From 515b93297ff6b96e6368070f3eb3f3a833112bc0 Mon Sep 17 00:00:00 2001 From: Michael Goulet Date: Thu, 5 Sep 2024 07:53:49 -0400 Subject: [PATCH] Document things a bit more carefully, also account for coercion in check_expr_has_type_or_error --- compiler/rustc_hir_typeck/src/coercion.rs | 1 + compiler/rustc_hir_typeck/src/expr.rs | 16 +++++++++++++++- tests/mir-opt/uninhabited_not_read.rs | 1 - .../match/pattern-matching-should-fail.stderr | 6 ++---- 4 files changed, 18 insertions(+), 6 deletions(-) diff --git a/compiler/rustc_hir_typeck/src/coercion.rs b/compiler/rustc_hir_typeck/src/coercion.rs index 1705421c2f4..96bff04dbc7 100644 --- a/compiler/rustc_hir_typeck/src/coercion.rs +++ b/compiler/rustc_hir_typeck/src/coercion.rs @@ -186,6 +186,7 @@ fn coerce(&self, a: Ty<'tcx>, b: Ty<'tcx>) -> CoerceResult<'tcx> { if self.coerce_never { return success(simple(Adjust::NeverToAny)(b), b, vec![]); } else { + // Otherwise the only coercion we can do is unification. return self.unify_and(a, b, identity); } } diff --git a/compiler/rustc_hir_typeck/src/expr.rs b/compiler/rustc_hir_typeck/src/expr.rs index 870a260c9fe..604bec95701 100644 --- a/compiler/rustc_hir_typeck/src/expr.rs +++ b/compiler/rustc_hir_typeck/src/expr.rs @@ -62,7 +62,7 @@ pub(crate) fn check_expr_has_type_or_error( // While we don't allow *arbitrary* coercions here, we *do* allow // coercions from ! to `expected`. - if ty.is_never() { + if ty.is_never() && self.expr_constitutes_read(expr) { if let Some(_) = self.typeck_results.borrow().adjustments().get(expr.hir_id) { let reported = self.dcx().span_delayed_bug( expr.span, @@ -260,6 +260,20 @@ pub(super) fn check_expr_with_expectation_and_args( ty } + /// Whether this expression constitutes a read of value of the type that + /// it evaluates to. + /// + /// This is used to determine if we should consider the block to diverge + /// if the expression evaluates to `!`, and if we should insert a `NeverToAny` + /// coercion for values of type `!`. + /// + /// This function generally returns `false` if the expression is a place + /// expression and the *parent* expression is the scrutinee of a match or + /// the pointee of an `&` addr-of expression, since both of those parent + /// expressions take a *place* and not a value. + /// + /// This function is unfortunately a bit heuristical, though it is certainly + /// far sounder than the prior status quo: . pub(super) fn expr_constitutes_read(&self, expr: &'tcx hir::Expr<'tcx>) -> bool { // We only care about place exprs. Anything else returns an immediate // which would constitute a read. We don't care about distinguishing diff --git a/tests/mir-opt/uninhabited_not_read.rs b/tests/mir-opt/uninhabited_not_read.rs index 39ffdbdbb33..15769cdd75b 100644 --- a/tests/mir-opt/uninhabited_not_read.rs +++ b/tests/mir-opt/uninhabited_not_read.rs @@ -24,4 +24,3 @@ fn main() { let _ = *x; } } - diff --git a/tests/ui/closures/2229_closure_analysis/match/pattern-matching-should-fail.stderr b/tests/ui/closures/2229_closure_analysis/match/pattern-matching-should-fail.stderr index f8ed792e3c6..e0d900a1eb5 100644 --- a/tests/ui/closures/2229_closure_analysis/match/pattern-matching-should-fail.stderr +++ b/tests/ui/closures/2229_closure_analysis/match/pattern-matching-should-fail.stderr @@ -7,14 +7,12 @@ LL | let c1 = || match x { }; | ^ `x` used here but it isn't initialized error[E0381]: used binding `x` isn't initialized - --> $DIR/pattern-matching-should-fail.rs:15:14 + --> $DIR/pattern-matching-should-fail.rs:15:23 | LL | let x: !; | - binding declared here but left uninitialized LL | let c2 = || match x { _ => () }; - | ^^ - borrow occurs due to use in closure - | | - | `x` used here but it isn't initialized + | ^ `x` used here but it isn't initialized error[E0381]: used binding `variant` isn't initialized --> $DIR/pattern-matching-should-fail.rs:27:13