Fix FormattingOptions instantiation with Default

The `fill` value by default should be set to `' '` (space), but the current implementation uses `#[derive(Default)]` which sets it to `\0`

library/core/src/fmt/mod.rs

@@ -288,7 +288,7 @@ pub enum DebugAsHex {
 ///
 /// `FormattingOptions` is a [`Formatter`] without an attached [`Write`] trait.
 /// It is mainly used to construct `Formatter` instances.
-#[derive(Copy, Clone, Debug, PartialEq, Eq, Default)]
+#[derive(Copy, Clone, Debug, PartialEq, Eq)]
 #[unstable(feature = "formatting_options", issue = "118117")]
 pub struct FormattingOptions {
     flags: u32,
@@ -508,6 +508,15 @@ impl FormattingOptions {
     }
 }
 
+#[unstable(feature = "formatting_options", issue = "118117")]
+impl Default for FormattingOptions {
+    /// Same as [`FormattingOptions::new()`].
+    fn default() -> Self {
+        // The `#[derive(Default)]` implementation would set `fill` to `\0` instead of space.
+        Self::new()
+    }
+}
+
 /// Configuration for formatting.
 ///
 /// A `Formatter` represents various options related to formatting. Users do not

library/core/tests/fmt/mod.rs

@@ -51,6 +51,12 @@ fn test_maybe_uninit_short() {
     assert_eq!(format!("{x:?}"), "MaybeUninit<u32>");
 }
 
+#[test]
+fn formatting_options_ctor() {
+    use core::fmt::FormattingOptions;
+    assert_eq!(FormattingOptions::new(), FormattingOptions::default());
+}
+
 #[test]
 fn formatting_options_flags() {
     use core::fmt::*;