`module_path!` expansion incorrect for modules without canonical path

When module_path! is used inside a module without a canonical path, it expands to an incorrect module path.

fn main() {
    print_inner_path(); // prints `<CRATE_NAME>::fn_inner`
    // according to `module_path` this should work; obviously it shouldn't and doesn't
    //dbg!(crate::fn_inner::PATH);
}
fn print_inner_path() {
    pub mod fn_inner {
        pub const PATH: &str = module_path!();
    }
    println!("module path: {}", fn_inner::PATH);
}

Playground

I consider this expansion incorrect because it doesn't indicate that fn_inner is in fact a nested module in a function and is not externally reachable. There's also the possibility that there exists a reachable module at the same level with the same name, from which module_path! would expand to the same thing despite being two distinct modules. See comment.

I'm not sure what the appropriate output/action would be in this situation. A placeholder text or empty string maybe? Although this sounds very wrong. Or maybe compile error. Regardless, at minimum I think the documentation for module_path! should be updated to address this limitation/bug.

Meta

rustc --version --verbose:

rustc 1.92.0-nightly (52618eb33 2025-09-14)
binary: rustc
commit-hash: 52618eb338609df44978b0ca4451ab7941fd1c7a
commit-date: 2025-09-14
host: x86_64-unknown-linux-gnu
release: 1.92.0-nightly
LLVM version: 21.1.1

Start with the provided Playground reproduction and the module_path! documentation and canonical-paths reference. Investigate how the macro expands for a module declared inside a function and compare that with the documented behavior. Done requires a decided behavior for non-canonical modules and either an implementation with coverage or documentation that clearly explains the limitation.