1 use crate::check::coercion::CoerceMany;
2 use crate::check::{Diverges, Expectation, FnCtxt, Needs};
3 use rustc::hir::{self, ExprKind};
4 use rustc::infer::type_variable::{TypeVariableOrigin, TypeVariableOriginKind};
5 use rustc::traits::ObligationCauseCode;
6 use rustc::traits::{IfExpressionCause, MatchExpressionArmCause, ObligationCause};
10 impl<'a, 'tcx> FnCtxt<'a, 'tcx> {
13 expr: &'tcx hir::Expr<'tcx>,
14 scrut: &'tcx hir::Expr<'tcx>,
15 arms: &'tcx [hir::Arm<'tcx>],
16 expected: Expectation<'tcx>,
17 match_src: hir::MatchSource,
21 use hir::MatchSource::*;
22 let (source_if, if_no_else, force_scrutinee_bool) = match match_src {
23 IfDesugar { contains_else_clause } => (true, !contains_else_clause, true),
24 IfLetDesugar { contains_else_clause } => (true, !contains_else_clause, false),
25 WhileDesugar => (false, false, true),
26 _ => (false, false, false),
29 // Type check the descriminant and get its type.
30 let scrut_ty = if force_scrutinee_bool {
31 // Here we want to ensure:
33 // 1. That default match bindings are *not* accepted in the condition of an
34 // `if` expression. E.g. given `fn foo() -> &bool;` we reject `if foo() { .. }`.
36 // 2. By expecting `bool` for `expr` we get nice diagnostics for e.g. `if x = y { .. }`.
38 // FIXME(60707): Consider removing hack with principled solution.
39 self.check_expr_has_type_or_error(scrut, self.tcx.types.bool, |_| {})
41 self.demand_scrutinee_type(arms, scrut)
44 // If there are no arms, that is a diverging match; a special case.
46 self.diverges.set(self.diverges.get() | Diverges::always(expr.span));
47 return tcx.types.never;
50 self.warn_arms_when_scrutinee_diverges(arms, match_src);
52 // Otherwise, we have to union together the types that the
53 // arms produce and so forth.
54 let scrut_diverges = self.diverges.get();
55 self.diverges.set(Diverges::Maybe);
57 // rust-lang/rust#55810: Typecheck patterns first (via eager
58 // collection into `Vec`), so we get types for all bindings.
59 let all_arm_pats_diverge: Vec<_> = arms
62 let mut all_pats_diverge = Diverges::WarnedAlways;
63 self.diverges.set(Diverges::Maybe);
64 self.check_pat_top(&arm.pat, scrut_ty, Some(scrut.span), true);
65 all_pats_diverge &= self.diverges.get();
67 // As discussed with @eddyb, this is for disabling unreachable_code
68 // warnings on patterns (they're now subsumed by unreachable_patterns
70 match all_pats_diverge {
71 Diverges::Maybe => Diverges::Maybe,
72 Diverges::Always { .. } | Diverges::WarnedAlways => Diverges::WarnedAlways,
77 // Now typecheck the blocks.
79 // The result of the match is the common supertype of all the
80 // arms. Start out the value as bottom, since it's the, well,
81 // bottom the type lattice, and we'll be moving up the lattice as
82 // we process each arm. (Note that any match with 0 arms is matching
83 // on any empty type and is therefore unreachable; should the flow
84 // of execution reach it, we will panic, so bottom is an appropriate
86 let mut all_arms_diverge = Diverges::WarnedAlways;
88 let expected = expected.adjust_for_branches(self);
91 let coerce_first = match expected {
92 // We don't coerce to `()` so that if the match expression is a
93 // statement it's branches can have any consistent type. That allows
94 // us to give better error messages (pointing to a usually better
95 // arm for inconsistent arms or to the whole match when a `()` type
97 Expectation::ExpectHasType(ety) if ety != self.tcx.mk_unit() => ety,
98 _ => self.next_ty_var(TypeVariableOrigin {
99 kind: TypeVariableOriginKind::MiscVariable,
103 CoerceMany::with_coercion_sites(coerce_first, arms)
106 let mut other_arms = vec![]; // used only for diagnostics
107 let mut prior_arm_ty = None;
108 for (i, (arm, pats_diverge)) in arms.iter().zip(all_arm_pats_diverge).enumerate() {
109 if let Some(g) = &arm.guard {
110 self.diverges.set(pats_diverge);
112 hir::Guard::If(e) => {
113 self.check_expr_has_type_or_error(e, tcx.types.bool, |_| {})
118 self.diverges.set(pats_diverge);
119 let arm_ty = if source_if
122 && self.if_fallback_coercion(expr.span, &arms[0].body, &mut coercion)
126 // Only call this if this is not an `if` expr with an expected type and no `else`
127 // clause to avoid duplicated type errors. (#60254)
128 self.check_expr_with_expectation(&arm.body, expected)
130 all_arms_diverge &= self.diverges.get();
132 let then_expr = &arms[0].body;
133 match (i, if_no_else) {
134 (0, _) => coercion.coerce(self, &self.misc(expr.span), &arm.body, arm_ty),
135 (_, true) => {} // Handled above to avoid duplicated type errors (#60254).
