Check that closure's by-value captures are sized

compiler/rustc_hir_typeck/src/upvar.rs

@@ -41,6 +41,7 @@ use rustc_hir::intravisit::{self, Visitor};
 use rustc_infer::infer::UpvarRegion;
 use rustc_middle::hir::place::{Place, PlaceBase, PlaceWithHirId, Projection, ProjectionKind};
 use rustc_middle::mir::FakeReadCause;
+use rustc_middle::traits::ObligationCauseCode;
 use rustc_middle::ty::{
     self, ClosureSizeProfileData, Ty, TyCtxt, TypeckResults, UpvarArgs, UpvarCapture,
 };
@@ -295,6 +296,20 @@ impl<'a, 'tcx> FnCtxt<'a, 'tcx> {
         let final_upvar_tys = self.final_upvar_tys(closure_def_id);
         debug!(?closure_hir_id, ?args, ?final_upvar_tys);
 
+        if self.tcx.features().unsized_locals || self.tcx.features().unsized_fn_params {
+            for capture in
+                self.typeck_results.borrow().closure_min_captures_flattened(closure_def_id)
+            {
+                if let UpvarCapture::ByValue = capture.info.capture_kind {
+                    self.require_type_is_sized(
+                        capture.place.ty(),
+                        capture.get_path_span(self.tcx),
+                        ObligationCauseCode::SizedClosureCapture(closure_def_id),
+                    );
+                }
+            }
+        }
+
         // Build a tuple (U0..Un) of the final upvar types U0..Un
         // and unify the upvar tuple type in the closure with it:
         let final_tupled_upvars_type = Ty::new_tup(self.tcx, &final_upvar_tys);

compiler/rustc_middle/src/traits/mod.rs

@@ -299,6 +299,8 @@ pub enum ObligationCauseCode<'tcx> {
     SizedYieldType,
     /// Inline asm operand type must be `Sized`.
     InlineAsmSized,
+    /// Captured closure type type must be `Sized`.
+    SizedClosureCapture(LocalDefId),
     /// `[expr; N]` requires `type_of(expr): Copy`.
     RepeatElementCopy {
         /// If element is a `const fn` we display a help message suggesting to move the

compiler/rustc_trait_selection/src/traits/error_reporting/suggestions.rs

@@ -3007,6 +3007,17 @@ impl<'tcx> TypeErrCtxtExt<'tcx> for TypeErrCtxt<'_, 'tcx> {
             ObligationCauseCode::InlineAsmSized => {
                 err.note("all inline asm arguments must have a statically known size");
             }
+            ObligationCauseCode::SizedClosureCapture(closure_def_id) => {
+                err.note("all values captured by value by a closure must have a statically known size");
+                let hir::ExprKind::Closure(closure) = self.tcx.hir().get_by_def_id(closure_def_id).expect_expr().kind else {
+                    bug!("expected closure in SizedClosureCapture obligation");
+                };
+                if let hir::CaptureBy::Value = closure.capture_clause
+                    && let Some(span) = closure.fn_arg_span
+                {
+                    err.span_label(span, "this closure captures all values by move");
+                }
+            }
             ObligationCauseCode::ConstPatternStructural => {
                 err.note("constants used for pattern-matching must derive `PartialEq` and `Eq`");
             }

tests/ui/closures/capture-unsized-by-move.rs

@@ -0,0 +1,10 @@
+// compile-flags: --crate-type=lib
+
+#![feature(unsized_fn_params)]
+
+pub fn f(k: dyn std::fmt::Display) {
+    let k2 = move || {
+        k.to_string();
+        //~^ ERROR the size for values of type `(dyn std::fmt::Display + 'static)` cannot be known at compilation time
+    };
+}

tests/ui/closures/capture-unsized-by-move.stderr

@@ -0,0 +1,14 @@
+error[E0277]: the size for values of type `(dyn std::fmt::Display + 'static)` cannot be known at compilation time
+  --> $DIR/capture-unsized-by-move.rs:7:9
+   |
+LL |     let k2 = move || {
+   |                   -- this closure captures all values by move
+LL |         k.to_string();
+   |         ^ doesn't have a size known at compile-time
+   |
+   = help: the trait `Sized` is not implemented for `(dyn std::fmt::Display + 'static)`
+   = note: all values captured by value by a closure must have a statically known size
+
+error: aborting due to previous error
+
+For more information about this error, try `rustc --explain E0277`.

tests/ui/closures/capture-unsized-by-ref.rs

@@ -0,0 +1,10 @@
+// build-pass
+// compile-flags: --crate-type=lib
+
+#![feature(unsized_fn_params)]
+
+pub fn f(k: dyn std::fmt::Display) {
+    let k2 = || {
+        k.to_string();
+    };
+}