1 use crate::check
::coercion
::{AsCoercionSite, CoerceMany}
;
2 use crate::check
::{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, ToPredicate, Ty, TypeFoldable}
;
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);
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_ty
= None
;
79 for (i
, arm
) in arms
.iter().enumerate() {
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
= self.opt_suggest_box_span(arm_ty
, orig_expected
);
99 let (arm_span
, semi_span
) =
100 self.get_appropriate_arm_semicolon_removal_span(&arms
, i
, prior_arm_ty
, arm_ty
);
101 let (span
, code
) = match i
{
102 // The reason for the first arm to fail is not that the match arms diverge,
103 // but rather that there's a prior obligation that doesn't hold.
104 0 => (arm_span
, ObligationCauseCode
::BlockTailExpression(arm
.body
.hir_id
)),
107 ObligationCauseCode
::MatchExpressionArm(Box
::new(MatchExpressionArmCause
{
109 scrut_span
: scrut
.span
,
112 prior_arms
: other_arms
.clone(),
113 last_ty
: prior_arm_ty
.unwrap(),
114 scrut_hir_id
: scrut
.hir_id
,
115 opt_suggest_box_span
,
119 let cause
= self.cause(span
, code
);
121 // This is the moral equivalent of `coercion.coerce(self, cause, arm.body, arm_ty)`.
122 // We use it this way to be able to expand on the potential error and detect when a
123 // `match` tail statement could be a tail expression instead. If so, we suggest
124 // removing the stray semicolon.
125 coercion
.coerce_inner(
130 Some(&mut |err
: &mut Diagnostic
| {
131 let Some(ret
) = self.ret_type_span
else {
134 let Expectation
::IsLast(stmt
) = orig_expected
else {
137 let can_coerce_to_return_ty
= match self.ret_coercion
.as_ref() {
138 Some(ret_coercion
) if self.in_tail_expr
=> {
139 let ret_ty
= ret_coercion
.borrow().expected_ty();
140 let ret_ty
= self.inh
.infcx
.shallow_resolve(ret_ty
);
141 self.can_coerce(arm_ty
, ret_ty
)
142 && prior_arm_ty
.map_or(true, |t
| self.can_coerce(t
, ret_ty
))
143 // The match arms need to unify for the case of `impl Trait`.
144 && !matches
!(ret_ty
.kind(), ty
::Opaque(..))
148 if !can_coerce_to_return_ty
{
152 let semi_span
= expr
.span
.shrink_to_hi().with_hi(stmt
.hi());
153 let mut ret_span
: MultiSpan
= semi_span
.into();
154 ret_span
.push_span_label(
156 "this could be implicitly returned but it is a statement, not a \
160 ret_span
.push_span_label(
162 "the `match` arms can conform to this return type".to_owned(),
164 ret_span
.push_span_label(
166 "the `match` is a statement because of this semicolon, consider \
172 "you might have meant to return the `match` expression",
174 err
.tool_only_span_suggestion(
176 "remove this semicolon",
178 Applicability
::MaybeIncorrect
,
184 other_arms
.push(arm_span
);
185 if other_arms
.len() > 5 {
186 other_arms
.remove(0);
188 prior_arm_ty
= Some(arm_ty
);
191 // If all of the arms in the `match` diverge,
192 // and we're dealing with an actual `match` block
193 // (as opposed to a `match` desugared from something else'),
194 // we can emit a better note. Rather than pointing
195 // at a diverging expression in an arbitrary arm,
196 // we can point at the entire `match` expression
197 if let (Diverges
::Always { .. }
, hir
::MatchSource
::Normal
) = (all_arms_diverge
, match_src
) {
198 all_arms_diverge
= Diverges
::Always
{
201 "any code following this `match` expression is unreachable, as all arms diverge",
206 // We won't diverge unless the scrutinee or all arms diverge.
