Line numbers in diagnostics for doctests are off by number of preceding hidden lines that aren't `use core;`

Code

/// ```
/// # use playground::f;
/// x
/// ```
pub fn f()
{
    todo!()
}

Note that the expression x occurs on line 3 in the source code.

Current output

error[E0425]: cannot find value `x` in this scope
 --> src/lib.rs:4:1
  |
6 | x
  | ^ help: a function with a similar name exists: `f`
  |
 ::: /playground/src/lib.rs:5:1
  |
5 | pub fn f()
  | ---------- similarly named function `f` defined here

Note that the diagnostic refers to line 4.

Desired output

error[E0425]: cannot find value `x` in this scope
 --> src/lib.rs:3:1
  |
6 | x
  | ^ help: a function with a similar name exists: `f`
  |
 ::: /playground/src/lib.rs:5:1
  |
5 | pub fn f()
  | ---------- similarly named function `f` defined here

Rationale and extra context

No response

Other cases

No response

Rust Version

$ rustc --version --verbose
rustc 1.77.0 (aedd173a2 2024-03-17)
binary: rustc
commit-hash: aedd173a2c086e558c2b66d3743b344f977621a7
commit-date: 2024-03-17
host: x86_64-unknown-linux-gnu
release: 1.77.0
LLVM version: 17.0.6

Anything else?

This doesn't always seem to happen, for example in the following example it does not:

/// ```
/// # use core;
/// x
/// ```
pub fn f()
{
}

error[E0425]: cannot find value `x` in this scope
 --> src/lib.rs:3:1
  |
4 | x
  | ^ not found in this scope

I'm not sure what makes the difference. The only difference in the code is that one imports playground::f and the other imports core.

Start by reproducing the doctest example from the issue and compare diagnostic line mapping when the hidden line imports playground::f versus core. Trace the doctest source transformation and line-offset handling, then add a regression test that expects the diagnostic for x to point to source line 3.