diff options
Diffstat (limited to 'compiler/rustc_hir_analysis/src/check/_match.rs')
| -rw-r--r-- | compiler/rustc_hir_analysis/src/check/_match.rs | 557 |
1 files changed, 557 insertions, 0 deletions
diff --git a/compiler/rustc_hir_analysis/src/check/_match.rs b/compiler/rustc_hir_analysis/src/check/_match.rs new file mode 100644 index 00000000000..201927091a6 --- /dev/null +++ b/compiler/rustc_hir_analysis/src/check/_match.rs @@ -0,0 +1,557 @@ +use crate::check::coercion::{AsCoercionSite, CoerceMany}; +use crate::check::{Diverges, Expectation, FnCtxt, Needs}; +use rustc_errors::{Applicability, MultiSpan}; +use rustc_hir::{self as hir, ExprKind}; +use rustc_infer::infer::type_variable::{TypeVariableOrigin, TypeVariableOriginKind}; +use rustc_infer::traits::Obligation; +use rustc_middle::ty::{self, ToPredicate, Ty}; +use rustc_span::Span; +use rustc_trait_selection::traits::query::evaluate_obligation::InferCtxtExt; +use rustc_trait_selection::traits::{ + IfExpressionCause, MatchExpressionArmCause, ObligationCause, ObligationCauseCode, +}; + +impl<'a, 'tcx> FnCtxt<'a, 'tcx> { + #[instrument(skip(self), level = "debug", ret)] + pub fn check_match( + &self, + expr: &'tcx hir::Expr<'tcx>, + scrut: &'tcx hir::Expr<'tcx>, + arms: &'tcx [hir::Arm<'tcx>], + orig_expected: Expectation<'tcx>, + match_src: hir::MatchSource, + ) -> Ty<'tcx> { + let tcx = self.tcx; + + let acrb = arms_contain_ref_bindings(arms); + let scrutinee_ty = self.demand_scrutinee_type(scrut, acrb, arms.is_empty()); + debug!(?scrutinee_ty); + + // If there are no arms, that is a diverging match; a special case. + if arms.is_empty() { + self.diverges.set(self.diverges.get() | Diverges::always(expr.span)); + return tcx.types.never; + } + + self.warn_arms_when_scrutinee_diverges(arms); + + // Otherwise, we have to union together the types that the arms produce and so forth. + let scrut_diverges = self.diverges.replace(Diverges::Maybe); + + // #55810: Type check patterns first so we get types for all bindings. + let scrut_span = scrut.span.find_ancestor_inside(expr.span).unwrap_or(scrut.span); + for arm in arms { + self.check_pat_top(&arm.pat, scrutinee_ty, Some(scrut_span), true); + } + + // Now typecheck the blocks. + // + // The result of the match is the common supertype of all the + // arms. Start out the value as bottom, since it's the, well, + // bottom the type lattice, and we'll be moving up the lattice as + // we process each arm. (Note that any match with 0 arms is matching + // on any empty type and is therefore unreachable; should the flow + // of execution reach it, we will panic, so bottom is an appropriate + // type in that case) + let mut all_arms_diverge = Diverges::WarnedAlways; + + let expected = orig_expected.adjust_for_branches(self); + debug!(?expected); + + let mut coercion = { + let coerce_first = match expected { + // We don't coerce to `()` so that if the match expression is a + // statement it's branches can have any consistent type. That allows + // us to give better error messages (pointing to a usually better + // arm for inconsistent arms or to the whole match when a `()` type + // is required). + Expectation::ExpectHasType(ety) if ety != self.tcx.mk_unit() => ety, + _ => self.next_ty_var(TypeVariableOrigin { + kind: TypeVariableOriginKind::MiscVariable, + span: expr.span, + }), + }; + CoerceMany::with_coercion_sites(coerce_first, arms) + }; + + let mut other_arms = vec![]; // Used only for diagnostics. + let mut prior_arm = None; + for arm in arms { + if let Some(g) = &arm.guard { + self.diverges.set(Diverges::Maybe); + match g { + hir::Guard::If(e) => { + self.check_expr_has_type_or_error(e, tcx.types.bool, |_| {}); + } + hir::Guard::IfLet(l) => { + self.check_expr_let(l); + } + }; + } + + self.diverges.set(Diverges::Maybe); + + let arm_ty = self.check_expr_with_expectation(&arm.body, expected); + all_arms_diverge &= self.diverges.get(); + + let opt_suggest_box_span = prior_arm.and_then(|(_, prior_arm_ty, _)| { + self.opt_suggest_box_span(prior_arm_ty, arm_ty, orig_expected) + }); + + let (arm_block_id, arm_span) = if let hir::ExprKind::Block(blk, _) = arm.body.kind { + (Some(blk.hir_id), self.find_block_span(blk)) + } else { + (None, arm.body.span) + }; + + let (span, code) = match prior_arm { + // The reason for the first arm to fail is not that the match arms diverge, + // but rather that there's a prior obligation that doesn't hold. + None => (arm_span, ObligationCauseCode::BlockTailExpression(arm.body.hir_id)), + Some((prior_arm_block_id, prior_arm_ty, prior_arm_span)) => ( + expr.span, + ObligationCauseCode::MatchExpressionArm(Box::new(MatchExpressionArmCause { + arm_block_id, + arm_span, + arm_ty, + prior_arm_block_id, + prior_arm_ty, + prior_arm_span, + scrut_span: scrut.span, + source: match_src, + prior_arms: other_arms.clone(), + scrut_hir_id: scrut.hir_id, + opt_suggest_box_span, + })), + ), + }; + let cause = self.cause(span, code); + + // This is the moral equivalent of `coercion.coerce(self, cause, arm.body, arm_ty)`. + // We use it this way to be able to expand on the potential error and detect when a + // `match` tail statement could be a tail expression instead. If so, we suggest + // removing the stray semicolon. + coercion.coerce_inner( + self, + &cause, + Some(&arm.body), + arm_ty, + Some(&mut |err| { + let Some(ret) = self + .tcx + .hir() + .find_by_def_id(self.body_id.owner.def_id) + .and_then(|owner| owner.fn_decl()) + .map(|decl| decl.output.span()) + else { return; }; + let Expectation::IsLast(stmt) = orig_expected else { + return + }; + let can_coerce_to_return_ty = match self.ret_coercion.as_ref() { + Some(ret_coercion) if self.in_tail_expr => { + let ret_ty = ret_coercion.borrow().expected_ty(); + let ret_ty = self.inh.infcx.shallow_resolve(ret_ty); + self.can_coerce(arm_ty, ret_ty) + && prior_arm.map_or(true, |(_, t, _)| self.can_coerce(t, ret_ty)) + // The match arms need to unify for the case of `impl Trait`. + && !matches!(ret_ty.kind(), ty::Opaque(..)) + } + _ => false, + }; + if !can_coerce_to_return_ty { + return; + } + + let semi_span = expr.span.shrink_to_hi().with_hi(stmt.hi()); + let mut ret_span: MultiSpan = semi_span.into(); + ret_span.push_span_label( + expr.span, + "this could be implicitly returned but it is a statement, not a \ + tail expression", + ); + ret_span + .push_span_label(ret, "the `match` arms can conform to this return type"); + ret_span.push_span_label( + semi_span, + "the `match` is a statement because of this semicolon, consider \ + removing it", + ); + err.span_note( + ret_span, + "you might have meant to return the `match` expression", + ); + err.tool_only_span_suggestion( + semi_span, + "remove this semicolon", + "", + Applicability::MaybeIncorrect, + ); + }), + false, + ); + + other_arms.push(arm_span); + if other_arms.len() > 5 { + other_arms.remove(0); + } + + prior_arm = Some((arm_block_id, arm_ty, arm_span)); + } + + // If all of the arms in the `match` diverge, + // and we're dealing with an actual `match` block + // (as opposed to a `match` desugared from something else'), + // we can emit a better