E0277: the trait bound `*const _: From<&Foo>` is not satisfied

[tyilo]

I tried this code:

[package]
name = "foo"
version = "0.1.0"
edition = "2024"

[dependencies]
security-framework-sys = { version = "=2.17.0", default-features = false }

#[allow(unused)]
use security_framework_sys;

#[repr(u8)]
pub enum Foo {
    A,
    B,
    C,
}

impl Foo {
    pub fn discriminant(self) -> u8 {
        // SAFETY: Because `Foo` is marked `repr(u8)`, its layout is a `repr(C)`
        // `union` between `repr(C)` structs, each of which has the `u8`
        // discriminant as its first field, so we can read the discriminant
        // without offsetting the pointer.
        unsafe { *<*const _>::from(&self).cast::<u8>() }
    }
}

I expected to see this happen: The code compiles.

Instead, this happened: The fails to compile with the following error:

error[E0277]: the trait bound `*const _: From<&Foo>` is not satisfied
  --> src/lib.rs:17:20
   |
17 |         unsafe { *<*const _>::from(&self).cast::<u8>() }
   |                    ^^^^^^^^ the trait `From<&Foo>` is not implemented for `*const _`

error[E0282]: type annotations needed
  --> src/lib.rs:17:19
   |
17 |         unsafe { *<*const _>::from(&self).cast::<u8>() }
   |                   ^^^^^^^^^^^^^^^^^^^^^^^ ---- cannot call a method on a raw pointer with an unknown pointee type
   |                   |
   |                   cannot infer type

Some errors have detailed explanations: E0277, E0282.
For more information about an error, try `rustc --explain E0277`.
error: could not compile `aaa` (lib) due to 2 previous errors

On nightly I only get the first error:

error[E0277]: the trait bound `*const _: From<&Foo>` is not satisfied
  --> src/lib.rs:17:20
   |
17 |         unsafe { *<*const _>::from(&self).cast::<u8>() }
   |                    ^^^^^^^^ the trait `From<&Foo>` is not implemented for `*const _`

For more information about this error, try `rustc --explain E0277`.
error: could not compile `aaa` (lib) due to 1 previous error

Meta

rustc --version --verbose:

rustc 1.93.1 (01f6ddf75 2026-02-11)
binary: rustc
commit-hash: 01f6ddf7588f42ae2d7eb0a2f21d44e8e96674cf
commit-date: 2026-02-11
host: aarch64-apple-darwin
release: 1.93.1
LLVM version: 21.1.8

rustc +nightly --version --verbose

rustc 1.96.0-nightly (80381278a 2026-03-01)
binary: rustc
commit-hash: 80381278a08582356c13b0f52af92d27c567c230
commit-date: 2026-03-01
host: aarch64-apple-darwin
release: 1.96.0-nightly
LLVM version: 22.1.0

I can also reproduce it on Linux:
rustc --version --verbose:

rustc 1.93.1 (01f6ddf75 2026-02-11)
binary: rustc
commit-hash: 01f6ddf7588f42ae2d7eb0a2f21d44e8e96674cf
commit-date: 2026-02-11
host: x86_64-unknown-linux-gnu
release: 1.93.1
LLVM version: 21.1.8

rustc +nightly --version --verbose

rustc 1.96.0-nightly (80381278a 2026-03-01)
binary: rustc
commit-hash: 80381278a08582356c13b0f52af92d27c567c230
commit-date: 2026-03-01
host: x86_64-unknown-linux-gnu
release: 1.96.0-nightly
LLVM version: 22.1.0

---

I originally discovered this in a project that uses reqwest. The relevant code have nothing to do with reqwest and the compile error only happened on macOS, not on Linux.

[theemathas]

I'm confused. What makes you think this code should compile? What pointer type are you expecting?

This code ought to not compile on all platforms.

[eggyal]

I don't understand why you'd expect this to compile? *const T only has one implementation of From (besides the blanket reflexive one) and that's From<*mut T>. Perhaps you meant to cast from an &T reference using as?

[tyilo]

I'm confused. What makes you think this code should compile? What pointer type are you expecting?

This code ought to not compile on all platforms.

It compiles without the security_framework_sys import. I expect it to convert it to *const Foo.

[tyilo]

It also work if you replace _ with Foo or Self.

[theemathas]

Ah right. I'm sorry. I think this is an instance of #44345

Somewhere in your dependency tree, some crate defined something like impl<T> From<&T> for *const Something. When this crate was added as a dependency, rust started getting confused on which From implementation you wanted. Rust was unable to figure out what type to use as the _ type, and produced an error.

Unfortunately I don't think this is getting fixed any time soon.

[tyilo]

But shouldn't the error in that case indicate that there multiple possible types for _ and not say that there are none?

[theemathas]

When you didn't have the dependency, there was only one applicable From impl. So, the compiler picked that impl.

[theemathas]

I suppose the error message isn't great, yeah 😅

[tyilo]

Here is a reproducer without any dependencies:

#[repr(u8)]
pub enum Foo {
    A,
    B,
    C,
}

impl Foo {
    pub fn discriminant(self) -> u8 {
        // SAFETY: Because `Foo` is marked `repr(u8)`, its layout is a `repr(C)`
        // `union` between `repr(C)` structs, each of which has the `u8`
        // discriminant as its first field, so we can read the discriminant
        // without offsetting the pointer.
        unsafe { *<*const _>::from(&self).cast::<u8>() }
    }
}

enum Bar {
    X,
    Y,
    Z,
}

impl From<Bar> for *const () {
    fn from(_bar: Bar) -> Self {
        todo!()
    }
}

This seems like a bug in Rust, as *const () can't be created from a &Foo, but from a Bar.

[theemathas]

That's... odd