Relative paths by default.

text/0000-relative-paths-by-default.md

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+- Start Date: 2015-01-09
+- RFC PR: (leave this empty)
+- Rust Issue: (leave this empty)
+
+# Summary
+
+All paths should be relative by default. The current design requires the
+`self::` prefix to be relative in `use` statements.
+
+Absolute paths should require a new `crate::` prefix. The current design has
+absolute paths by default in `use` statements.
+
+# Motivation
+
+Currently, `use` statements have absolute paths by default, but other paths
+are relative paths by default. This causes confusion for beginners due to
+the inconsistencies.
+
+Additionally, this encourages misusing absolute paths when relative paths
+are more logical (e.g. when importing from a sibling module). This makes
+refactoring such as renaming modules harder. The cases that require
+absolute paths should be rarer than the ones that require relative paths,
+given a reasonable module structure.
+
+Note that this creates a nice analogy with the file system, with `::`
+instead of `/`.
+
+# Detailed design
+
+The original grammar for paths (from the reference) is:
+
+```
+expr_path : [ "::" ] ident [ "::" expr_path_tail ] + ;
+```
+
+The new grammar (which includes some clean up) is:
+
+```
+expr_path : [ "crate::" | "super::" + ] ? ident [ "::" expr_path_tail ] + ;
+```
+
+The behavior is simple:
+
+- The `crate::` prefix makes the path an absolute path (i.e. starts from
+  the crate root).
+- The `super::` prefix makes the path start from the nth ancestor module,
+  where n is the number of `super::` prefixes.
+- Having no prefix makes the path a relative path (i.e. starts from the
+  current module).
+
+Note that `use foo::bar` should behave as if the contents of `foo::bar`
+(`self::foo::bar` in the current syntax) is copied into the current module
+(`self`). The analogy to the file system is creating a soft link named `bar`
+pointing to `foo/bar`. As a consequence, `use foo::bar` followed by
+`use bar::baz` is valid.
+
+# Drawbacks
+
+This breaks *a lot* of code.
+
+# Alternatives
+
+- Use the existing `::` prefix instead of the `crate::` prefix. However, I
+think that this might cause some confusion for beginners.
+- The status quo has problems as mentioned in the Motivation section.
+
+# Unresolved questions
+
+Unknown