Auto merge of rust-lang#86998 - m-ou-se:const-panic-fmt-as-str, r=oli-obk

Make const panic!("..") work in Rust 2021.

During const eval, this replaces calls to core::panicking::panic_fmt and std::panicking::being_panic_fmt with a call to a new const fn: core::panicking::const_panic_fmt. That function uses fmt::Arguments::as_str() to get the str and calls panic_str with that instead.

panic!() invocations with formatting arguments are still not accepted, as the creation of such a fmt::Arguments cannot be done in constant functions right now.

r? `@RalfJung`

Author: bors

core/src/fmt/mod.rs

@@ -337,7 +337,8 @@ impl<'a> Arguments<'a> {
     #[doc(hidden)]
     #[inline]
     #[unstable(feature = "fmt_internals", reason = "internal to format_args!", issue = "none")]
-    pub fn new_v1(pieces: &'a [&'static str], args: &'a [ArgumentV1<'a>]) -> Arguments<'a> {
+    #[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 }
     }
 
@@ -350,7 +351,8 @@ impl<'a> Arguments<'a> {
     #[doc(hidden)]
     #[inline]
     #[unstable(feature = "fmt_internals", reason = "internal to format_args!", issue = "none")]
-    pub fn new_v1_formatted(
+    #[rustc_const_unstable(feature = "const_fmt_arguments_new", issue = "none")]
+    pub const fn new_v1_formatted(
         pieces: &'a [&'static str],
         args: &'a [ArgumentV1<'a>],
         fmt: &'a [rt::v1::Argument],

core/src/lib.rs

@@ -73,6 +73,7 @@
 #![feature(cfg_target_has_atomic)]
 #![feature(const_heap)]
 #![feature(const_alloc_layout)]
+#![feature(const_arguments_as_str)]
 #![feature(const_assert_type)]
 #![feature(const_discriminant)]
 #![feature(const_cell_into_inner)]

core/src/macros/mod.rs

@@ -837,6 +837,31 @@ pub(crate) mod builtin {
         ($fmt:expr, $($args:tt)*) => {{ /* compiler built-in */ }};
     }
 
+    /// Same as `format_args`, but can be used in some const contexts.
+    ///
+    /// This macro is used by the panic macros for the `const_panic` feature.
+    ///
+    /// This macro will be removed once `format_args` is allowed in const contexts.
+    #[cfg(not(bootstrap))]
+    #[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 can be used in some const contexts.
+    #[cfg(bootstrap)]
+    #[unstable(feature = "const_format_args", issue = "none")]
+    #[macro_export]
+    macro_rules! const_format_args {
+        ($($t:tt)*) => {
+            $crate::format_args!($($t)*)
+        }
+    }
+
     /// 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)+))
     ),
 }
 

core/src/panicking.rs

@@ -74,6 +74,7 @@ fn panic_bounds_check(index: usize, len: usize) -> ! {
 #[cfg_attr(not(feature = "panic_immediate_abort"), inline(never))]
 #[cfg_attr(feature = "panic_immediate_abort", inline)]
 #[track_caller]
+#[cfg_attr(not(bootstrap), lang = "panic_fmt")] // needed for const-evaluated panics
 pub fn panic_fmt(fmt: fmt::Arguments<'_>) -> ! {
     if cfg!(feature = "panic_immediate_abort") {
         super::intrinsics::abort()
@@ -92,6 +93,20 @@ pub fn panic_fmt(fmt: fmt::Arguments<'_>) -> ! {
     unsafe { panic_impl(&pi) }
 }
 
+/// This function is used instead of panic_fmt in const eval.
+#[cfg(not(bootstrap))]
+#[lang = "const_panic_fmt"]
+pub const fn const_panic_fmt(fmt: fmt::Arguments<'_>) -> ! {
+    if let Some(msg) = fmt.as_str() {
+        panic_str(msg);
+    } else {
+        // SAFETY: This is only evaluated at compile time, which reliably
+        // handles this UB (in case this branch turns out to be reachable
+        // somehow).
+        unsafe { crate::hint::unreachable_unchecked() };
+    }
+}
+
 #[derive(Debug)]
 #[doc(hidden)]
 pub enum AssertKind {

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)]
@@ -555,9 +556,9 @@ 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)+))
     }),
 }
 

std/src/panicking.rs

@@ -448,6 +448,7 @@ pub fn panicking() -> bool {
 #[cfg_attr(not(feature = "panic_immediate_abort"), track_caller)]
 #[cfg_attr(not(feature = "panic_immediate_abort"), inline(never))]
 #[cfg_attr(feature = "panic_immediate_abort", inline)]
+#[cfg_attr(all(not(bootstrap), not(test)), lang = "begin_panic_fmt")]
 pub fn begin_panic_fmt(msg: &fmt::Arguments<'_>) -> ! {
     if cfg!(feature = "panic_immediate_abort") {
         intrinsics::abort()