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

compiler/rustc_builtin_macros/src/lib.rs

@@ -72,6 +72,7 @@ pub fn register_builtin_macros(resolver: &mut dyn ResolverExpand) {
         file: source_util::expand_file,
         format_args_nl: format::expand_format_args_nl,
         format_args: format::expand_format_args,
+        const_format_args: format::expand_format_args,
         global_asm: asm::expand_global_asm,
         include_bytes: source_util::expand_include_bytes,
         include_str: source_util::expand_include_str,

compiler/rustc_span/src/symbol.rs

@@ -410,6 +410,7 @@ symbols! {
         const_fn_transmute,
         const_fn_union,
         const_fn_unsize,
+        const_format_args,
         const_generic_defaults,
         const_generics,
         const_generics_defaults,

library/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>],

library/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",

library/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 {
@@ -21,13 +21,13 @@ 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 {
@@ -35,7 +35,7 @@ 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)+))
     ),
 }
 

library/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,
+    assert, assert_matches, cfg, column, compile_error, concat, concat_idents, const_format_args,
-    format_args, format_args_nl, include, include_bytes, include_str, line, llvm_asm, log_syntax,
+    env, file, format_args, format_args_nl, include, include_bytes, include_str, line, llvm_asm,
-    module_path, option_env, stringify, trace_macros,
+    log_syntax, module_path, option_env, stringify, trace_macros,
 };
 
 #[stable(feature = "core_primitive", since = "1.43.0")]

library/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)+))
     }),
 }
 

src/test/ui/borrowck/issue-64453.rs

@@ -3,6 +3,7 @@ struct Value;
 
 static settings_dir: String = format!("");
 //~^ ERROR calls in statics are limited to constant functions
+//~| ERROR is not yet stable as a const
 
 fn from_string(_: String) -> Value {
     Value

src/test/ui/borrowck/issue-64453.stderr

@@ -1,9 +1,18 @@
 error[E0507]: cannot move out of static item `settings_dir`
-  --> $DIR/issue-64453.rs:13:37
+  --> $DIR/issue-64453.rs:14:37
    |
 LL |     let settings_data = from_string(settings_dir);
    |                                     ^^^^^^^^^^^^ move occurs because `settings_dir` has type `String`, which does not implement the `Copy` trait
 
+error: `Arguments::<'a>::new_v1` is not yet stable as a const fn
+  --> $DIR/issue-64453.rs:4:31
+   |
+LL | static settings_dir: String = format!("");
+   |                               ^^^^^^^^^^^
+   |
+   = help: add `#![feature(const_fmt_arguments_new)]` to the crate attributes to enable
+   = note: this error originates in the macro `$crate::__export::format_args` (in Nightly builds, run with -Z macro-backtrace for more info)
+
 error[E0015]: calls in statics are limited to constant functions, tuple structs and tuple variants
   --> $DIR/issue-64453.rs:4:31
    |
@@ -12,7 +21,7 @@ LL | static settings_dir: String = format!("");
    |
    = note: this error originates in the macro `format` (in Nightly builds, run with -Z macro-backtrace for more info)
 
-error: aborting due to 2 previous errors
+error: aborting due to 3 previous errors
 
 Some errors have detailed explanations: E0015, E0507.
 For more information about an error, try `rustc --explain E0015`.