"encountered dangling pointer in final value of constant" solved by nested const { const {} }

[m-ou-se]

    let a = const { [        "asdf".as_ptr()  ].as_ptr() }; // error
    let a = const { [const { "asdf".as_ptr() }].as_ptr() }; // okay

I think either both of these should pass, or both of these should error.

However, only the first line results in:

error: encountered dangling pointer in final value of constant

---

Fwiw, inlining as_ptr() results in both lines being accepted:

    let a = const { [        "asdf" as *const str as *const u8  ].as_ptr() }; // okay
    let a = const { [const { "asdf" as *const str as *const u8 }].as_ptr() }; // okay

Meta

rustc --version --verbose:

rustc 1.86.0-nightly (a567209da 2025-02-13)
binary: rustc
commit-hash: a567209daab72b7ea59eac533278064396bb0534
commit-date: 2025-02-13
host: x86_64-unknown-linux-gnu
release: 1.86.0-nightly
LLVM version: 19.1.7

[theemathas]

I suspect that, in the second version of the code, the array is being static-promoted, and the method call outside a const block prevents the array from being static-promoted. If this is the case, the difference in behavior would be expected behavior.

[compiler-errors]

Yeah, I don't think it's intuitive to expect a const block to act differently w.r.t. promoting things that are nested into the body but aren't subject to normal promotion rules. Like, I wouldn't expect this code to work:

const { let s = String::new(); &s }

[m-ou-se]

I thought it was unexpected that nesting a const block in a const block had any effect. But if that's expected, feel free to close this.

[saethlin]

This is at least the second time I have seen someone surprised that putting a const block in a const block makes this error go away.

Do we have documentation for how promotion works? Or documentation that at least confirms that promotion has made this pointer not dangling as opposed to just hiding the dangling from the compiler?