Confusing error message when including a module recursively

[jethrogb]

Executing the following:

cargo new --bin x
cd x
touch src/a.rs
echo 'mod a;' >> src/main.rs 
echo 'mod main;' >> src/main.rs 
cargo build

Results in this error message:

src/main.rs:4:5: 4:6 error: cannot declare a new module at this location
src/main.rs:4 mod a;
                  ^
src/main.rs:4:5: 4:6 note: maybe move this module `main` to its own directory via `main/mod.rs`
src/main.rs:4 mod a;
                  ^
src/main.rs:4:5: 4:6 note: ... or maybe `use` the module `a` instead of possibly redeclaring it
src/main.rs:4 mod a;
                  ^

None of which indicates that main.rs is being included recursively. Removing the mod main; line builds succesfully.

[jseyfried]

Closing as duplicate of #36146.