A directory cannot be a module on its own: there is no mod.mtl equivalent
Gap
Rust lets a directory be its own module through mod.rs (util/mod.rs
declares util). Metel has no counterpart. :: maps directly to /, so a
module path names exactly one file, and import root::util::answer; looks for
util.mtl, never for util/mod.mtl. Reproduced against current develop:
// main.mtl
import root::util::answer;
fun main() { assert(answer() == 42); }
with the module in util/mod.mtl:
[P0001] parse error: cannot find module file for `util::answer`
The same program passes with the module in util.mtl. A directory module that
needs a public facade is written as name.mtl alongside the name/ directory
(parser.mtl re-exporting from parser/ast.mtl, parser/lexer.mtl); the two
are distinct paths and coexist. The spec states this directly: there is no
name/mod.mtl convention.
Impact
A module cannot keep its root file inside its own directory. Moving or renaming
a module means moving a file and a directory that sit side by side, and the
directory cannot be relocated as a self-contained unit the way a mod.rs
directory can. Programs that group submodules under a directory must add the
facade file next to it.
Affects
Resolution
Accepted as designed: RFC-0030 (module system redesign, implemented) chose
":: maps to /, no special directory module file", and the spec records that
as Legality Rule file-to-module-mapping.legality-1, covered by a fixture.
Reopen if directory-local module roots are wanted; that would be a new RFC.