Rollup merge of #129017 - its-the-shrimp:core_fmt_from_fn, r=Noratrieb

Replace `std::fmt:FormatterFn` with `std::fmt::from_fn`

Modelled after the suggestion made in [this](https://github.com/rust-lang/rust/issues/117729#issuecomment-1837628559) comment, this should bring this functionality in line with the existing `array::from_fn` & `iter::from_fn`

library/alloc/src/fmt.rs

@@ -581,7 +581,7 @@
 #[stable(feature = "rust1", since = "1.0.0")]
 pub use core::fmt::Error;
 #[unstable(feature = "debug_closure_helpers", issue = "117729")]
-pub use core::fmt::FormatterFn;
+pub use core::fmt::{from_fn, FromFn};
 #[stable(feature = "rust1", since = "1.0.0")]
 pub use core::fmt::{write, Arguments};
 #[stable(feature = "rust1", since = "1.0.0")]

library/core/src/fmt/builders.rs

@@ -1018,7 +1018,8 @@ fn is_pretty(&self) -> bool {
     }
 }
 
-/// Implements [`fmt::Debug`] and [`fmt::Display`] using a function.
+/// Creates a type whose [`fmt::Debug`] and [`fmt::Display`] impls are provided with the function
+/// `f`.
 ///
 /// # Examples
 ///
@@ -1030,17 +1031,25 @@ fn is_pretty(&self) -> bool {
 /// assert_eq!(format!("{}", value), "a");
 /// assert_eq!(format!("{:?}", value), "'a'");
 ///
-/// let wrapped = fmt::FormatterFn(|f| write!(f, "{value:?}"));
+/// let wrapped = fmt::from_fn(|f| write!(f, "{value:?}"));
 /// assert_eq!(format!("{}", wrapped), "'a'");
 /// assert_eq!(format!("{:?}", wrapped), "'a'");
 /// ```
 #[unstable(feature = "debug_closure_helpers", issue = "117729")]
-pub struct FormatterFn<F>(pub F)
+pub fn from_fn<F: Fn(&mut fmt::Formatter<'_>) -> fmt::Result>(f: F) -> FromFn<F> {
+    FromFn(f)
+}
+
+/// Implements [`fmt::Debug`] and [`fmt::Display`] using a function.
+///
+/// Created with [`from_fn`].
+#[unstable(feature = "debug_closure_helpers", issue = "117729")]
+pub struct FromFn<F>(F)
 where
     F: Fn(&mut fmt::Formatter<'_>) -> fmt::Result;
 
 #[unstable(feature = "debug_closure_helpers", issue = "117729")]
-impl<F> fmt::Debug for FormatterFn<F>
+impl<F> fmt::Debug for FromFn<F>
 where
     F: Fn(&mut fmt::Formatter<'_>) -> fmt::Result,
 {
@@ -1050,7 +1059,7 @@ fn fmt(&self, f: &mut fmt::Formatter<'_>) -> fmt::Result {
 }
 
 #[unstable(feature = "debug_closure_helpers", issue = "117729")]
-impl<F> fmt::Display for FormatterFn<F>
+impl<F> fmt::Display for FromFn<F>
 where
     F: Fn(&mut fmt::Formatter<'_>) -> fmt::Result,
 {

library/core/src/fmt/mod.rs

@@ -34,7 +34,7 @@ pub enum Alignment {
 }
 
 #[unstable(feature = "debug_closure_helpers", issue = "117729")]
-pub use self::builders::FormatterFn;
+pub use self::builders::{from_fn, FromFn};
 #[stable(feature = "debug_builders", since = "1.2.0")]
 pub use self::builders::{DebugList, DebugMap, DebugSet, DebugStruct, DebugTuple};