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.

Author: m-ou-se

core/src/fmt/mod.rs

@@ -337,7 +337,7 @@ 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> {
+    pub const fn new_v1(pieces: &'a [&'static str], args: &'a [ArgumentV1<'a>]) -> Arguments<'a> {
         Arguments { pieces, fmt: None, args }
     }
 
@@ -350,7 +350,7 @@ impl<'a> Arguments<'a> {
     #[doc(hidden)]
     #[inline]
     #[unstable(feature = "fmt_internals", reason = "internal to format_args!", issue = "none")]
-    pub fn new_v1_formatted(
+    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/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 +93,17 @@ 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 {
+        panic_str("???");
+    }
+}
+
 #[derive(Debug)]
 #[doc(hidden)]
 pub enum AssertKind {

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()