207 self.diverges
.set(scrut_diverges
| all_arms_diverge
);
209 let match_ty
= coercion
.complete(self);
214 fn get_appropriate_arm_semicolon_removal_span(
216 arms
: &'tcx
[hir
::Arm
<'tcx
>],
218 prior_arm_ty
: Option
<Ty
<'tcx
>>,
220 ) -> (Span
, Option
<(Span
, StatementAsExpression
)>) {
222 let (arm_span
, mut semi_span
) = if let hir
::ExprKind
::Block(blk
, _
) = &arm
.body
.kind
{
223 self.find_block_span(blk
, prior_arm_ty
)
225 (arm
.body
.span
, None
)
227 if semi_span
.is_none() && i
> 0 {
228 if let hir
::ExprKind
::Block(blk
, _
) = &arms
[i
- 1].body
.kind
{
229 let (_
, semi_span_prev
) = self.find_block_span(blk
, Some(arm_ty
));
230 semi_span
= semi_span_prev
;
233 (arm_span
, semi_span
)
236 /// When the previously checked expression (the scrutinee) diverges,
237 /// warn the user about the match arms being unreachable.
238 fn warn_arms_when_scrutinee_diverges(&self, arms
: &'tcx
[hir
::Arm
<'tcx
>]) {
240 self.warn_if_unreachable(arm
.body
.hir_id
, arm
.body
.span
, "arm");
244 /// Handle the fallback arm of a desugared if(-let) like a missing else.
246 /// Returns `true` if there was an error forcing the coercion to the `()` type.
247 pub(super) fn if_fallback_coercion
<T
>(
250 then_expr
: &'tcx hir
::Expr
<'tcx
>,
251 coercion
: &mut CoerceMany
<'tcx
, '_
, T
>,
256 // If this `if` expr is the parent's function return expr,
257 // the cause of the type coercion is the return type, point at it. (#25228)
258 let ret_reason
= self.maybe_get_coercion_reason(then_expr
.hir_id
, span
);
259 let cause
= self.cause(span
, ObligationCauseCode
::IfExpressionWithNoElse
);
260 let mut error
= false;
261 coercion
.coerce_forced_unit(
265 if let Some((span
, msg
)) = &ret_reason
{
266 err
.span_label(*span
, msg
.as_str());
267 } else if let ExprKind
::Block(block
, _
) = &then_expr
.kind
268 && let Some(expr
) = &block
.expr
270 err
.span_label(expr
.span
, "found here".to_string());
272 err
.note("`if` expressions without `else` evaluate to `()`");
273 err
.help("consider adding an `else` block that evaluates to the expected type");
276 ret_reason
.is_none(),
281 fn maybe_get_coercion_reason(&self, hir_id
: hir
::HirId
, sp
: Span
) -> Option
<(Span
, String
)> {
283 let rslt
= self.tcx
.hir().get_parent_node(self.tcx
.hir().get_parent_node(hir_id
));
284 self.tcx
.hir().get(rslt
)
286 if let hir
::Node
::Block(block
) = node
{
287 // check that the body's parent is an fn
291 .get(self.tcx
.hir().get_parent_node(self.tcx
.hir().get_parent_node(block
.hir_id
)));
292 if let (Some(expr
), hir
::Node
::Item(hir
::Item { kind: hir::ItemKind::Fn(..), .. }
)) =
293 (&block
.expr
, parent
)
295 // check that the `if` expr without `else` is the fn body's expr
297 return self.get_fn_decl(hir_id
).and_then(|(fn_decl
, _
)| {
298 let span
= fn_decl
.output
.span();
299 let snippet
= self.tcx
.sess
.source_map().span_to_snippet(span
).ok()?