note. Rather than pointing + // at a diverging expression in an arbitrary arm, + // we can point at the entire `match` expression + if let (Diverges::Always { .. }, hir::MatchSource::Normal) = (all_arms_diverge, match_src) { + all_arms_diverge = Diverges::Always { + span: expr.span, + custom_note: Some( + "any code following this `match` expression is unreachable, as all arms diverge", + ), + }; + } + + // We won't diverge unless the scrutinee or all arms diverge. + self.diverges.set(scrut_diverges | all_arms_diverge); + + coercion.complete(self) + } + + /// When the previously checked expression (the scrutinee) diverges, + /// warn the user about the match arms being unreachable. + fn warn_arms_when_scrutinee_diverges(&self, arms: &'tcx [hir::Arm<'tcx>]) { + for arm in arms { + self.warn_if_unreachable(arm.body.hir_id, arm.body.span, "arm"); + } + } + + /// Handle the fallback arm of a desugared if(-let) like a missing else. + /// + /// Returns `true` if there was an error forcing the coercion to the `()` type. + pub(super) fn if_fallback_coercion<T>( + &self, + span: Span, + then_expr: &'tcx hir::Expr<'tcx>, + coercion: &mut CoerceMany<'tcx, '_, T>, + ) -> bool + where + T: AsCoercionSite, + { + // If this `if` expr is the parent's function return expr, + // the cause of the type coercion is the return type, point at it. (#25228) + let ret_reason = self.maybe_get_coercion_reason(then_expr.hir_id, span); + let cause = self.cause(span, ObligationCauseCode::IfExpressionWithNoElse); + let mut error = false; + coercion.coerce_forced_unit( + self, + &cause, + &mut |err| { + if let Some((span, msg)) = &ret_reason { + err.span_label(*span, msg); + } else if let ExprKind::Block(block, _) = &then_expr.kind + && let Some(expr) = &block.expr + { + err.span_label(expr.span, "found here"); + } + err.note("`if` expressions without `else` evaluate to `()`"); + err.help("consider adding an `else` block that evaluates to the expected type"); + error = true; + }, + ret_reason.is_none(), + ); + error + } + + fn maybe_get_coercion_reason(&self, hir_id: hir::HirId, sp: Span) -> Option<(Span, String)> { + let node = { + let rslt = self.tcx.hir().get_parent_node(self.tcx.hir().get_parent_node(hir_id)); + self.tcx.hir().get(rslt) + }; + if let hir::Node::Block(block) = node { + // check that the body's parent is an fn + let parent = self + .tcx + .hir() + .get(self.tcx.hir().get_parent_node(self.tcx.hir().get_parent_node(block.hir_id))); + if let (Some(expr), hir::Node::Item(hir::Item { kind: hir::ItemKind::Fn(..), .. })) = + (&block.expr, parent) + { + // check that the `if` expr without `else` is the fn body's expr + if expr.span == sp { + return self.get_fn_decl(hir_id).and_then(|(fn_decl, _)| { + let span = fn_decl.output.span(); + let snippet = self.tcx.sess.source_map().span_to_snippet(span).ok()?; + Some((span, format!("expected `{snippet}` because of this return type"))) + }); + } + } + } + if let hir::Node::Local(hir::Local { ty: Some(_), pat, .. }) = node { + return Some((pat.span, "expected because of this assignment".to_string())); + } + None + } + + pub(crate) fn if_cause( + &self, + span: Span, + cond_span: Span, + then_expr: &'tcx hir::Expr<'tcx>, + else_expr: &'tcx hir::Expr<'tcx>, + then_ty: Ty<'tcx>, + else_ty: Ty<'tcx>, + opt_suggest_box_span: Option<Span>, + ) -> ObligationCause<'tcx> { + let mut outer_span = if self.tcx.sess.source_map().is_multiline(span) { + // The `if`/`else` isn't in one line in the output, include some context to make it + // clear it is an if/else expression: + // ``` + // LL | let x = if true { + // | _____________- + // LL || 10i32 + // || ----- expected because of this + // LL || } else { + // LL || 10u32 + // || ^^^^^ expected `i32`, found `u32` + // LL || }; + // ||_____- `if` and `else` have incompatible types + // ``` + Some(span) + } else { + // The entire expression is in one line, only point at the arms + // ``` + // LL | let x = if true { 10i32 } else { 10u32 }; + // | ----- ^^^^^ expected `i32`, found `u32` + // | | + // | expected because of this + // ``` + None + }; + + let (error_sp, else_id) = if let ExprKind::Block(block, _) = &else_expr.kind { + let block = block.innermost_block(); + + // Avoid overlapping spans that aren't as readable: + // ``` + // 2 | let x = if true { + // | _____________- + // 3 | | 3 + // | | - expected because of this + // 4 | | } else { + // | |____________^ + // 5 | || + // 6 | || }; + // | || ^ + // | ||_____| + // | |______if and else have incompatible types + // | expected integer, found `()` + // ``` + // by not pointing at the entire expression: + // ``` + // 2 | let x = if true { + // | ------- `if` and `else` have incompatible types + // 3 | 3 + // | - expected because of this + // 4 | } else { + // | ____________^ + // 5 | | + // 6 | | }; + // | |_____^ expected integer, found `()` + // ``` + if block.expr.is_none() && block.stmts.is_empty() + && let Some(outer_span) = &mut outer_span + && let Some(cond_span) = cond_span.find_ancestor_inside(*outer_span) + { + *outer_span = outer_span.with_hi(cond_span.hi()) + } + + (self.find_block_span(block), block.hir_id) + } else { + (else_expr.span, else_expr.hir_id) + }; + + let then_id = if let ExprKind::Block(block, _) = &then_expr.kind { + let block = block.innermost_block(); + // Exclude overlapping spans + if block.expr.is_none() && block.stmts.is_empty() { + outer_span = None; + } + block.hir_id + } else { + then_expr.hir_id + }; + + // Finally construct the cause: + self.cause( + error_sp, + ObligationCauseCode::IfExpression(Box::new(IfExpressionCause { + else_id, + then_id, + then_ty, + else_ty, + outer_span, + opt_suggest_box_span, + })), + ) + } + + pub(super) fn demand_scrutinee_type( + &self, + scrut: &'tcx hir::Expr<'tcx>, + contains_ref_bindings: Option<hir::Mutability>, + no_arms: bool, + ) -> Ty<'tcx> { + // Not entirely obvious: if matches may create ref bindings, we want to + // use the *precise* type of the scrutinee, *not* some supertype, as + // the "scrutinee type" (issue #23116). + // + // arielb1 [writes here in this comment thread][c] that there + // is certainly *some* potential danger, e.g., for an example + // like: + // + // [c]: https://github.com/rust-lang/rust/pull/43399#discussion_r130223956 + // + // ``` + // let Foo(x) = f()[0]; + // ``` + // + // Then if the pattern matches by reference, we want to match + // `f()[0]` as a lexpr, so we can't allow it to be + // coerced. But if the pattern matches by value, `f()[0]` is + // still syntactically a lexpr, but we *do* want to allow + // coercions. + // + // However, *likely* we are ok with allowing coercions to + // happen if there are no explicit ref mut patterns - all + // implicit ref mut patterns must occur behind a reference, so + // they will have the "correct" variance and lifetime. + // + // This does mean that the following pattern would be legal: + // + // ``` + // struct Foo(Bar); + // struct Bar(u32); + // impl Deref for Foo { + // type Target = Bar; + // fn deref(&self) -> &Bar { &self.0 } + // } + // impl DerefMut for Foo { + // fn deref_mut(&mut self) -> &mut Bar { &mut self.0 } + // } + // fn foo(x: &mut Foo) { + // { + // let Bar(z): &mut Bar = x; + // *z = 42; + // } + // assert_eq!