Change attribute syntax to @foo

Author: sfackler

text/0000-attribute-syntax.md

@@ -0,0 +1,90 @@
+- Start Date: 2014-11-25
+- RFC PR:
+- Rust Issue:
+
+# Summary
+
+Change attribute syntax from `#[foo]` and `#![foo]` to `@foo` and `@!foo`.
+
+# Motivation
+
+Other languages like Java and Python use `@` for their equivalents of
+attributes. In addition, `@foo` is easier to type than `#[foo]`, and is
+arguably less noisy looking.
+
+This change was proposed as part of [RFC
+386](https://github.com/rust-lang/rfcs/pull/386/files) and was generally
+well-received.
+
+# Detailed design
+
+Attributes and inner attributes would be written in one of the following forms
+(BNF):
+
+```
+ATTR       = '@' [!] META
+META       = ID
+           | ID '(' META_SEQ ')'
+META_SEQ   = META_ITEM {',' META_ITEM}
+META_ITEM  = META
+           | ID '=' STRING_LITERAL
+```
+
+Here are some examples of legal syntax:
+
+* `@inline`
+* `@!inline`
+* `@deprecated(reason = "foo")`
+* `@deriving(Eq)`
+
+Note that some attributes which are legal today have no equivalent:
+
+* `#[deprecated = "foo"]` becomes `@deprecated(reason = "foo")`
+
+## Implementation
+
+The parser will be adjusted to accept `@`-attributes in addition to current
+attributes. The internal data structures will remain the same. Once a snapshot
+lands, the Rust codebase can be converted, and parsing support for
+`#`-attributes can be removed with an error message added explaining how to fix
+code.
+
+# Drawbacks
+
+It's a large change that will cause a ton of churn very close to 1.0. Since the
+only compiler changes required will be to the parser and pretty printer, it's
+relatively low risk (compared to resolve or typeck changes for example).
+
+# Alternatives
+
+We can punt on this until after 1.0. `@`-attributes and `#`-attributes will
+have to coexist to avoid breaking backwards compatibility, but that won't be
+all that hard to deal with.
+
+We can leave the syntax as is, which is also not that bad.
+
+Support for `#[deprecated = "reason"]` style attributes is removed because
+`@deprecated = "reason"` is a bit visually confusing since there are no
+delimiters wrapping the attribute. There are a couple of alternatives here.
+One is to just allow that syntax. It's not grammatically ambiguous, after all.
+
+Another is to change `foo = "bar'` to `foo("bar")`:
+```
+ATTR     = '@' [!] META
+META     = ID
+         | ID '(' STRING_LITERAL ')
+         | ID '(' META_SEQ ')'
+META_SEQ = META {',' META}
+```
+
+For example:
+* `@deprecated("foo")`
+* `@cfg(all(target_os("linux"), feature("my_cargo_feature")))`
+
+This allows for a more convenient syntax for deprecation, but does add even
+more parentheses to attribute invocations like the `cfg` example above.
+
+# Unresolved questions
+
+Should there be a "deprecation period" where the `#` syntax is accepted with a
+warning?