Unsatisfied implicit trait bound is not made explicit in E0277

[MDLC01]

Code

trait Trait {
    fn foo(&self);
}

// Implicitly requires `T: Sized`.
// Adding a `?Sized` bound makes the error go away.
impl<T> Trait for T {
    fn foo(&self) {}
}

// `[u8]` does not implement `Sized`.
fn test(x: &[u8]) {
    Trait::foo(x)
}

Current output

error[E0277]: the trait bound `[u8]: Trait` is not satisfied
  --> src/lib.rs:13:16
   |
13 |     Trait::foo(x)
   |     ---------- ^ the trait `Trait` is not implemented for `[u8]`
   |     |
   |     required by a bound introduced by this call
   |
note: required for `[u8]` to implement `Trait`
  --> src/lib.rs:7:9
   |
 7 | impl<T> Trait for T {
   |      -  ^^^^^     ^
   |      |
   |      unsatisfied trait bound introduced here
help: consider borrowing here
   |
13 |     Trait::foo(&x)
   |                +
13 |     Trait::foo(&mut x)
   |                ++++

For more information about this error, try `rustc --explain E0277`.

Desired output

error[E0277]: the trait bound `[u8]: Trait` is not satisfied
  --> src/lib.rs:12:16
   |
13 |     Trait::foo(x)
   |     ---------- ^ the trait `Sized` is not implemented for `[u8]`
   |     |
   |     required by a bound introduced by this call
   |
note: required for `[u8]` to implement `Trait`
  --> src/lib.rs:6:9
   |
 7 | impl<T> Trait for T {
   |      -  ^^^^^     ^
   |      |
   |      unsatisfied trait bound introduced here
help: consider relaxing the implicit `Sized` bound
   |
 7 | impl<T: ?Sized> Trait for T {
   |       ++++++++

For more information about this error, try `rustc --explain E0277`.

Rationale and extra context

No response

Other cases

Rust Version

$ rustc --version --verbose
rustc 1.90.0 (1159e78c4 2025-09-14)
binary: rustc
commit-hash: 1159e78c4747b02ef996e55082b704c09b970588
commit-date: 2025-09-14
host: x86_64-pc-windows-gnu
release: 1.90.0
LLVM version: 20.1.8

Anything else?

No response

[Daniel-B-Smith]

What would you think of changing the output to:

error[E0277]: the trait bound `[u8]: Trait` is not satisfied
  --> src/lib.rs:12:16
   |
13 |     Trait::foo(x)
   |     ---------- ^ the trait `Sized` is not implemented for `[u8]`
   |     |
   |     required by a bound introduced by this call
   |
note: required for `[u8]` to implement `Trait`
  --> src/lib.rs:7:9
   |
 7 | impl<T> Trait for T {
   |      -  ^^^^^     ^
   |      |
   |      unsatisfied trait bound introduced here
help: consider borrowing here
   |
13 |     Trait::foo(&x)
   |                +
13 |     Trait::foo(&mut x)
   |                ++++

For more information about this error, try `rustc --explain E0277`.

I agree that not mentioning Sized is confusing. However, the borrowing advice is the only local change that will fix the compile error. My personal opinion is that the compiler should not recommend relaxing the Sized trait bound since it is not possible for the compiler to know if it is possible to relax that constraint, since that requires knowing about all implementations (and arguably potential implementations) of the trait.

[MDLC01]

My main point was that Sized should be mentioned somewhere, so I'm fine with this.

A possible improvement is the following:

error[E0277]: the trait bound `[u8]: Trait` is not satisfied
  --> src/lib.rs:12:16
   |
13 |     Trait::foo(x)
   |     ---------- ^ the trait `Sized` is not implemented for `[u8]`
   |     |
   |     required by a bound introduced by this call
   |
note: required for `[u8]` to implement `Trait`
  --> src/lib.rs:7:9
   |
 7 | impl<T> Trait for T {
   |      -  ^^^^^     ^
   |      |
   |      unsatisfied trait bound implicitly introduced here
help: consider borrowing here
   |
13 |     Trait::foo(&x)
   |                +
13 |     Trait::foo(&mut x)
   |                ++++

For more information about this error, try `rustc --explain E0277`.

which makes it clear to newer users that the trait bound is implicitly introduced, and that the compiler is not pointing at the wrong place.