(foo.0.0, 42); + // } + // ``` + // + // FIXME(tschottdorf): don't call contains_explicit_ref_binding, which + // is problematic as the HIR is being scraped, but ref bindings may be + // implicit after #42640. We need to make sure that pat_adjustments + // (once introduced) is populated by the time we get here. + // + // See #44848. + if let Some(m) = contains_ref_bindings { + self.check_expr_with_needs(scrut, Needs::maybe_mut_place(m)) + } else if no_arms { + self.check_expr(scrut) + } else { + // ...but otherwise we want to use any supertype of the + // scrutinee. This is sort of a workaround, see note (*) in + // `check_pat` for some details. + let scrut_ty = self.next_ty_var(TypeVariableOrigin { + kind: TypeVariableOriginKind::TypeInference, + span: scrut.span, + }); + self.check_expr_has_type_or_error(scrut, scrut_ty, |_| {}); + scrut_ty + } + } + + /// When we have a `match` as a tail expression in a `fn` with a returned `impl Trait` + /// we check if the different arms would work with boxed trait objects instead and + /// provide a structured suggestion in that case. + pub(crate) fn opt_suggest_box_span( + &self, + first_ty: Ty<'tcx>, + second_ty: Ty<'tcx>, + orig_expected: Expectation<'tcx>, + ) -> Option<Span> { + // FIXME(compiler-errors): This really shouldn't need to be done during the + // "good" path of typeck, but here we are. + match orig_expected { + Expectation::ExpectHasType(expected) => { + let TypeVariableOrigin { + span, + kind: TypeVariableOriginKind::OpaqueTypeInference(rpit_def_id), + .. + } = self.type_var_origin(expected)? else { return None; }; + + let sig = *self + .typeck_results + .borrow() + .liberated_fn_sigs() + .get(hir::HirId::make_owner(self.body_id.owner.def_id))?; + + let substs = sig.output().walk().find_map(|arg| { + if let ty::GenericArgKind::Type(ty) = arg.unpack() + && let ty::Opaque(def_id, substs) = *ty.kind() + && def_id == rpit_def_id + { + Some(substs) + } else { + None + } + })?; + let opaque_ty = self.tcx.mk_opaque(rpit_def_id, substs); + + if !self.can_coerce(first_ty, expected) || !self.can_coerce(second_ty, expected) { + return None; + } + + for ty in [first_ty, second_ty] { + for pred in self.tcx.bound_explicit_item_bounds(rpit_def_id).transpose_iter() { + let pred = pred.map_bound(|(pred, _)| *pred).subst(self.tcx, substs); + let pred = match pred.kind().skip_binder() { + ty::PredicateKind::Trait(mut trait_pred) => { + assert_eq!(trait_pred.trait_ref.self_ty(), opaque_ty); + trait_pred.trait_ref.substs = + self.tcx.mk_substs_trait(ty, &trait_pred.trait_ref.substs[1..]); + pred.kind().rebind(trait_pred).to_predicate(self.tcx) + } + ty::PredicateKind::Projection(mut proj_pred) => { + assert_eq!(proj_pred.projection_ty.self_ty(), opaque_ty); + proj_pred.projection_ty.substs = self + .tcx + .mk_substs_trait(ty, &proj_pred.projection_ty.substs[1..]); + pred.kind().rebind(proj_pred).to_predicate(self.tcx) + } + _ => continue, + }; + if !self.predicate_must_hold_modulo_regions(&Obligation::new( + ObligationCause::misc(span, self.body_id), + self.param_env, + pred, + )) { + return None; + } + } + } + + Some(span) + } + _ => None, + } + } +} + +fn arms_contain_ref_bindings<'tcx>(arms: &'tcx [hir::Arm<'tcx>]) -> Option<hir::Mutability> { + arms.iter().filter_map(|a| a.pat.contains_explicit_ref_binding()).max_by_key(|m| match *m { + hir::Mutability::Mut => 1, + hir::Mutability::Not => 0, + }) +} |