1 use crate::check::coercion::CoerceMany;
2 use crate::check::{Diverges, Expectation, FnCtxt, Needs};
4 use rustc_hir::ExprKind;
5 use rustc_infer::infer::type_variable::{TypeVariableOrigin, TypeVariableOriginKind};
6 use rustc_middle::ty::Ty;
8 use rustc_trait_selection::traits::ObligationCauseCode;
9 use rustc_trait_selection::traits::{IfExpressionCause, MatchExpressionArmCause, ObligationCause};
11 impl<'a, 'tcx> FnCtxt<'a, 'tcx> {
14 expr: &'tcx hir::Expr<'tcx>,
15 scrut: &'tcx hir::Expr<'tcx>,
16 arms: &'tcx [hir::Arm<'tcx>],
17 expected: Expectation<'tcx>,
18 match_src: hir::MatchSource,
22 use hir::MatchSource::*;
23 let (source_if, if_no_else, force_scrutinee_bool) = match match_src {
24 IfDesugar { contains_else_clause } => (true, !contains_else_clause, true),
25 IfLetDesugar { contains_else_clause } => (true, !contains_else_clause, false),
26 WhileDesugar => (false, false, true),
27 _ => (false, false, false),
30 // Type check the descriminant and get its type.
31 let scrut_ty = if force_scrutinee_bool {
32 // Here we want to ensure:
34 // 1. That default match bindings are *not* accepted in the condition of an
35 // `if` expression. E.g. given `fn foo() -> &bool;` we reject `if foo() { .. }`.
37 // 2. By expecting `bool` for `expr` we get nice diagnostics for e.g. `if x = y { .. }`.
39 // FIXME(60707): Consider removing hack with principled solution.
40 self.check_expr_has_type_or_error(scrut, self.tcx.types.bool, |_| {})
42 self.demand_scrutinee_type(arms, scrut)
45 // If there are no arms, that is a diverging match; a special case.
47 self.diverges.set(self.diverges.get() | Diverges::always(expr.span));
48 return tcx.types.never;
51 self.warn_arms_when_scrutinee_diverges(arms, match_src);
53 // Otherwise, we have to union together the types that the arms produce and so forth.
54 let scrut_diverges = self.diverges.replace(Diverges::Maybe);
56 // #55810: Type check patterns first so we get types for all bindings.
58 self.check_pat_top(&arm.pat, scrut_ty, Some(scrut.span), true);
61 // Now typecheck the blocks.
63 // The result of the match is the common supertype of all the
64 // arms. Start out the value as bottom, since it's the, well,
65 // bottom the type lattice, and we'll be moving up the lattice as
66 // we process each arm. (Note that any match with 0 arms is matching
67 // on any empty type and is therefore unreachable; should the flow
68 // of execution reach it, we will panic, so bottom is an appropriate
70 let mut all_arms_diverge = Diverges::WarnedAlways;
72 let expected = expected.adjust_for_branches(self);
75 let coerce_first = match expected {
76 // We don't coerce to `()` so that if the match expression is a
77 // statement it's branches can have any consistent type. That allows
78 // us to give better error messages (pointing to a usually better
79 // arm for inconsistent arms or to the whole match when a `()` type
81 Expectation::ExpectHasType(ety) if ety != self.tcx.mk_unit() => ety,
82 _ => self.next_ty_var(TypeVariableOrigin {
83 kind: TypeVariableOriginKind::MiscVariable,
87 CoerceMany::with_coercion_sites(coerce_first, arms)
90 let mut other_arms = vec![]; // Used only for diagnostics.
91 let mut prior_arm_ty = None;
92 for (i, arm) in arms.iter().enumerate() {
93 if let Some(g) = &arm.guard {
94 self.diverges.set(Diverges::Maybe);
96 hir::Guard::If(e) => {
97 self.check_expr_has_type_or_error(e, tcx.types.bool, |_| {})
102 self.diverges.set(Diverges::Maybe);
103 let arm_ty = if source_if
106 && self.if_fallback_coercion(expr.span, &arms[0].body, &mut coercion)
110 // Only call this if this is not an `if` expr with an expected type and no `else`
111 // clause to avoid duplicated type errors. (#60254)
112 self.check_expr_with_expectation(&arm.body, expected)
114 all_arms_diverge &= self.diverges.get();
116 let then_expr = &arms[0].body;
117 match (i, if_no_else) {
118 (0, _) => coercion.coerce(self, &self.misc(expr.span), &arm.body, arm_ty),
119 (_, true) => {} // Handled above to avoid duplicated type errors (#60254).
121 let then_ty = prior_arm_ty.unwrap();
122 let cause = self.if_cause(expr.span, then_expr, &arm.body, then_ty, arm_ty);
123 coercion.coerce(self, &cause, &arm.body, arm_ty);
127 let arm_span = if let hir::ExprKind::Block(blk, _) = &arm.body.kind {
128 // Point at the block expr instead of the entire block
129 blk.expr.as_ref().map(|e| e.span).unwrap_or(arm.body.span)
133 let (span, code) = match i {
134 // The reason for the first arm to fail is not that the match arms diverge,
135 // but rather that there's a prior obligation that doesn't hold.
