RFC to require impl MyStruct to be in the same module as the definition of MyStruct

This fixes #15060 and #3785.

Author: apoelstra

active/0000-anonymous-impl-only-in-same-module.md

@@ -0,0 +1,136 @@
+- Start Date: 2014-07-04
+- RFC PR #: (leave this empty)
+- Rust Issue #: (leave this empty)
+
+# Summary
+
+Require "anonymous traits", i.e. `impl MyStruct` to occur only in the same module that `MyStruct` is defined.
+
+# Motivation
+
+Before I can explain the motivation for this, I should provide some background
+as to how anonymous traits are implemented, and the sorts of bugs we see with
+the current behaviour. The conclusion will be that we effectively already only
+support `impl MyStruct` in the same module that `MyStruct` is defined, and
+making this a rule will simply give cleaner error messages.
+
+- The compiler first sees `impl MyStruct` during the resolve phase, specifically
+  in `Resolver::build_reduced_graph()`, called by `Resolver::resolve()` in
+  `src/librustc/middle/resolve.rs`. This is before any type checking (or type
+  resolution, for that matter) is done, so the compiler trusts for now that
+  `MyStruct` is a valid type.
+- If `MyStruct` is a path with more than one segment, such as `mymod::MyStruct`,
+  it is silently ignored (how was this not flagged when the code was written??),
+  which effectively causes static methods in such `impl`s to be dropped on the
+  floor. A silver lining here is that nothing is added to the current module
+  namespace, so the shadowing bugs demonstrated in the next bullet point do not
+  apply here. (To locate this bug in the code, find the `match` immediately following
+  the `FIXME (#3785)` comment in `resolve.rs`.) This leads to the following
+````
+mod break1 {
+    pub struct MyGuy;
+
+    impl MyGuy {
+        pub fn do1() { println!("do 1"); }
+    }
+}
+
+impl break1::MyGuy {
+    fn do2() { println!("do 2"); }
+}
+
+fn main() {
+    break1::MyGuy::do1();
+    break1::MyGuy::do2();
+}
+````
+````
+<anon>:15:5: 15:23 error: unresolved name `break1::MyGuy::do2`.
+<anon>:15     break1::MyGuy::do2();
+````
+  as noticed by @huonw in https://github.com/rust-lang/rust/issues/15060 .
+- If one does not exist, the compiler creates a submodule `MyStruct` of the
+  current module, with `kind` `ImplModuleKind`. Static methods are placed into
+  this module. If such a module already exists, the methods are appended to it,
+  to support multiple `impl MyStruct` blocks within the same module. If a module
+  exists that is not `ImplModuleKind`, the compiler signals a duplicate module
+  definition error.
+- Notice at this point that if there is a `use MyStruct`, the compiler will act
+  as though it is unaware of this. This is because imports are not resolved yet
+  (they are in `Resolver::resolve_imports()` called immediately after
+  `Resolver::build_reduced_graph()` is called). In the final resolution step,
+  `MyStruct` will be searched in the namespace of the current module, checking
+  imports only as a fallback (and only in some contexts), so the `use MyStruct` is
+  effectively shadowed. If there is an `impl MyStruct` in the file being imported
+  from, the user expects that the new `impl MyStruct` will append to that one,
+  same as if they are in the original file. This leads to the original bug report
+  https://github.com/rust-lang/rust/issues/15060 .
+- In fact, even if no methods from the import are used, the name `MyStruct` will
+  not be associated to a type, so that
+````
+trait T {}
+impl<U: T> Vec<U> {
+    fn from_slice<'a>(x: &'a [uint]) -> Vec<uint> {
+        fail!()
+    }
+}
+fn main() { let r = Vec::from_slice(&[1u]); }
+````
+````
+error: found module name used as a type: impl Vec<U>::Vec<U> (id=5)
+impl<U: T> Vec<U>
+````
+  which @Ryman noticed in https://github.com/rust-lang/rust/issues/15060 . The
+  reason for this is that in `Resolver::resolve_crate()`, the final step of
+  `Resolver::resolve()`, the type of an anonymous `impl` is determined by
+  `NameBindings::def_for_namespace(TypeNS)`. This function searches the namespace
+  `TypeNS` (which is <i>not</i> affected by imports) for a type; failing that it
+  tries for a module; failing that it returns `None`. The result is that when
+  typeck runs, it sees `impl [module name]` instead of `impl [type name]`.
+  
+
+The main motivation of this RFC is to clear out these bugs, which do not make
+sense to a user of the language (and had me confused for quite a while).
+
+A secondary motivation is to enforce consistency in code layout; anonymous traits
+are used the way that class methods are used in other languages, and the data
+and methods of a struct should be defined nearby.
+
+# Detailed design
+
+I propose three changes to the language:
+
+- `impl` on multiple-ident paths such as `impl mymod::MyStruct` is disallowed.
+  Since this currently suprises the user by having absolutely no effect for
+  static methods, support for this is already broken.
+- `impl MyStruct` must occur in the same module that `MyStruct` is defined.
+  This is to prevent the above problems with `impl`-across-modules.
+  Migration path is for users to just move code between source files.
+
+# Drawbacks
+
+Static methods on `impl`s-away-from-definition never worked, while non-static
+methods can be implemented using non-anonymous traits. So there is no loss in
+expressivity. However, using a trait where before there was none may be clumsy,
+since it might not have a sensible name, and it must be explicitly imported by
+all users of the trait methods.
+
+For example, in the stdlib `src/libstd/io/fs.rs` we see the code `impl path::Path`
+to attach (non-static) filesystem-related methods to the `Path` type. This would
+have to be done via a `FsPath` trait which is implemented on `Path` and exported
+alongside `Path` in the prelude.
+
+It is worth noting that this is the only instance of this RFC conflicting with
+current usage in the stdlib or compiler.
+
+# Alternatives
+
+- Leaving this alone and fixing the bugs directly. This is really hard. To do it
+  properly, we would need to seriously refactor resolve.
+
+# Unresolved questions
+
+None.
+
+
+