;
300 Some((span
, format
!("expected `{snippet}` because of this return type")))
305 if let hir
::Node
::Local(hir
::Local { ty: Some(_), pat, .. }
) = node
{
306 return Some((pat
.span
, "expected because of this assignment".to_string()));
311 pub(crate) fn if_cause(
314 then_expr
: &'tcx hir
::Expr
<'tcx
>,
315 else_expr
: &'tcx hir
::Expr
<'tcx
>,
318 opt_suggest_box_span
: Option
<Span
>,
319 ) -> ObligationCause
<'tcx
> {
320 let mut outer_sp
= if self.tcx
.sess
.source_map().is_multiline(span
) {
321 // The `if`/`else` isn't in one line in the output, include some context to make it
322 // clear it is an if/else expression:
324 // LL | let x = if true {
327 // || ----- expected because of this
330 // || ^^^^^ expected `i32`, found `u32`
332 // ||_____- `if` and `else` have incompatible types
336 // The entire expression is in one line, only point at the arms
338 // LL | let x = if true { 10i32 } else { 10u32 };
339 // | ----- ^^^^^ expected `i32`, found `u32`
341 // | expected because of this
346 let mut remove_semicolon
= None
;
347 let error_sp
= if let ExprKind
::Block(block
, _
) = &else_expr
.kind
{
348 let (error_sp
, semi_sp
) = self.find_block_span(block
, Some(then_ty
));
349 remove_semicolon
= semi_sp
;
350 if block
.expr
.is_none() && block
.stmts
.is_empty() {
351 // Avoid overlapping spans that aren't as readable:
353 // 2 | let x = if true {
356 // | | - expected because of this
363 // | |______if and else have incompatible types
364 // | expected integer, found `()`
366 // by not pointing at the entire expression:
368 // 2 | let x = if true {
369 // | ------- `if` and `else` have incompatible types
371 // | - expected because of this
376 // | |_____^ expected integer, found `()`
378 if outer_sp
.is_some() {
379 outer_sp
= Some(self.tcx
.sess
.source_map().guess_head_span(span
));
384 // shouldn't happen unless the parser has done something weird
388 // Compute `Span` of `then` part of `if`-expression.
389 let then_sp
= if let ExprKind
::Block(block
, _
) = &then_expr
.kind
{
390 let (then_sp
, semi_sp
) = self.find_block_span(block
, Some(else_ty
));
391 remove_semicolon
= remove_semicolon
.or(semi_sp
);
392 if block
.expr
.is_none() && block
.stmts
.is_empty() {
393 outer_sp
= None
; // same as in `error_sp`; cleanup output
397 // shouldn't happen unless the parser has done something weird
401 // Finally construct the cause:
404 ObligationCauseCode
::IfExpression(Box
::new(IfExpressionCause
{
408 semicolon
: remove_semicolon
,
409 opt_suggest_box_span
,
414 pub(super) fn demand_scrutinee_type(
416 scrut
: &'tcx hir
::Expr
<'tcx
>,
417 contains_ref_bindings
: Option
<hir
::Mutability
>,
420 // Not entirely obvious: if matches may create ref bindings, we want to
421 // use the *precise* type of the scrutinee, *not* some supertype, as
422 // the "scrutinee type" (issue #23116).
424 // arielb1 [writes here in this comment thread][c] that there
425 // is certainly *some* potential danger, e.g., for an example
428 // [c]: https://github.com/rust-lang/rust/pull/43399#discussion_r130223956
431 // let Foo(x) = f()[0];
434 // Then if the pattern matches by reference, we want to match
435 // `f()[0]` as a lexpr, so we can't allow it to be
436 // coerced. But if the pattern matches by value, `f()[0]` is
437 // still syntactically a lexpr, but we *do* want to allow
440 // However, *likely* we are ok with allowing coercions to
441 // happen if there are no explicit ref mut patterns - all
442 // implicit ref mut patterns must occur behind a reference, so
443 // they will have the "correct" variance and lifetime.
