Add new const_format_args!() macro and use it in panics.

Author: m-ou-se

core/src/fmt/mod.rs

@@ -337,6 +337,7 @@ impl<'a> Arguments<'a> {
     #[doc(hidden)]
     #[inline]
     #[unstable(feature = "fmt_internals", reason = "internal to format_args!", issue = "none")]
+    #[rustc_const_unstable(feature = "const_fmt_arguments_new", issue = "none")]
     pub const fn new_v1(pieces: &'a [&'static str], args: &'a [ArgumentV1<'a>]) -> Arguments<'a> {
         Arguments { pieces, fmt: None, args }
     }
@@ -350,6 +351,7 @@ impl<'a> Arguments<'a> {
     #[doc(hidden)]
     #[inline]
     #[unstable(feature = "fmt_internals", reason = "internal to format_args!", issue = "none")]
+    #[rustc_const_unstable(feature = "const_fmt_arguments_new", issue = "none")]
     pub const fn new_v1_formatted(
         pieces: &'a [&'static str],
         args: &'a [ArgumentV1<'a>],

core/src/macros/mod.rs

@@ -837,6 +837,16 @@ pub(crate) mod builtin {
         ($fmt:expr, $($args:tt)*) => {{ /* compiler built-in */ }};
     }
 
+    /// Same as `format_args`, but can be used in some const contexts.
+    #[unstable(feature = "const_format_args", issue = "none")]
+    #[allow_internal_unstable(fmt_internals, const_fmt_arguments_new)]
+    #[rustc_builtin_macro]
+    #[macro_export]
+    macro_rules! const_format_args {
+        ($fmt:expr) => {{ /* compiler built-in */ }};
+        ($fmt:expr, $($args:tt)*) => {{ /* compiler built-in */ }};
+    }
+
     /// Same as `format_args`, but adds a newline in the end.
     #[unstable(
         feature = "format_args_nl",

core/src/panic.rs

@@ -7,7 +7,7 @@ use crate::fmt;
 
 #[doc(hidden)]
 #[unstable(feature = "edition_panic", issue = "none", reason = "use panic!() instead")]
-#[allow_internal_unstable(core_panic)]
+#[allow_internal_unstable(core_panic, const_format_args)]
 #[rustc_diagnostic_item = "core_panic_2015_macro"]
 #[rustc_macro_transparency = "semitransparent"]
 pub macro panic_2015 {
@@ -21,21 +21,21 @@ pub macro panic_2015 {
         $crate::panicking::panic_str($msg)
     ),
     ($fmt:expr, $($arg:tt)+) => (
-        $crate::panicking::panic_fmt($crate::format_args!($fmt, $($arg)+))
+        $crate::panicking::panic_fmt($crate::const_format_args!($fmt, $($arg)+))
     ),
 }
 
 #[doc(hidden)]
 #[unstable(feature = "edition_panic", issue = "none", reason = "use panic!() instead")]
-#[allow_internal_unstable(core_panic)]
+#[allow_internal_unstable(core_panic, const_format_args)]
 #[rustc_diagnostic_item = "core_panic_2021_macro"]
 #[rustc_macro_transparency = "semitransparent"]
 pub macro panic_2021 {
     () => (
         $crate::panicking::panic("explicit panic")
     ),
     ($($t:tt)+) => (
-        $crate::panicking::panic_fmt($crate::format_args!($($t)+))
+        $crate::panicking::panic_fmt($crate::const_format_args!($($t)+))
     ),
 }
 

std/src/lib.rs

@@ -247,6 +247,7 @@
 #![feature(const_fn_floating_point_arithmetic)]
 #![feature(const_fn_fn_ptr_basics)]
 #![cfg_attr(bootstrap, feature(const_fn_transmute))]
+#![feature(const_format_args)]
 #![feature(const_io_structs)]
 #![feature(const_ip)]
 #![feature(const_ipv4)]
@@ -556,9 +557,9 @@ pub use core::{
 #[stable(feature = "builtin_macro_prelude", since = "1.38.0")]
 #[allow(deprecated)]
 pub use core::{
-    assert, assert_matches, cfg, column, compile_error, concat, concat_idents, env, file,
-    format_args, format_args_nl, include, include_bytes, include_str, line, llvm_asm, log_syntax,
-    module_path, option_env, stringify, trace_macros,
+    assert, assert_matches, cfg, column, compile_error, concat, concat_idents, const_format_args,
+    env, file, format_args, format_args_nl, include, include_bytes, include_str, line, llvm_asm,
+    log_syntax, module_path, option_env, stringify, trace_macros,
 };
 
 #[stable(feature = "core_primitive", since = "1.43.0")]

std/src/panic.rs

@@ -20,7 +20,7 @@ use crate::thread::Result;
 
 #[doc(hidden)]
 #[unstable(feature = "edition_panic", issue = "none", reason = "use panic!() instead")]
-#[allow_internal_unstable(libstd_sys_internals)]
+#[allow_internal_unstable(libstd_sys_internals, const_format_args)]
 #[cfg_attr(not(test), rustc_diagnostic_item = "std_panic_2015_macro")]
 #[rustc_macro_transparency = "semitransparent"]
 pub macro panic_2015 {
@@ -31,7 +31,7 @@ pub macro panic_2015 {
         $crate::rt::begin_panic($msg)
     }),
     ($fmt:expr, $($arg:tt)+) => ({
-        $crate::rt::begin_panic_fmt(&$crate::format_args!($fmt, $($arg)+))
+        $crate::rt::begin_panic_fmt(&$crate::const_format_args!($fmt, $($arg)+))
     }),
 }