How to fix `definition of unknown language item 'panic_fmt'`? [duplicate] - rust

Question

This question already has an answer here:
What is a crate attribute and where do I add it?
(1 answer)
Closed 4 years ago.
For a no_std application there are a few language items defined in lang_items.rs, one of them being the panic_fmt language item (to specify the behavior of panic! in this no_std context) defined like:
#[lang = "panic_fmt"] #[no_mangle] pub extern fn panic_fmt() -> ! { loop{} }
When compiling, I receive this error:
error[E0522]: definition of an unknown language item: `panic_fmt`
--> src/lang_items.rs:3:1
|
3 | #[lang = "panic_fmt"] #[no_mangle] pub extern fn panic_fmt() -> ! { loop{} }
| ^^^^^^^^^^^^^^^^^^^^^ definition of unknown language item `panic_fmt`
error: `#[panic_implementation]` function required, but not found
After reading RFC 2070 I learned there was a recent breaking change for no_std/embedded programs. While it's recommended that I use the #[panic_implementation] attributes, a recently added feature, I still receive an error, doing so like:
#[panic_implementation] #[no_mangle] pub extern fn panic_fmt() -> ! { loop{} }
Gives the error:
error[E0658]: #[panic_implementation] is an unstable feature (see issue #44489)
--> src/lang_items.rs:4:1
|
4 | #[panic_implementation] #[no_mangle] pub extern fn panic_fmt() -> ! { loop{} }
| ^^^^^^^^^^^^^^^^^^^^^^^
|
= help: add #![feature(panic_implementation)] to the crate attributes to enable
Following their suggestion of adding #![feature(panic_implementation)] to the top of the lang_items.rs file doesn't seem to do the trick, as I'm getting the same error. How do I enable this unstable feature properly so that I can compile this no_std application with behavior for panic! defined?

Answer 1

Okay, it was a simple mistake I was making. Here is a link to the playground containing my code. First of all, I needed to add the crate attribute to the top of my crate root (which, for me, was lib.rs). lang_items.rs was just a file that was used in lib.rs like so:
#![feature(compiler_builtins_lib, lang_items, asm, panic_implementation, core_intrinsics)]
#![no_builtins]
#![no_std]
pub mod lang_items;
const GPIO_BASE: usize = 0x3F000000 + 0x200000;
const GPIO_FSEL1: *mut u32 = (GPIO_BASE + 0x04) as *mut u32;
const GPIO_SET0: *mut u32 = (GPIO_BASE + 0x1C) as *mut u32;
const GPIO_CLR0: *mut u32 = (GPIO_BASE + 0x28) as *mut u32;
#[inline(never)]
fn spin_sleep_ms(ms: usize) {
for _ in 0..(ms * 600) {
unsafe { asm!("nop" :::: "volatile"); }
}
}
#[no_mangle]
pub unsafe extern "C" fn kmain() {
// STEP 1: Set GPIO Pin 16 as output.
GPIO_FSEL1.write_volatile(0x1 << 0x12);
// STEP 2: Continuously set and clear GPIO 16.
loop {
GPIO_SET0.write_volatile(0x1 << 0x10);
spin_sleep_ms(256);
GPIO_CLR0.write_volatile(0x1 << 0x10);
spin_sleep_ms(256);
}
}