445 // This does mean that the following pattern would be legal:
450 // impl Deref for Foo {
451 // type Target = Bar;
452 // fn deref(&self) -> &Bar { &self.0 }
454 // impl DerefMut for Foo {
455 // fn deref_mut(&mut self) -> &mut Bar { &mut self.0 }
457 // fn foo(x: &mut Foo) {
459 // let Bar(z): &mut Bar = x;
462 // assert_eq!(foo.0.0, 42);
466 // FIXME(tschottdorf): don't call contains_explicit_ref_binding, which
467 // is problematic as the HIR is being scraped, but ref bindings may be
468 // implicit after #42640. We need to make sure that pat_adjustments
469 // (once introduced) is populated by the time we get here.
472 if let Some(m
) = contains_ref_bindings
{
473 self.check_expr_with_needs(scrut
, Needs
::maybe_mut_place(m
))
475 self.check_expr(scrut
)
477 // ...but otherwise we want to use any supertype of the
478 // scrutinee. This is sort of a workaround, see note (*) in
479 // `check_pat` for some details.
480 let scrut_ty
= self.next_ty_var(TypeVariableOrigin
{
481 kind
: TypeVariableOriginKind
::TypeInference
,
484 self.check_expr_has_type_or_error(scrut
, scrut_ty
, |_
| {}
);
491 block
: &'tcx hir
::Block
<'tcx
>,
492 expected_ty
: Option
<Ty
<'tcx
>>,
493 ) -> (Span
, Option
<(Span
, StatementAsExpression
)>) {
494 if let Some(expr
) = &block
.expr
{
496 } else if let Some(stmt
) = block
.stmts
.last() {
497 // possibly incorrect trailing `;` in the else arm
498 (stmt
.span
, expected_ty
.and_then(|ty
| self.could_remove_semicolon(block
, ty
)))
500 // empty block; point at its entirety
505 // When we have a `match` as a tail expression in a `fn` with a returned `impl Trait`
506 // we check if the different arms would work with boxed trait objects instead and
507 // provide a structured suggestion in that case.
508 pub(crate) fn opt_suggest_box_span(
511 orig_expected
: Expectation
<'tcx
>,
513 match orig_expected
{
514 Expectation
::ExpectHasType(expected
)
516 && self.ret_coercion
.as_ref()?
.borrow().merged_ty().has_opaque_types()
517 && self.can_coerce(outer_ty
, expected
) =>
519 let obligations
= self.fulfillment_cx
.borrow().pending_obligations();
520 let mut suggest_box
= !obligations
.is_empty();
521 for o
in obligations
{
522 match o
.predicate
.kind().skip_binder() {
523 ty
::PredicateKind
::Trait(t
) => {
525 ty
::Binder
::dummy(ty
::PredicateKind
::Trait(ty
::TraitPredicate
{
526 trait_ref
: ty
::TraitRef
{
528 substs
: self.infcx
.tcx
.mk_substs_trait(outer_ty
, &[]),
530 constness
: t
.constness
,
531 polarity
: t
.polarity
,
533 let obl
= Obligation
::new(
536 pred
.to_predicate(self.infcx
.tcx
),
538 suggest_box
&= self.infcx
.predicate_must_hold_modulo_regions(&obl
);
540 // We've encountered some obligation that didn't hold, so the
541 // return expression can't just be boxed. We don't need to
542 // evaluate the rest of the obligations.
549 // If all the obligations hold (or there are no obligations) the tail expression
550 // we can suggest to return a boxed trait object instead of an opaque type.
551 if suggest_box { self.ret_type_span }
else { None }
558 fn arms_contain_ref_bindings
<'tcx
>(arms
: &'tcx
[hir
::Arm
<'tcx
>]) -> Option
<hir
::Mutability
> {
559 arms
.iter().filter_map(|a
| a
.pat
.contains_explicit_ref_binding()).max_by_key(|m
| match *m
{
560 hir
::Mutability
::Mut
=> 1,
561 hir
::Mutability
::Not
=> 0,