137 let then_ty = prior_arm_ty.unwrap();
138 let cause = self.if_cause(expr.span, then_expr, &arm.body, then_ty, arm_ty);
139 coercion.coerce(self, &cause, &arm.body, arm_ty);
143 let arm_span = if let hir::ExprKind::Block(blk, _) = &arm.body.kind {
144 // Point at the block expr instead of the entire block
145 blk.expr.as_ref().map(|e| e.span).unwrap_or(arm.body.span)
149 let (span, code) = match i {
150 // The reason for the first arm to fail is not that the match arms diverge,
151 // but rather that there's a prior obligation that doesn't hold.
152 0 => (arm_span, ObligationCauseCode::BlockTailExpression(arm.body.hir_id)),
155 ObligationCauseCode::MatchExpressionArm(box MatchExpressionArmCause {
158 prior_arms: other_arms.clone(),
159 last_ty: prior_arm_ty.unwrap(),
160 scrut_hir_id: scrut.hir_id,
164 let cause = self.cause(span, code);
165 coercion.coerce(self, &cause, &arm.body, arm_ty);
166 other_arms.push(arm_span);
167 if other_arms.len() > 5 {
168 other_arms.remove(0);
171 prior_arm_ty = Some(arm_ty);
174 // If all of the arms in the `match` diverge,
175 // and we're dealing with an actual `match` block
176 // (as opposed to a `match` desugared from something else'),
177 // we can emit a better note. Rather than pointing
178 // at a diverging expression in an arbitrary arm,
179 // we can point at the entire `match` expression
180 if let (Diverges::Always { .. }, hir::MatchSource::Normal) = (all_arms_diverge, match_src) {
181 all_arms_diverge = Diverges::Always {
184 "any code following this `match` expression is unreachable, as all arms diverge",
189 // We won't diverge unless the scrutinee or all arms diverge.
190 self.diverges.set(scrut_diverges | all_arms_diverge);
192 coercion.complete(self)
195 /// When the previously checked expression (the scrutinee) diverges,
196 /// warn the user about the match arms being unreachable.
197 fn warn_arms_when_scrutinee_diverges(
199 arms: &'tcx [hir::Arm<'tcx>],
200 source: hir::MatchSource,
202 if self.diverges.get().is_always() {
203 use hir::MatchSource::*;
204 let msg = match source {
205 IfDesugar { .. } | IfLetDesugar { .. } => "block in `if` expression",
206 WhileDesugar { .. } | WhileLetDesugar { .. } => "block in `while` expression",
210 self.warn_if_unreachable(arm.body.hir_id, arm.body.span, msg);
215 /// Handle the fallback arm of a desugared if(-let) like a missing else.
217 /// Returns `true` if there was an error forcing the coercion to the `()` type.
218 fn if_fallback_coercion(
221 then_expr: &'tcx hir::Expr<'tcx>,
222 coercion: &mut CoerceMany<'tcx, '_, rustc::hir::Arm<'tcx>>,
224 // If this `if` expr is the parent's function return expr,
225 // the cause of the type coercion is the return type, point at it. (#25228)
226 let ret_reason = self.maybe_get_coercion_reason(then_expr.hir_id, span);
227 let cause = self.cause(span, ObligationCauseCode::IfExpressionWithNoElse);
228 let mut error = false;
229 coercion.coerce_forced_unit(
233 if let Some((span, msg)) = &ret_reason {
234 err.span_label(*span, msg.as_str());
235 } else if let ExprKind::Block(block, _) = &then_expr.kind {
236 if let Some(expr) = &block.expr {
237 err.span_label(expr.span, "found here".to_string());
240 err.note("`if` expressions without `else` evaluate to `()`");
241 err.help("consider adding an `else` block that evaluates to the expected type");
244 ret_reason.is_none(),
249 fn maybe_get_coercion_reason(&self, hir_id: hir::HirId, span: Span) -> Option<(Span, String)> {
250 use hir::Node::{Block, Item, Local};
252 let hir = self.tcx.hir();
253 let arm_id = hir.get_parent_node(hir_id);
254 let match_id = hir.get_parent_node(arm_id);
255 let containing_id = hir.get_parent_node(match_id);
257 let node = hir.get(containing_id);
258 if let Block(block) = node {
259 // check that the body's parent is an fn
260 let parent = hir.get(hir.get_parent_node(hir.get_parent_node(block.hir_id)));
261 if let (Some(expr), Item(hir::Item { kind: hir::ItemKind::Fn(..), .. })) =
262 (&block.expr, parent)
264 // check that the `if` expr without `else` is the fn body's expr
265 if expr.span == span {
266 return self.get_fn_decl(hir_id).map(|(fn_decl, _)| {
268 fn_decl.output.span(),
269 format!("expected `{}` because of this return type", fn_decl.output),
275 if let Local(hir::Local { ty: Some(_), pat, .. }) = node {
276 return Some((pat.span, "expected because of this assignment".to_string()));
284 then_expr: &'tcx hir::Expr<'tcx>,
285 else_expr: &'tcx hir::Expr<'tcx>,
288 ) -> ObligationCause<'tcx> {
