Error on moving unsized values rather than ICE'ing

4 files changed, 35 insertions, 2 deletions

diff --git a/compiler/rustc_codegen_ssa/src/mir/mod.rs b/compiler/rustc_codegen_ssa/src/mir/mod.rs
index 4e35b143173..fa69820d5d2 100644
--- a/compiler/rustc_codegen_ssa/src/mir/mod.rs
+++ b/compiler/rustc_codegen_ssa/src/mir/mod.rs
@@ -141,7 +141,12 @@ enum LocalRef<'tcx, V> {
     /// `UnsizedPlace(p)`: `p` itself is a thin pointer (indirect place).
     /// `*p` is the wide pointer that references the actual unsized place.
     ///
-    /// Rust has no alloca and thus no ability to move the unsized place.
+    /// MIR only supports unsized args, not dynamically-sized locals, so
+    /// new unsized temps don't exist and we must reuse the referred-to place.
+    ///
+    /// FIXME: Since the removal of unsized locals in <https://github.com/rust-lang/rust/pull/142911>,
+    /// can we maybe use `Place` here? Or refactor it in another way? There are quite a few
+    /// `UnsizedPlace => bug` branches now.
     UnsizedPlace(PlaceRef<'tcx, V>),
     /// The backend [`OperandValue`] has already been generated.
     Operand(OperandRef<'tcx, V>),
diff --git a/compiler/rustc_hir_typeck/src/fn_ctxt/checks.rs b/compiler/rustc_hir_typeck/src/fn_ctxt/checks.rs
index eeb8d33ef65..eb8d671c939 100644
--- a/compiler/rustc_hir_typeck/src/fn_ctxt/checks.rs
+++ b/compiler/rustc_hir_typeck/src/fn_ctxt/checks.rs
@@ -241,6 +241,8 @@ impl<'a, 'tcx> FnCtxt<'a, 'tcx> {
                 arg_expr.span,
                 ObligationCauseCode::WellFormed(None),
             );
+
+            self.check_place_expr_if_unsized(fn_input_ty, arg_expr);
         }
 
         // First, let's unify the formal method signature with the expectation eagerly.
@@ -543,6 +545,21 @@ impl<'a, 'tcx> FnCtxt<'a, 'tcx> {
         }
     }
 
+    /// If `unsized_fn_params` is active, check that unsized values are place expressions. Since
+    /// the removal of `unsized_locals` in <https://github.com/rust-lang/rust/pull/142911> we can't
+    /// store them in MIR locals as temporaries.
+    ///
+    /// If `unsized_fn_params` is inactive, this will be checked in borrowck instead.
+    fn check_place_expr_if_unsized(&self, ty: Ty<'tcx>, expr: &'tcx hir::Expr<'tcx>) {
+        if self.tcx.features().unsized_fn_params() && !expr.is_syntactic_place_expr() {
+            self.require_type_is_sized(
+                ty,
+                expr.span,
+                ObligationCauseCode::UnsizedNonPlaceExpr(expr.span),
+            );
+        }
+    }
+
     fn report_arg_errors(
         &self,
         compatibility_diagonal: IndexVec<ProvidedIdx, Compatibility<'tcx>>,
@@ -1873,7 +1890,8 @@ impl<'a, 'tcx> FnCtxt<'a, 'tcx> {
                 });
             }
             hir::StmtKind::Semi(expr) => {
-                self.check_expr(expr);
+                let ty = self.check_expr(expr);
+                self.check_place_expr_if_unsized(ty, expr);
             }
         }
 
diff --git a/compiler/rustc_middle/src/traits/mod.rs b/compiler/rustc_middle/src/traits/mod.rs
index 1a5a9765ce7..5bdde3a514e 100644
--- a/compiler/rustc_middle/src/traits/mod.rs
+++ b/compiler/rustc_middle/src/traits/mod.rs
@@ -412,6 +412,10 @@ pub enum ObligationCauseCode<'tcx> {
 
     /// Obligations emitted during the normalization of a free type alias.
     TypeAlias(ObligationCauseCodeHandle<'tcx>, Span, DefId),
+
+    /// Only reachable if the `unsized_fn_params` feature is used. Unsized function arguments must
+    /// be place expressions because we can't store them in MIR locals as temporaries.
+    UnsizedNonPlaceExpr(Span),
 }
 
 /// Whether a value can be extracted into a const.
diff --git a/compiler/rustc_trait_selection/src/error_reporting/traits/suggestions.rs b/compiler/rustc_trait_selection/src/error_reporting/traits/suggestions.rs
index 362052e9fdb..c5b85c95ba2 100644
--- a/compiler/rustc_trait_selection/src/error_reporting/traits/suggestions.rs
+++ b/compiler/rustc_trait_selection/src/error_reporting/traits/suggestions.rs
@@ -3624,6 +3624,12 @@ impl<'a, 'tcx> TypeErrCtxt<'a, 'tcx> {
                 );
                 suggest_remove_deref(err, &expr);
             }
+            ObligationCauseCode::UnsizedNonPlaceExpr(span) => {
+                err.span_note(
+                    span,
+                    "unsized values must be place expressions and cannot be put in temporaries",
+                );
+            }
         }
     }