Adding upstream version 1.64.0+dfsg1.upstream/1.64.0+dfsg1

Signed-off-by: Daniel Baumann <daniel.baumann@progress-linux.org>

1 files changed, 529 insertions, 0 deletions

diff --git a/compiler/rustc_typeck/src/check/_match.rs b/compiler/rustc_typeck/src/check/_match.rs
new file mode 100644
index 000000000..1b13c98e4
--- /dev/null
+++ b/compiler/rustc_typeck/src/check/_match.rs
@@ -0,0 +1,529 @@
+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, TypeVisitable};
+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")]
+    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 = self.opt_suggest_box_span(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.ret_type_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);
+
+        let match_ty = coercion.complete(self);
+        debug!(?match_ty);
+        match_ty
+    }
+
+    /// 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,
+        outer_ty: Ty<'tcx>,
+        orig_expected: Expectation<'tcx>,
+    ) -> Option<Span> {
+        match orig_expected {
+            Expectation::ExpectHasType(expected)
+                if self.in_tail_expr
+                    && self.ret_coercion.as_ref()?.borrow().merged_ty().has_opaque_types()
+                    && self.can_coerce(outer_ty, expected) =>
+            {
+                let obligations = self.fulfillment_cx.borrow().pending_obligations();
+                let mut suggest_box = !obligations.is_empty();
+                for o in obligations {
+                    match o.predicate.kind().skip_binder() {
+                        ty::PredicateKind::Trait(t) => {
+                            let pred =
+                                ty::Binder::dummy(ty::PredicateKind::Trait(ty::TraitPredicate {
+                                    trait_ref: ty::TraitRef {
+                                        def_id: t.def_id(),
+                                        substs: self.tcx.mk_substs_trait(outer_ty, &[]),
+                                    },
+                                    constness: t.constness,
+                                    polarity: t.polarity,
+                                }));
+                            let obl = Obligation::new(
+                                o.cause.clone(),
+                                self.param_env,
+                                pred.to_predicate(self.tcx),
+                            );
+                            suggest_box &= self.predicate_must_hold_modulo_regions(&obl);
+                            if !suggest_box {
+                                // We've encountered some obligation that didn't hold, so the
+                                // return expression can't just be boxed. We don't need to
+                                // evaluate the rest of the obligations.
+                                break;
+                            }
+                        }
+                        _ => {}
+                    }
+                }
+                // If all the obligations hold (or there are no obligations) the tail expression
+                // we can suggest to return a boxed trait object instead of an opaque type.
+                if suggest_box { self.ret_type_span } else { None }
+            }
+            _ => 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,
+    })
+}