136 0 => (arm_span, ObligationCauseCode::BlockTailExpression(arm.body.hir_id)),
139 ObligationCauseCode::MatchExpressionArm(box MatchExpressionArmCause {
142 prior_arms: other_arms.clone(),
143 last_ty: prior_arm_ty.unwrap(),
144 scrut_hir_id: scrut.hir_id,
148 let cause = self.cause(span, code);
149 coercion.coerce(self, &cause, &arm.body, arm_ty);
150 other_arms.push(arm_span);
151 if other_arms.len() > 5 {
152 other_arms.remove(0);
155 prior_arm_ty = Some(arm_ty);
158 // If all of the arms in the `match` diverge,
159 // and we're dealing with an actual `match` block
160 // (as opposed to a `match` desugared from something else'),
161 // we can emit a better note. Rather than pointing
162 // at a diverging expression in an arbitrary arm,
163 // we can point at the entire `match` expression
164 if let (Diverges::Always { .. }, hir::MatchSource::Normal) = (all_arms_diverge, match_src) {
165 all_arms_diverge = Diverges::Always {
168 "any code following this `match` expression is unreachable, as all arms diverge",
173 // We won't diverge unless the scrutinee or all arms diverge.
174 self.diverges.set(scrut_diverges | all_arms_diverge);
176 coercion.complete(self)
179 /// When the previously checked expression (the scrutinee) diverges,
180 /// warn the user about the match arms being unreachable.
181 fn warn_arms_when_scrutinee_diverges(
183 arms: &'tcx [hir::Arm<'tcx>],
184 source: hir::MatchSource,
186 use hir::MatchSource::*;
187 let msg = match source {
188 IfDesugar { .. } | IfLetDesugar { .. } => "block in `if` expression",
189 WhileDesugar { .. } | WhileLetDesugar { .. } => "block in `while` expression",
193 self.warn_if_unreachable(arm.body.hir_id, arm.body.span, msg);
197 /// Handle the fallback arm of a desugared if(-let) like a missing else.
199 /// Returns `true` if there was an error forcing the coercion to the `()` type.
200 fn if_fallback_coercion(
203 then_expr: &'tcx hir::Expr<'tcx>,
204 coercion: &mut CoerceMany<'tcx, '_, rustc_hir::Arm<'tcx>>,
206 // If this `if` expr is the parent's function return expr,
207 // the cause of the type coercion is the return type, point at it. (#25228)
208 let ret_reason = self.maybe_get_coercion_reason(then_expr.hir_id, span);
209 let cause = self.cause(span, ObligationCauseCode::IfExpressionWithNoElse);
210 let mut error = false;
211 coercion.coerce_forced_unit(
215 if let Some((span, msg)) = &ret_reason {
216 err.span_label(*span, msg.as_str());
217 } else if let ExprKind::Block(block, _) = &then_expr.kind {
218 if let Some(expr) = &block.expr {
219 err.span_label(expr.span, "found here".to_string());
222 err.note("`if` expressions without `else` evaluate to `()`");
223 err.help("consider adding an `else` block that evaluates to the expected type");
226 ret_reason.is_none(),
231 fn maybe_get_coercion_reason(&self, hir_id: hir::HirId, span: Span) -> Option<(Span, String)> {
232 use hir::Node::{Block, Item, Local};
234 let hir = self.tcx.hir();
235 let arm_id = hir.get_parent_node(hir_id);
236 let match_id = hir.get_parent_node(arm_id);
237 let containing_id = hir.get_parent_node(match_id);
239 let node = hir.get(containing_id);
240 if let Block(block) = node {
241 // check that the body's parent is an fn
242 let parent = hir.get(hir.get_parent_node(hir.get_parent_node(block.hir_id)));
243 if let (Some(expr), Item(hir::Item { kind: hir::ItemKind::Fn(..), .. })) =
244 (&block.expr, parent)
246 // check that the `if` expr without `else` is the fn body's expr
247 if expr.span == span {
248 return self.get_fn_decl(hir_id).and_then(|(fn_decl, _)| {
249 let span = fn_decl.output.span();
250 let snippet = self.tcx.sess.source_map().span_to_snippet(span).ok()?;
251 Some((span, format!("expected `{}` because of this return type", snippet)))
256 if let Local(hir::Local { ty: Some(_), pat, .. }) = node {
257 return Some((pat.span, "expected because of this assignment".to_string()));
265 then_expr: &'tcx hir::Expr<'tcx>,
266 else_expr: &'tcx hir::Expr<'tcx>,
269 ) -> ObligationCause<'tcx> {
270 let mut outer_sp = if self.tcx.sess.source_map().is_multiline(span) {
271 // The `if`/`else` isn't in one line in the output, include some context to make it
272 // clear it is an if/else expression:
274 // LL | let x = if true {
277 // || ----- expected because of this
