Added struct fmt::FormattingOptions

This allows to build custom `std::Formatter`s at runtime.

Also added some related enums and two related methods on `std::Formatter`.

alloc/src/fmt.rs

@@ -596,6 +596,8 @@ pub use core::fmt::{Arguments, write};
 pub use core::fmt::{Binary, Octal};
 #[stable(feature = "rust1", since = "1.0.0")]
 pub use core::fmt::{Debug, Display};
+#[unstable(feature = "formatting_options", issue = "118117")]
+pub use core::fmt::{DebugAsHex, FormattingOptions, Sign};
 #[stable(feature = "rust1", since = "1.0.0")]
 pub use core::fmt::{DebugList, DebugMap, DebugSet, DebugStruct, DebugTuple};
 #[stable(feature = "rust1", since = "1.0.0")]

alloc/src/lib.rs

@@ -117,6 +117,7 @@
 #![feature(extend_one_unchecked)]
 #![feature(fmt_internals)]
 #![feature(fn_traits)]
+#![feature(formatting_options)]
 #![feature(hasher_prefixfree_extras)]
 #![feature(inplace_iteration)]
 #![feature(iter_advance_by)]

alloc/src/string.rs

@@ -43,6 +43,7 @@
 #![stable(feature = "rust1", since = "1.0.0")]
 
 use core::error::Error;
+use core::fmt::FormattingOptions;
 use core::iter::FusedIterator;
 #[cfg(not(no_global_oom_handling))]
 use core::iter::from_fn;
@@ -2682,7 +2683,7 @@ impl<T: fmt::Display + ?Sized> ToString for T {
     #[inline]
     default fn to_string(&self) -> String {
         let mut buf = String::new();
-        let mut formatter = core::fmt::Formatter::new(&mut buf);
+        let mut formatter = core::fmt::Formatter::new(&mut buf, FormattingOptions::new());
         // Bypass format_args!() to avoid write_str with zero-length strs
         fmt::Display::fmt(self, &mut formatter)
             .expect("a Display implementation returned an error unexpectedly");