Diagnostic for non-inline modules inside blocks should encourage moving it out of the block

[shepmaster]

Code

fn main () {
    mod secret;
}

Current output

error: cannot declare a non-inline module inside a block unless it has a path attribute
 --> src/main.rs:2:5
  |
2 |     mod secret;
  |     ^^^^^^^^^^^

Desired output

error: cannot declare a non-inline module inside a block unless it has a path attribute. 
       Modules are usually placed outside of blocks, at the top level of the file.
 --> src/main.rs:2:5
  |
2 |     mod secret;
  |     ^^^^^^^^^^^

Rationale and extra context

Using #[path] is generally discouraged. The current message presents #[path] as the only solution.

Rust Version

rustc 1.92.0-nightly (5c7ae0c7e 2025-10-02)
binary: rustc
commit-hash: 5c7ae0c7ed184c603e5224604a9f33ca0e8e0b36
commit-date: 2025-10-02
host: aarch64-apple-darwin
release: 1.92.0-nightly
LLVM version: 21.1.2

Anything else?

Discovered while giving training to newcomers to Rust.

[chenyukang]

maybe it's better to add a proper note for Modules are usually placed outside of blocks, at the top level of the file.

rust/compiler/rustc_expand/messages.ftl

Lines 120 to 122 in 7950f24

	expand_module_in_block =
	cannot declare a non-inline module inside a block unless it has a path attribute
	.note = maybe `use` the module `{$name}` instead of redeclaring it

[hkBst]

Why is it "non-inline"?

[Kivooeo]

non-inline means that this module is defined somewhere else than this file, so example of inline module is mod notAsecret {}

[hkBst]

Hmm, thanks for the explanation. That seems like an unfortunate way of naming these things...

[Kivooeo]

Yeah, I also think that something like "external module" would be a more intuitive than "non-inline module"

[hkBst]

Yeah, or "mod declaration" vs "mod definition".

[reddevilmidzy]

I’d like to work on this issue! @rustbot claim