280 // || ^^^^^ expected `i32`, found `u32`
282 // ||_____- `if` and `else` have incompatible types
286 // The entire expression is in one line, only point at the arms
288 // LL | let x = if true { 10i32 } else { 10u32 };
289 // | ----- ^^^^^ expected `i32`, found `u32`
291 // | expected because of this
296 let mut remove_semicolon = None;
297 let error_sp = if let ExprKind::Block(block, _) = &else_expr.kind {
298 if let Some(expr) = &block.expr {
300 } else if let Some(stmt) = block.stmts.last() {
301 // possibly incorrect trailing `;` in the else arm
302 remove_semicolon = self.could_remove_semicolon(block, then_ty);
305 // empty block; point at its entirety
306 // Avoid overlapping spans that aren't as readable:
308 // 2 | let x = if true {
311 // | | - expected because of this
318 // | |______if and else have incompatible types
319 // | expected integer, found `()`
321 // by not pointing at the entire expression:
323 // 2 | let x = if true {
324 // | ------- `if` and `else` have incompatible types
326 // | - expected because of this
331 // | |_____^ expected integer, found `()`
333 if outer_sp.is_some() {
334 outer_sp = Some(self.tcx.sess.source_map().guess_head_span(span));
339 // shouldn't happen unless the parser has done something weird
343 // Compute `Span` of `then` part of `if`-expression.
344 let then_sp = if let ExprKind::Block(block, _) = &then_expr.kind {
345 if let Some(expr) = &block.expr {
347 } else if let Some(stmt) = block.stmts.last() {
348 // possibly incorrect trailing `;` in the else arm
349 remove_semicolon = remove_semicolon.or(self.could_remove_semicolon(block, else_ty));
352 // empty block; point at its entirety
353 outer_sp = None; // same as in `error_sp`; cleanup output
357 // shouldn't happen unless the parser has done something weird
361 // Finally construct the cause:
364 ObligationCauseCode::IfExpression(box IfExpressionCause {
367 semicolon: remove_semicolon,
372 fn demand_scrutinee_type(
374 arms: &'tcx [hir::Arm<'tcx>],
375 scrut: &'tcx hir::Expr<'tcx>,
377 // Not entirely obvious: if matches may create ref bindings, we want to
378 // use the *precise* type of the scrutinee, *not* some supertype, as
379 // the "scrutinee type" (issue #23116).
381 // arielb1 [writes here in this comment thread][c] that there
382 // is certainly *some* potential danger, e.g., for an example
385 // [c]: https://github.com/rust-lang/rust/pull/43399#discussion_r130223956
388 // let Foo(x) = f()[0];
391 // Then if the pattern matches by reference, we want to match
392 // `f()[0]` as a lexpr, so we can't allow it to be
393 // coerced. But if the pattern matches by value, `f()[0]` is
394 // still syntactically a lexpr, but we *do* want to allow
397 // However, *likely* we are ok with allowing coercions to
398 // happen if there are no explicit ref mut patterns - all
399 // implicit ref mut patterns must occur behind a reference, so
400 // they will have the "correct" variance and lifetime.
402 // This does mean that the following pattern would be legal:
407 // impl Deref for Foo {
408 // type Target = Bar;
409 // fn deref(&self) -> &Bar { &self.0 }
411 // impl DerefMut for Foo {
412 // fn deref_mut(&mut self) -> &mut Bar { &mut self.0 }
414 // fn foo(x: &mut Foo) {
416 // let Bar(z): &mut Bar = x;
419 // assert_eq!(foo.0.0, 42);
423 // FIXME(tschottdorf): don't call contains_explicit_ref_binding, which
424 // is problematic as the HIR is being scraped, but ref bindings may be
425 // implicit after #42640. We need to make sure that pat_adjustments
426 // (once introduced) is populated by the time we get here.
429 let contains_ref_bindings = arms
431 .filter_map(|a| a.pat.contains_explicit_ref_binding())
432 .max_by_key(|m| match *m {
433 hir::Mutability::Mut => 1,
434 hir::Mutability::Not => 0,
437 if let Some(m) = contains_ref_bindings {
438 self.check_expr_with_needs(scrut, Needs::maybe_mut_place(m))
439 } else if arms.is_empty() {
440 self.check_expr(scrut)
442 // ...but otherwise we want to use any supertype of the
443 // scrutinee. This is sort of a workaround, see note (*) in
444 // `check_pat` for some details.
445 let scrut_ty = self.next_ty_var(TypeVariableOrigin {
446 kind: TypeVariableOriginKind::TypeInference,
449 self.check_expr_has_type_or_error(scrut, scrut_ty, |_| {});