Merge pull request #450 from ehuss/mod-rs

Document mod.rs changes.

Author: matthewjasper

src/crates-and-source-files.md

@@ -14,7 +14,7 @@
 
 > Note: Although Rust, like any other language, can be implemented by an
 > interpreter as well as a compiler, the only existing implementation is a
-> compiler,and the language has always been designed to be compiled. For these
+> compiler, and the language has always been designed to be compiled. For these
 > reasons, this section assumes a compiler.
 
 Rust's semantics obey a *phase distinction* between compile-time and
@@ -42,10 +42,10 @@ extension `.rs`.
 
 A Rust source file describes a module, the name and location of which —
 in the module tree of the current crate — are defined from outside the
-source file: either by an explicit `mod_item` in a referencing source file, or
-by the name of the crate itself. Every source file is a module, but not every
-module needs its own source file: [module definitions][module] can be nested
-within one file.
+source file: either by an explicit [`mod` item][module] in a referencing
+source file, or by the name of the crate itself. Every source file is a
+module, but not every module needs its own source file: [module
+definitions][module] can be nested within one file.
 
 Each source file contains a sequence of zero or more `item` definitions, and
 may optionally begin with any number of [attributes]

src/items/modules.md

@@ -1,6 +1,6 @@
 # Modules
 
-> **<sup>Syntax:<sup>**\
+> **<sup>Syntax:</sup>**\
 > _Module_ :\
 > &nbsp;&nbsp; &nbsp;&nbsp; `mod` [IDENTIFIER] `;`\
 > &nbsp;&nbsp; | `mod` [IDENTIFIER] `{`\
@@ -40,34 +40,88 @@ struct, enumeration, union, type parameter or crate can't shadow the name of a
 module in scope, or vice versa. Items brought into scope with `use` also have
 this restriction.
 
-A module without a body is loaded from an external file, by default with the
-same name as the module, plus the `.rs` extension. When a nested submodule is
-loaded from an external file, it is loaded from a subdirectory path that
-mirrors the module hierarchy.
+## Module Source Filenames
+
+A module without a body is loaded from an external file. When the module does
+not have a `path` attribute, the path to the file mirrors the logical [module
+path]. Ancestor module path components are directories, and the module's
+contents are in a file with the name of the module plus the `.rs` extension.
+For example, the following module structure can have this corresponding
+filesystem structure:
+
+Module Path               | Filesystem Path  | File Contents
+------------------------- | ---------------  | -------------
+`crate`                   | `lib.rs`         | `mod util;`
+`crate::util`             | `util.rs`        | `mod config;`
+`crate::util::config`     | `util/config.rs` |
+
+Module filenames may also be the name of the module as a directory with the
+contents in a file named `mod.rs` within that directory. The above example can
+alternately be expressed with `crate::util`'s contents in a file named
+`util/mod.rs`. It is not allowed to have both `util.rs` and `util/mod.rs`.
+
+> **Note**: Previous to `rustc` 1.30, using `mod.rs` files was the way to load
+> a module with nested children. It is encouraged to use the new naming
+> convention as it is more consistent, and avoids having many files named
+> `mod.rs` within a project.
+
+### `path` Attribute
+
+The directories and files used for loading external file modules can be
+influenced with the `path` attribute.
+
+For `path` attributes on modules not inside inline module blocks, the file
+path is relative to the directory the source file is located. For example, the
+following code snippet would use the paths shown based on where it is located:
 
 ```rust,ignore
-// Load the `vec` module from `vec.rs`
-mod vec;
+#[path = "foo.rs"]
+mod c;
+```
 
-mod thread {
-    // Load the `local_data` module from `thread/local_data.rs`
-    // or `thread/local_data/mod.rs`.
-    mod local_data;
+Source File    | `c`'s File Location | `c`'s Module Path
+-------------- | ------------------- | ----------------------
+`src/a/b.rs`   | `src/a/foo.rs`      | `crate::a::b::c`
+`src/a/mod.rs` | `src/a/foo.rs`      | `crate::a::c`
+
+For `path` attributes inside inline module blocks, the relative location of
+the file path depends on the kind of source file the `path` attribute is
+located in. "mod-rs" source files are root modules (such as `lib.rs` or
+`main.rs`) and modules with files named `mod.rs`. "non-mod-rs" source files
+are all other module files. Paths for `path` attributes inside inline module
+blocks in a mod-rs file are relative to the directory of the mod-rs file
+including the inline module components as directories. For non-mod-rs files,
+it is the same except the path starts with a directory with the name of the
+non-mod-rs module. For example, the following code snippet would use the paths
+shown based on where it is located:
+
+```rust,ignore
+mod inline {
+    #[path = "other.rs"]
+    mod inner;
 }
 ```
 
-The directories and files used for loading external file modules can be
-influenced with the `path` attribute.
+Source File    | `inner`'s File Location   | `inner`'s Module Path
+-------------- | --------------------------| ----------------------------
+`src/a/b.rs`   | `src/a/b/inline/other.rs` | `crate::a::b::inline::inner`
+`src/a/mod.rs` | `src/a/inline/other.rs`   | `crate::a::inline::inner`
+
+An example of combining the above rules of `path` attributes on inline modules
+and nested modules within (applies to both mod-rs and non-mod-rs files):
 
 ```rust,ignore
 #[path = "thread_files"]
 mod thread {
-    // Load the `local_data` module from `thread_files/tls.rs`
+    // Load the `local_data` module from `thread_files/tls.rs` relative to
+    // this source file's directory.
     #[path = "tls.rs"]
     mod local_data;
 }
 ```
 
+## Prelude Items
+
 Modules implicitly have some names in scope. These name are to built-in types,
 macros imported with `#[macro_use]` on an extern crate, and by the crate's
 [prelude]. These names are all made of a single identifier. These names are not
@@ -94,5 +148,6 @@ The built-in attributes that have meaning on a function are [`cfg`],
 [IDENTIFIER]: identifiers.html
 [attribute]: attributes.html
 [items]: items.html
+[module path]: paths.html
 [prelude]: crates-and-source-files.html#preludes-and-no_std
 [the lint check attributes]: attributes.html#lint-check-attributes