const functions can assert! but cannot assert_eq!

This is probably a well-known limitation, but I couldn't find any other issues for it with a quick search. Apologies if it's a duplicate.

// compiles
pub const fn foo() {
    assert!(1 == 1);
}

// does not compile
pub const fn bar() {
    assert_eq!(1, 1);
}

error[E0015]: cannot call non-const fn `core::panicking::assert_failed::<i32, i32>` in constant functions
 --> src/lib.rs:6:5
  |
6 |     assert_eq!(1, 1);
  |     ^^^^^^^^^^^^^^^^
  |

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Can core::panicking::assert_failed be made const?

Hi!

I believe this is due to the fact that assert_eq (and other assert_* macros) end up calling core::panicking::assert_fail_inner. Which in turn uses formatting macros (println! with a {:?} placeholder) that are currently not usable in a const context which is known but I couldn't find any issue tracking progress on this front.

Yeah, I think this is blocked on const traits. assert_eq! can only be used when the Debug impls or the tail are guaranteed to work in const, and we don't have any way to express that yet.

We could use const_panic meanwhile?

rust/library/core/src/panic.rs

Lines 193 to 203 in 2783fc4

    
           /// Helper macro for panicking in a `const fn`. 
        
           /// Invoke as: 
        
           /// ```rust,ignore (just an example) 
        
           /// core::macros::const_panic!("boring message", "flavored message {a} {b:?}", a: u32 = foo.len(), b: Something = bar); 
        
           /// ``` 
        
           /// where the first message will be printed in const-eval, 
        
           /// and the second message will be printed at runtime. 
        
           // All uses of this macro are FIXME(const-hack). 
        
           #[unstable(feature = "panic_internals", issue = "none")] 
        
           #[doc(hidden)] 
        
           pub macro const_panic {

We don't need hacks, we can start marking Debug as const_trait and making impls const as we go. Then we need to figure out how to make more Formatter methods const

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const functions can assert! but cannot assert_eq! #119826

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	/// Helper macro for panicking in a `const fn`.
	/// Invoke as:
	/// ```rust,ignore (just an example)
	/// core::macros::const_panic!("boring message", "flavored message {a} {b:?}", a: u32 = foo.len(), b: Something = bar);
	/// ```
	/// where the first message will be printed in const-eval,
	/// and the second message will be printed at runtime.
	// All uses of this macro are FIXME(const-hack).
	#[unstable(feature = "panic_internals", issue = "none")]
	#[doc(hidden)]
	pub macro const_panic {

const functions can assert! but cannot assert_eq! #119826

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krtab commented on Jan 11, 2024

saethlin commented on Jan 27, 2024

Randl commented on Jul 8, 2025

oli-obk commented on Jul 8, 2025

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