Expose fmt::Arguments::from_str as unstable.

library/alloc/src/lib.rs

@@ -116,6 +116,7 @@
 #![feature(exact_size_is_empty)]
 #![feature(extend_one)]
 #![feature(extend_one_unchecked)]
+#![feature(fmt_arguments_from_str)]
 #![feature(fmt_internals)]
 #![feature(fn_traits)]
 #![feature(formatting_options)]

library/core/src/fmt/mod.rs

@@ -734,7 +734,21 @@ impl<'a> Arguments<'a> {
         unsafe { Arguments { template: mem::transmute(template), args: mem::transmute(args) } }
     }
 
+    // Same as `from_str`, but not const.
+    // Used by format_args!() expansion when arguments are inlined,
+    // e.g. format_args!("{}", 123), which is not allowed in const.
     #[inline]
+    pub fn from_str_nonconst(s: &'static str) -> Arguments<'a> {
+        Arguments::from_str(s)
+    }
+}
+
+impl<'a> Arguments<'a> {
+    /// Create a `fmt::Arguments` object for a single static string.
+    ///
+    /// Formatting this `fmt::Arguments` will just produce the string as-is.
+    #[inline]
+    #[unstable(feature = "fmt_arguments_from_str", issue = "148905")]
     pub const fn from_str(s: &'static str) -> Arguments<'a> {
         // SAFETY: This is the "static str" representation of fmt::Arguments; see above.
         unsafe {
@@ -744,14 +758,6 @@ impl<'a> Arguments<'a> {
             }
         }
     }
-
-    // Same as `from_str`, but not const.
-    // Used by format_args!() expansion when arguments are inlined,
-    // e.g. format_args!("{}", 123), which is not allowed in const.
-    #[inline]
-    pub fn from_str_nonconst(s: &'static str) -> Arguments<'a> {
-        Arguments::from_str(s)
-    }
 }
 
 #[doc(hidden)]

library/core/src/macros/mod.rs

@@ -991,7 +991,7 @@ pub(crate) mod builtin {
     #[stable(feature = "rust1", since = "1.0.0")]
     #[rustc_diagnostic_item = "format_args_macro"]
     #[allow_internal_unsafe]
-    #[allow_internal_unstable(fmt_internals)]
+    #[allow_internal_unstable(fmt_internals, fmt_arguments_from_str)]
     #[rustc_builtin_macro]
     #[macro_export]
     macro_rules! format_args {
@@ -1005,7 +1005,7 @@ pub(crate) mod builtin {
     ///
     /// This macro will be removed once `format_args` is allowed in const contexts.
     #[unstable(feature = "const_format_args", issue = "none")]
-    #[allow_internal_unstable(fmt_internals, const_fmt_arguments_new)]
+    #[allow_internal_unstable(fmt_internals, fmt_arguments_from_str)]
     #[rustc_builtin_macro]
     #[macro_export]
     macro_rules! const_format_args {
@@ -1020,7 +1020,7 @@ pub(crate) mod builtin {
         reason = "`format_args_nl` is only for internal \
                   language use and is subject to change"
     )]
-    #[allow_internal_unstable(fmt_internals)]
+    #[allow_internal_unstable(fmt_internals, fmt_arguments_from_str)]
     #[rustc_builtin_macro]
     #[doc(hidden)]
     #[macro_export]