Do not use unsized_fn_params in patterns

compiler/rustc_typeck/src/check/gather_locals.rs

@@ -129,7 +129,9 @@ impl<'a, 'tcx> Visitor<'tcx> for GatherLocalsVisitor<'a, 'tcx> {
                 var_ty
             );
         }
+        let old_within_fn_param = mem::replace(&mut self.within_fn_param, false);
         intravisit::walk_pat(self, p);
+        self.within_fn_param = old_within_fn_param;
     }
 
     // Don't descend into the bodies of nested closures.

src/test/ui/unsized-locals/unsized-local-pat.rs

@@ -0,0 +1,11 @@
+#![feature(box_patterns)]
+#![feature(unsized_fn_params)]
+
+#[allow(dead_code)]
+fn f1(box box _b: Box<Box<[u8]>>) {}
+//~^ ERROR: the size for values of type `[u8]` cannot be known at compilation time [E0277]
+
+fn f2((_x, _y): (i32, [i32])) {}
+//~^ ERROR: the size for values of type `[i32]` cannot be known at compilation time [E0277]
+
+fn main() {}

src/test/ui/unsized-locals/unsized-local-pat.stderr

@@ -0,0 +1,23 @@
+error[E0277]: the size for values of type `[u8]` cannot be known at compilation time
+  --> $DIR/unsized-local-pat.rs:5:15
+   |
+LL | fn f1(box box _b: Box<Box<[u8]>>) {}
+   |               ^^ doesn't have a size known at compile-time
+   |
+   = help: the trait `Sized` is not implemented for `[u8]`
+   = note: all local variables must have a statically known size
+   = help: unsized locals are gated as an unstable feature
+
+error[E0277]: the size for values of type `[i32]` cannot be known at compilation time
+  --> $DIR/unsized-local-pat.rs:8:12
+   |
+LL | fn f2((_x, _y): (i32, [i32])) {}
+   |            ^^ doesn't have a size known at compile-time
+   |
+   = help: the trait `Sized` is not implemented for `[i32]`
+   = note: all local variables must have a statically known size
+   = help: unsized locals are gated as an unstable feature
+
+error: aborting due to 2 previous errors
+
+For more information about this error, try `rustc --explain E0277`.

src/test/ui/unsized-locals/unsized-parameters.rs

@@ -5,7 +5,7 @@
 
 pub fn f0(_f: dyn FnOnce()) {}
 pub fn f1(_s: str) {}
-pub fn f2((_x, _y): (i32, [i32])) {}
+pub fn f2(_x: i32, _y: [i32]) {}
 
 fn main() {
     let foo = "foo".to_string().into_boxed_str();