Error when autoderef of smart pointer occurs in `const fn` is unclear

[SNCPlay42]

Given the following code:

use std::ops::Deref;

struct Foo;

impl Foo {
    const fn foo(&self) {}
}

struct SmartPointer(Foo);

impl Deref for SmartPointer {
    type Target = Foo;

    fn deref(&self) -> &Self::Target {
        &self.0
    }
}

pub const fn bar() {
    Foo.foo(); // fine
    SmartPointer(Foo).foo(); // fails since we can't constly deref
}

The current output is:

error[E0015]: calls in constant functions are limited to constant functions, tuple structs and tuple variants
  --> src/lib.rs:21:5
   |
21 |     SmartPointer(Foo).foo(); // fails since we can't constly deref
   |     ^^^^^^^^^^^^^^^^^^^^^^^

It's unclear from the diagnostic that the problem is not the call of foo itself (which is a const fn) but that SmartPointer would have to be dereferenced.

Ideally the output should look like:

error[E0015]: `SmartPointer` cannot be dereferenced in constant functions
  --> src/lib.rs:21:5
   |
21 |     SmartPointer(Foo).foo(); // fails since we can't constly deref
   |     ^^^^^^^^^^^^^^^^^^^^^^^ dereference occurs due to method call
   |
   = note: operators in constant functions are limited to intrinsic operations

Perhaps more generally, the compiler should avoid referring to non-intrinsic operators as "calls" in const errors, or explain that it means calling the operator method. This is usually less of a problem because an operator appears in the span.

@rustbot label A-const-fn

[djc]

I think this is the same problem:

match &self.0 {
    Cow::Borrowed(s) => s,
    Cow::Owned(s) => &*s,
}

Fails with

error[E0015]: calls in constant functions are limited to constant functions, tuple structs and tuple variants
  --> opentelemetry-api/src/common.rs:61:9
   |
61 | /         match &self.0 {
62 | |             Cow::Borrowed(s) => s,
63 | |             Cow::Owned(s) => &*s,
64 | |         }
   | |_________^

Despite there clearly being no calls in this function.

[SNCPlay42]

Output on current nightly, after #90532:

error[E0015]: cannot perform deref coercion on `SmartPointer` in constant functions
  --> src/lib.rs:21:5
   |
21 |     SmartPointer(Foo).foo(); // fails since we can't constly deref
   |     ^^^^^^^^^^^^^^^^^^^^^^^
   |
   = note: attempting to deref into `Foo`
note: deref defined here
  --> src/lib.rs:12:5
   |
12 |     type Target = Foo;
   |     ^^^^^^^^^^^^^^^^^^
note: impl defined here, but it is not `const`
  --> src/lib.rs:11:1
   |
11 | impl Deref for SmartPointer {
   | ^^^^^^^^^^^^^^^^^^^^^^^^^^^
   = note: calls in constant functions are limited to constant functions, tuple structs and tuple variants

Output from #91496 (comment), (as reproduced like this) has improved as well, although the span is still a bit confusing:

error[E0015]: cannot perform deref coercion on `String` in constant functions
    --> src/lib.rs:7:9
     |
7    | /         match &self.0 {
8    | |             Cow::Borrowed(s) => s,
9    | |             Cow::Owned(s) => &*s,
10   | |         }
     | |_________^
     |
     = note: attempting to deref into `str`
note: impl defined here, but it is not `const`
     = note: calls in constant functions are limited to constant functions, tuple structs and tuple variants

At any rate, I'm closing this as fixed/in favor of the broader #92380.