Ambiguous module files report unrelated errors

When there is a module name collision, rust-analyzer reports unrelated errors making it hard for the user to understand what is going on. I believe this is what we see on #19709.

A reproducible test case is available in https://github.com/stephanemagnenat/ra-ghost-module, the related Zed issue that surfaced this situation in the first place is reported here: zed-industries/zed#64231.

It could be good if rust-analyzer would report a clear user message like the Rust compiler do:

error[E0761]: file for module `ast` found at both "src/ast.rs" and "src/ast/mod.rs"
   --> src/lib.rs:1:1
    |
  1 | mod ast;
    | ^^^^^^^^
    |
    = help: delete or rename one of them to remove the ambiguity

instead of the current behaviour when errors related to common std types appear instead.

This issue is a follow up of issue #23366 but focusing on the error message reporting, which is the real problem here.

rust-analyzer version: 0.3.3049-standalone (release 2026-09-14, commit https://github.com/rust-lang/rust-analyzer/commit/682a84e95b5a52cf06e9822fcaa4f628738554bf)
rustc version: 1.98.0 (88d9e12ae 2026-08-18)
editor or extension: Zed 1.19.2 (df181c6f58d0)

Start by reproducing the module collision with the case in https://github.com/stephanemagnenat/ra-ghost-module and compare rust-analyzer's diagnostics with the Rust compiler's E0761 output. Trace the diagnostic entry point for the ambiguous module and make the reported message identify both conflicting paths and the module declaration; verify the unrelated std-type errors are replaced by the clear ambiguity diagnostic.