Always require closure parameters to be Sized

The `rust-call` ABI isn't compatible with
`#![feature(unsized_fn_params)]`, so trying to use that feature with
closures leads to an ICE (rust-lang#67981). This turns that ICE into a
type-check error.

compiler/rustc_hir_typeck/src/check.rs

@@ -32,6 +32,7 @@ pub(super) fn check_fn<'a, 'tcx>(
     fn_def_id: LocalDefId,
     body: &'tcx hir::Body<'tcx>,
     can_be_generator: Option<hir::Movability>,
+    params_can_be_unsized: bool,
 ) -> Option<GeneratorTypes<'tcx>> {
     let fn_id = fcx.tcx.hir().local_def_id_to_hir_id(fn_def_id);
 
@@ -94,7 +95,7 @@ pub(super) fn check_fn<'a, 'tcx>(
         // The check for a non-trivial pattern is a hack to avoid duplicate warnings
         // for simple cases like `fn foo(x: Trait)`,
         // where we would error once on the parameter as a whole, and once on the binding `x`.
-        if param.pat.simple_ident().is_none() && !tcx.features().unsized_fn_params {
+        if param.pat.simple_ident().is_none() && !params_can_be_unsized {
             fcx.require_type_is_sized(param_ty, param.pat.span, traits::SizedArgumentType(ty_span));
         }
 

compiler/rustc_hir_typeck/src/closure.rs

@@ -89,6 +89,8 @@ impl<'a, 'tcx> FnCtxt<'a, 'tcx> {
             expr_def_id,
             body,
             closure.movability,
+            // Closure "rust-call" ABI doesn't support unsized params
+            false,
         );
 
         let parent_substs = InternalSubsts::identity_for_item(

compiler/rustc_hir_typeck/src/lib.rs

@@ -212,7 +212,7 @@ fn typeck_with_fallback<'tcx>(
         let fn_sig = tcx.liberate_late_bound_regions(def_id.to_def_id(), fn_sig);
         let fn_sig = fcx.normalize(body.value.span, fn_sig);
 
-        check_fn(&mut fcx, fn_sig, decl, def_id, body, None);
+        check_fn(&mut fcx, fn_sig, decl, def_id, body, None, tcx.features().unsized_fn_params);
     } else {
         let expected_type = if let Some(&hir::Ty { kind: hir::TyKind::Infer, span, .. }) = body_ty {
             Some(fcx.next_ty_var(TypeVariableOrigin {

tests/ui/unsized-locals/issue-67981.rs

@@ -0,0 +1,9 @@
+#![feature(unsized_fn_params)]
+
+fn main() {
+    let f: fn([u8]) = |_| {};
+    //~^ERROR the size for values of type `[u8]` cannot be known at compilation time
+    let slice: Box<[u8]> = Box::new([1; 8]);
+
+    f(*slice);
+}

tests/ui/unsized-locals/issue-67981.stderr

@@ -0,0 +1,15 @@
+error[E0277]: the size for values of type `[u8]` cannot be known at compilation time
+  --> $DIR/issue-67981.rs:4:24
+   |
+LL |     let f: fn([u8]) = |_| {};
+   |                        ^ doesn't have a size known at compile-time
+   |
+   = help: the trait `Sized` is not implemented for `[u8]`
+help: function arguments must have a statically known size, borrowed types always have a known size
+   |
+LL |     let f: fn([u8]) = |&_| {};
+   |                        +
+
+error: aborting due to previous error
+
+For more information about this error, try `rustc --explain E0277`.