self.warn_if_unreachable(arm.body.hir_id, arm.body.span, msg);
283 /// Handle the fallback arm of a desugared if(-let) like a missing else.
285 /// Returns `true` if there was an error forcing the coercion to the `()` type.
286 fn if_fallback_coercion(
289 then_expr: &'tcx hir::Expr<'tcx>,
290 coercion: &mut CoerceMany<'tcx, '_, rustc_hir::Arm<'tcx>>,
292 // If this `if` expr is the parent's function return expr,
293 // the cause of the type coercion is the return type, point at it. (#25228)
294 let ret_reason = self.maybe_get_coercion_reason(then_expr.hir_id, span);
295 let cause = self.cause(span, ObligationCauseCode::IfExpressionWithNoElse);
296 let mut error = false;
297 coercion.coerce_forced_unit(
301 if let Some((span, msg)) = &ret_reason {
302 err.span_label(*span, msg.as_str());
303 } else if let ExprKind::Block(block, _) = &then_expr.kind {
304 if let Some(expr) = &block.expr {
305 err.span_label(expr.span, "found here".to_string());
308 err.note("`if` expressions without `else` evaluate to `()`");
309 err.help("consider adding an `else` block that evaluates to the expected type");
312 ret_reason.is_none(),
317 fn maybe_get_coercion_reason(&self, hir_id: hir::HirId, span: Span) -> Option<(Span, String)> {
318 use hir::Node::{Block, Item, Local};
320 let hir = self.tcx.hir();
321 let arm_id = hir.get_parent_node(hir_id);
322 let match_id = hir.get_parent_node(arm_id);
323 let containing_id = hir.get_parent_node(match_id);
325 let node = hir.get(containing_id);
326 if let Block(block) = node {
327 // check that the body's parent is an fn
328 let parent = hir.get(hir.get_parent_node(hir.get_parent_node(block.hir_id)));
329 if let (Some(expr), Item(hir::Item { kind: hir::ItemKind::Fn(..), .. })) =
330 (&block.expr, parent)
332 // check that the `if` expr without `else` is the fn body's expr
333 if expr.span == span {
334 return self.get_fn_decl(hir_id).and_then(|(fn_decl, _)| {
335 let span = fn_decl.output.span();
336 let snippet = self.tcx.sess.source_map().span_to_snippet(span).ok()?;
337 Some((span, format!("expected `{}` because of this return type", snippet)))
342 if let Local(hir::Local { ty: Some(_), pat, .. }) = node {
343 return Some((pat.span, "expected because of this assignment".to_string()));
351 then_expr: &'tcx hir::Expr<'tcx>,
352 else_expr: &'tcx hir::Expr<'tcx>,
355 opt_suggest_box_span: Option<Span>,
356 ) -> ObligationCause<'tcx> {
357 let mut outer_sp = if self.tcx.sess.source_map().is_multiline(span) {
358 // The `if`/`else` isn't in one line in the output, include some context to make it
359 // clear it is an if/else expression:
361 // LL | let x = if true {
364 // || ----- expected because of this
367 // || ^^^^^ expected `i32`, found `u32`
369 // ||_____- `if` and `else` have incompatible types
373 // The entire expression is in one line, only point at the arms
375 // LL | let x = if true { 10i32 } else { 10u32 };
376 // | ----- ^^^^^ expected `i32`, found `u32`
378 // | expected because of this
383 let mut remove_semicolon = None;
384 let error_sp = if let ExprKind::Block(block, _) = &else_expr.kind {
385 let (error_sp, semi_sp) = self.find_block_span(block, Some(then_ty));
386 remove_semicolon = semi_sp;
387 if block.expr.is_none() && block.stmts.is_empty() {
388 // Avoid overlapping spans that aren't as readable:
390 // 2 | let x = if true {
393 // | | - expected because of this
400 // | |______if and else have incompatible types
401 // | expected integer, found `()`
403 // by not pointing at the entire expression:
405 // 2 | let x = if true {
406 // | ------- `if` and `else` have incompatible types
408 // | - expected because of this
413 // | |_____^ expected integer, found `()`
415 if outer_sp.is_some() {
416 outer_sp = Some(self.tcx.sess.source_map().guess_head_span(span));
421 // shouldn't happen unless the parser has done something weird
425 // Compute `Span` of `then` part of `if`-expression.
