path-clarity: Document new relative paths variant

For the final Rust 2018 release, we can delete the section that doesn't
apply, and change the various qualified references like "the relative
paths variant of Rust 2018" to just "Rust 2018".

Author: joshtriplett

src/rust-2018/path-clarity.md

@@ -12,11 +12,23 @@ As such, the 2018 edition of Rust introduces a few new module system
 features, but they end up *simplifying* the module system, to make it more
 clear as to what is going on.
 
+Note: During the 2018 edition preview, there are two variants of the module
+system under consideration, the "absolute use paths" variant and the "relative
+paths" variant. Most of these changes apply to both variants; the two variant
+sections call out the differences between the two. We encourage testing of the
+new "relative paths" variant introduced in edition preview 2. The release of
+the 2018 edition will use one of these two variants.
+
 Here's a brief summary:
 
 * `extern crate` is no longer needed
-* Absolute paths begin with a crate name, where the keyword `crate`
-  refers to the current crate.
+* The `crate` keyword refers to the current crate.
+* Relative paths variant: Paths work uniformly in both `use` statements and in
+  other code, both in the top-level module and in submodules, and may use
+  either absolute paths or local names relative to the current module.
+* Absolute use paths variant: Paths in `use` statements are always absolute and
+  begin with a crate name (or `crate`); paths in other code may use absolute
+  paths or local names relative to the current module.
 * A `foo.rs` and `foo/` subdirectory may coexist; `mod.rs` is no longer needed
   when placing submodules in a subdirectory.
 
@@ -60,9 +72,142 @@ keep doing what you were doing there as well.
 One other use for `extern crate` was to import macros; that's no longer needed.
 Check [the macro section](2018/transitioning/modules/macros.html) for more.
 
-### Absolute paths begin with `crate` or the crate name
+### The `crate` keyword refers to the current crate.
+
+In `use` statements and in other code, you can refer to the root of the current
+crate with the `crate::` prefix. For instance, `crate::foo::bar` will always
+refer to the name `bar` inside the module `foo`, from anywhere else in the same
+crate.
+
+The prefix `::` previously referred to either the crate root or an external
+crate; it now unambiguously refers to an external crate. For instance,
+`::foo::bar` always refers to the name `bar` inside the external crate `foo`.
+
+### Relative paths variant
+
+The relative paths variant of Rust 2018 simplifies and unifies path handling
+compared to Rust 2015. In Rust 2015, paths work differently in use statements
+than they do elsewhere. In particular, paths in `use` statements would always
+start from the crate root, while paths in other code implicitly started from
+the current module. Those differences didn't have any effect in the top-level
+module, which meant that everything would seem straightforward until working on
+a project large enough to have submodules.
+
+In the relative paths variant of Rust 2018, paths in `use` statements and in
+other code always work the same way, both in the top-level module and in any
+submodule. You can either use a relative path from the current module, an
+absolute path from the top of the current crate (starting with `crate::`), or
+an absolute path starting from an external crate name.
+
+Code that looked like this:
+
+```rust,ignore
+// Rust 2015
+
+extern crate futures;
+
+use futures::Future;
+
+mod foo {
+    struct Bar;
+}
+
+use foo::Bar;
+
+fn my_poll() -> futures::Poll { ... }
+
+enum SomeEnum {
+    V1(usize),
+    V2(String),
+}
+
+fn func() {
+    let five = std::sync::Arc::new(5);
+    use SomeEnum::*;
+    match ... {
+        V1(i) => { ... }
+        V2(s) => { ... }
+    }
+}
+```
+
+will look exactly the same in Rust 2018, except that you can delete the `extern
+crate` line:
 
-In Rust 2018, paths in `use` statements *must* begin with one of:
+```rust,ignore
+// Rust 2018 (relative paths variant)
+
+use futures::Future;
+
+mod foo {
+    struct Bar;
+}
+
+use foo::Bar;
+
+fn my_poll() -> futures::Poll { ... }
+
+enum SomeEnum {
+    V1(usize),
+    V2(String),
+}
+
+fn func() {
+    let five = std::sync::Arc::new(5);
+    use SomeEnum::*;
+    match ... {
+        V1(i) => { ... }
+        V2(s) => { ... }
+    }
+}
+```
+
+With Rust 2018, however, the same code will also work completely unmodified in
+a submodule:
+
+```rust,ignore
+// Rust 2018 (relative paths variant)
+
+mod submodule {
+    use futures::Future;
+
+    mod foo {
+        struct Bar;
+    }
+
+    use foo::Bar;
+
+    fn my_poll() -> futures::Poll { ... }
+
+    enum SomeEnum {
+        V1(usize),
+        V2(String),
+    }
+
+    fn func() {
+        let five = std::sync::Arc::new(5);
+        use SomeEnum::*;
+        match ... {
+            V1(i) => { ... }
+            V2(s) => { ... }
+        }
+    }
+}
+```
+
+This makes it easy to move code around in a project, and avoids introducing
+additional complexity to multi-module projects.
+
+If a path is ambiguous, such as if you have an external crate and a local
+module or item with the same name, you'll get an error, and you'll need to
+either rename one of the conflicting names or explicitly disambiguate the path.
+To explicitly disambiguate a path, use `::name` for an external crate name, or
+`self::name` for a local module or item.
+
+### Absolute use paths variant
+
+In the absolute use paths variant of Rust 2018, paths in `use` statements
+*must* begin with one of:
 
 - A crate name
 - `crate` for the current crate's root
@@ -88,7 +233,7 @@ use foo::Bar;
 Now looks like this:
 
 ```rust,ignore
-// Rust 2018
+// Rust 2018 (absolute use paths variant)
 
 // 'futures' is the name of a crate
 use futures::Future;
@@ -142,7 +287,7 @@ it's fine in `main`, but it doesn't exist in the submodule at all.
 Let's look at how this change affects things:
 
 ```rust,ignore
-// Rust 2018
+// Rust 2018 (absolute use paths variant)
 
 // no more `extern crate futures;`
 
@@ -169,7 +314,6 @@ mod submodule {
 
 Much more straightforward.
 
-**Note**: an alternative syntax is also under consideration: writing `::some::Local` rather than `crate::some::Local`. If you have thoughts about this alternative, please leave a comment on [the tracking issue](https://github.com/rust-lang/rust/issues/44660) or start a thread on the [edition feedback category](https://internals.rust-lang.org/c/edition-2018-feedback).
 
 ### No more `mod.rs`