289 let mut outer_sp = if self.tcx.sess.source_map().is_multiline(span) {
290 // The `if`/`else` isn't in one line in the output, include some context to make it
291 // clear it is an if/else expression:
293 // LL | let x = if true {
296 // || ----- expected because of this
299 // || ^^^^^ expected `i32`, found `u32`
301 // ||_____- if and else have incompatible types
305 // The entire expression is in one line, only point at the arms
307 // LL | let x = if true { 10i32 } else { 10u32 };
308 // | ----- ^^^^^ expected `i32`, found `u32`
310 // | expected because of this
315 let mut remove_semicolon = None;
316 let error_sp = if let ExprKind::Block(block, _) = &else_expr.kind {
317 if let Some(expr) = &block.expr {
319 } else if let Some(stmt) = block.stmts.last() {
320 // possibly incorrect trailing `;` in the else arm
321 remove_semicolon = self.could_remove_semicolon(block, then_ty);
324 // empty block; point at its entirety
325 // Avoid overlapping spans that aren't as readable:
327 // 2 | let x = if true {
330 // | | - expected because of this
337 // | |______if and else have incompatible types
338 // | expected integer, found `()`
340 // by not pointing at the entire expression:
342 // 2 | let x = if true {
343 // | ------- if and else have incompatible types
345 // | - expected because of this
350 // | |_____^ expected integer, found `()`
352 if outer_sp.is_some() {
353 outer_sp = Some(self.tcx.sess.source_map().def_span(span));
358 // shouldn't happen unless the parser has done something weird
362 // Compute `Span` of `then` part of `if`-expression.
363 let then_sp = if let ExprKind::Block(block, _) = &then_expr.kind {
364 if let Some(expr) = &block.expr {
366 } else if let Some(stmt) = block.stmts.last() {
367 // possibly incorrect trailing `;` in the else arm
368 remove_semicolon = remove_semicolon.or(self.could_remove_semicolon(block, else_ty));
371 // empty block; point at its entirety
372 outer_sp = None; // same as in `error_sp`; cleanup output
376 // shouldn't happen unless the parser has done something weird
380 // Finally construct the cause:
383 ObligationCauseCode::IfExpression(box IfExpressionCause {
386 semicolon: remove_semicolon,
391 fn demand_scrutinee_type(
393 arms: &'tcx [hir::Arm<'tcx>],
394 scrut: &'tcx hir::Expr<'tcx>,
396 // Not entirely obvious: if matches may create ref bindings, we want to
397 // use the *precise* type of the scrutinee, *not* some supertype, as
398 // the "scrutinee type" (issue #23116).
400 // arielb1 [writes here in this comment thread][c] that there
401 // is certainly *some* potential danger, e.g., for an example
404 // [c]: https://github.com/rust-lang/rust/pull/43399#discussion_r130223956
407 // let Foo(x) = f()[0];
410 // Then if the pattern matches by reference, we want to match
411 // `f()[0]` as a lexpr, so we can't allow it to be
412 // coerced. But if the pattern matches by value, `f()[0]` is
413 // still syntactically a lexpr, but we *do* want to allow
416 // However, *likely* we are ok with allowing coercions to
417 // happen if there are no explicit ref mut patterns - all
418 // implicit ref mut patterns must occur behind a reference, so
419 // they will have the "correct" variance and lifetime.
421 // This does mean that the following pattern would be legal:
426 // impl Deref for Foo {
427 // type Target = Bar;
428 // fn deref(&self) -> &Bar { &self.0 }
430 // impl DerefMut for Foo {
431 // fn deref_mut(&mut self) -> &mut Bar { &mut self.0 }
433 // fn foo(x: &mut Foo) {
435 // let Bar(z): &mut Bar = x;
438 // assert_eq!(foo.0.0, 42);
442 // FIXME(tschottdorf): don't call contains_explicit_ref_binding, which
443 // is problematic as the HIR is being scraped, but ref bindings may be
444 // implicit after #42640. We need to make sure that pat_adjustments
445 // (once introduced) is populated by the time we get here.
448 let contains_ref_bindings = arms
450 .filter_map(|a| a.pat.contains_explicit_ref_binding())
451 .max_by_key(|m| match *m {
452 hir::Mutability::Mut => 1,
453 hir::Mutability::Not => 0,
456 if let Some(m) = contains_ref_bindings {
457 self.check_expr_with_needs(scrut, Needs::maybe_mut_place(m))
459 // ...but otherwise we want to use any supertype of the
460 // scrutinee. This is sort of a workaround, see note (*) in
461 // `check_pat` for some details.
462 let scrut_ty = self.next_ty_var(TypeVariableOrigin {
463 kind: TypeVariableOriginKind::TypeInference,
466 self.check_expr_has_type_or_error(scrut, scrut_ty, |_| {});