426 let then_sp = if let ExprKind::Block(block, _) = &then_expr.kind {
427 let (then_sp, semi_sp) = self.find_block_span(block, Some(else_ty));
428 remove_semicolon = remove_semicolon.or(semi_sp);
429 if block.expr.is_none() && block.stmts.is_empty() {
430 outer_sp = None; // same as in `error_sp`; cleanup output
434 // shouldn't happen unless the parser has done something weird
438 // Finally construct the cause:
441 ObligationCauseCode::IfExpression(box IfExpressionCause {
445 semicolon: remove_semicolon,
446 opt_suggest_box_span,
451 fn demand_scrutinee_type(
453 arms: &'tcx [hir::Arm<'tcx>],
454 scrut: &'tcx hir::Expr<'tcx>,
456 // Not entirely obvious: if matches may create ref bindings, we want to
457 // use the *precise* type of the scrutinee, *not* some supertype, as
458 // the "scrutinee type" (issue #23116).
460 // arielb1 [writes here in this comment thread][c] that there
461 // is certainly *some* potential danger, e.g., for an example
464 // [c]: https://github.com/rust-lang/rust/pull/43399#discussion_r130223956
467 // let Foo(x) = f()[0];
470 // Then if the pattern matches by reference, we want to match
471 // `f()[0]` as a lexpr, so we can't allow it to be
472 // coerced. But if the pattern matches by value, `f()[0]` is
473 // still syntactically a lexpr, but we *do* want to allow
476 // However, *likely* we are ok with allowing coercions to
477 // happen if there are no explicit ref mut patterns - all
478 // implicit ref mut patterns must occur behind a reference, so
479 // they will have the "correct" variance and lifetime.
481 // This does mean that the following pattern would be legal:
486 // impl Deref for Foo {
487 // type Target = Bar;
488 // fn deref(&self) -> &Bar { &self.0 }
490 // impl DerefMut for Foo {
491 // fn deref_mut(&mut self) -> &mut Bar { &mut self.0 }
493 // fn foo(x: &mut Foo) {
495 // let Bar(z): &mut Bar = x;
498 // assert_eq!(foo.0.0, 42);
502 // FIXME(tschottdorf): don't call contains_explicit_ref_binding, which
503 // is problematic as the HIR is being scraped, but ref bindings may be
504 // implicit after #42640. We need to make sure that pat_adjustments
505 // (once introduced) is populated by the time we get here.
508 let contains_ref_bindings = arms
510 .filter_map(|a| a.pat.contains_explicit_ref_binding())
511 .max_by_key(|m| match *m {
512 hir::Mutability::Mut => 1,
513 hir::Mutability::Not => 0,
516 if let Some(m) = contains_ref_bindings {
517 self.check_expr_with_needs(scrut, Needs::maybe_mut_place(m))
518 } else if arms.is_empty() {
519 self.check_expr(scrut)
521 // ...but otherwise we want to use any supertype of the
522 // scrutinee. This is sort of a workaround, see note (*) in
523 // `check_pat` for some details.
524 let scrut_ty = self.next_ty_var(TypeVariableOrigin {
525 kind: TypeVariableOriginKind::TypeInference,
528 self.check_expr_has_type_or_error(scrut, scrut_ty, |_| {});
535 block: &'tcx hir::Block<'tcx>,
536 expected_ty: Option<Ty<'tcx>>,
537 ) -> (Span, Option<(Span, StatementAsExpression)>) {
538 if let Some(expr) = &block.expr {
540 } else if let Some(stmt) = block.stmts.last() {
541 // possibly incorrect trailing `;` in the else arm
542 (stmt.span, expected_ty.and_then(|ty| self.could_remove_semicolon(block, ty)))
544 // empty block; point at its entirety