Sane module file system hierarchy, first cut.

active/0000-sane-module-file-system-hierarchy.md

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+- Start Date: 2014-05-02
+- RFC PR #: (leave this empty)
+- Rust Issue #: (leave this empty)
+
+# Summary
+
+The rules about the places `mod foo;` can be used are tightened to only permit
+its use in a crate root and in `mod.rs` files, to ensure a more sane
+correspondence between module structure and file system hierarchy. Most
+notably, this prevents a common newbie error where a module is loaded multiple
+times, leading to surprising incompatibility between them. This proposal does
+not take away one's ability to shoot oneself in the foot should one really
+desire to; it just removes almost all of the rope, leaving only mixed
+metaphors.
+
+# Motivation
+
+It is a common newbie mistake to write things like this:
+
+`lib.rs`:
+
+```rust
+mod foo;
+pub mod bar;
+```
+
+`foo.rs`:
+
+```rust
+mod baz;
+
+pub fn foo(_baz: baz::Baz) { }
+```
+
+`bar.rs`:
+
+```rust
+mod baz;
+use foo::foo;
+
+pub fn bar(baz: baz::Baz) {
+    foo(baz)
+}
+```
+
+`baz.rs`:
+
+```rust
+pub struct Baz;
+```
+
+This fails to compile because `foo::foo()` wants a `foo::baz::Baz`, while
+`bar::bar()` is giving it a `bar::baz::Baz`.
+
+Such a situation, importing one file multiple times, is exceedingly rarely what
+the user actually wanted to do, but the present design allows it to occur
+without warning the user. The alterations contained herein ensure that there is
+no situation where such double loading can occur without deliberate intent via
+`#[path = "….rs"]`.
+
+# Drawbacks
+
+None known.
+
+# Detailed design
+
+When a `mod foo;` statement is used, the compiler attempts to find a suitable
+file. At present, it just blindly seeks for `foo.rs` or `foo/mod.rs` (relative
+to the file under parsing).
+
+The new behaviour will only permit `mod foo;` if at least one of the following
+conditions hold:
+
+- The file under parsing is the crate root, or
+
+- The file under parsing is a `mod.rs`, or
+
+- `#[path]` is specified, e.g. `#[path = "foo.rs"] mod foo;`.
+
+In layman's terms, the file under parsing must "own" the directory, so to
+speak.
+
+# Alternatives
+
+The rationale is covered in the summary. This is the simplest repair to the
+current lack of structure; all alternatives would be more complex and invasive.
+
+One non-invasive alternative is a lint which would detect double loads. This is
+less desirable than the solution discussed in this RFC as it doesn't fix the
+underlying problem which can, fortunately, be fairly easily fixed.
+
+# Unresolved questions
+
+None.