Allow slices to be used in field

src/libcore/fmt/builders.rs

@@ -103,7 +103,8 @@ pub fn debug_struct_new<'a, 'b>(fmt: &'a mut fmt::Formatter<'b>,
 impl<'a, 'b: 'a> DebugStruct<'a, 'b> {
     /// Adds a new field to the generated struct output.
     #[stable(feature = "debug_builders", since = "1.2.0")]
-    pub fn field(&mut self, name: &str, value: &fmt::Debug) -> &mut DebugStruct<'a, 'b> {
+    pub fn field<V: fmt::Debug>(&mut self, name: &str, value: &V) -> &mut DebugStruct<'a, 'b>
+        where V: ?Sized {
         self.result = self.result.and_then(|_| {
             let prefix = if self.has_fields {
                 ","
@@ -195,7 +196,8 @@ pub fn debug_tuple_new<'a, 'b>(fmt: &'a mut fmt::Formatter<'b>, name: &str) -> D
 impl<'a, 'b: 'a> DebugTuple<'a, 'b> {
     /// Adds a new field to the generated tuple struct output.
     #[stable(feature = "debug_builders", since = "1.2.0")]
-    pub fn field(&mut self, value: &fmt::Debug) -> &mut DebugTuple<'a, 'b> {
+    pub fn field<V: fmt::Debug>(&mut self, value: &V) -> &mut DebugTuple<'a, 'b>
+        where V: ?Sized {
         self.result = self.result.and_then(|_| {
             let (prefix, space) = if self.fields > 0 {
                 (",", " ")

src/test/run-pass/fmt-arrays.rs

@@ -0,0 +1,39 @@
+// Copyright 2017 The Rust Project Developers. See the COPYRIGHT
+// file at the top-level directory of this distribution and at
+// http://rust-lang.org/COPYRIGHT.
+//
+// Licensed under the Apache License, Version 2.0 <LICENSE-APACHE or
+// http://www.apache.org/licenses/LICENSE-2.0> or the MIT license
+// <LICENSE-MIT or http://opensource.org/licenses/MIT>, at your
+// option. This file may not be copied, modified, or distributed
+// except according to those terms.
+
+use std::fmt;
+
+struct Foo {
+    v: [i32; 33],
+}
+
+struct Bar([i32; 33]);
+
+impl fmt::Debug for Foo {
+    fn fmt(&self, fmt: &mut fmt::Formatter) -> fmt::Result {
+        fmt.debug_struct("Foo")
+           .field("v", &self.v as &[i32])
+           .finish()
+     }
+}
+
+impl fmt::Debug for Bar {
+    fn fmt(&self, fmt: &mut fmt::Formatter) -> fmt::Result {
+        fmt.debug_tuple("Foo")
+           .field(&self.0 as &[i32])
+           .finish()
+     }
+}
+
+fn main() {
+    println!("{:?} {:?}",
+             Foo { v: [0,0,0,0,0,0,0,0,0,0,0,0,0,0,0,0,0,0,0,0,0,0,0,0,0,0,0,0,0,0,0,0,0] },
+             Bar([0,0,0,0,0,0,0,0,0,0,0,0,0,0,0,0,0,0,0,0,0,0,0,0,0,0,0,0,0,0,0,0,0]));
+}