RFC for attributes on match arms and statements.

Author: huonw

active/0000-more-attributes.md

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+- Start Date: 2014-03-20
+- RFC PR #: (leave this empty)
+- Rust Issue #: (leave this empty)
+
+# Summary
+
+Allow attributes on more places inside functions, such as match arms
+and statements.
+
+# Motivation
+
+One sometimes wishes to annotate things inside functions with, for
+example, lint `#[allow]`s, conditional compilation `#[cfg]`s, branch
+weight hints and even extra semantic (or otherwise) annotations for
+external tools.
+
+For the lints, one can currently only activate lints at the level of
+the function which is possibly larger than one needs, and so may allow
+other "bad" things to sneak through accidentally. E.g.
+
+```rust
+#[allow(uppercase_variable)]
+let L = List::new(); // lowercase looks like one or capital i
+```
+
+For the conditional compilation, the work-around is duplicating the
+whole containing function with a `#[cfg]`. A case study is
+[sfackler's bindings to OpenSSL](https://github.com/sfackler/rust-openssl),
+where many distributions remove SSLv2 support, and so that portion of
+Rust bindings needs to be conditionally disabled. The obvious way to
+support the various different SSL versions is an enum
+
+```rust
+pub enum SslMethod {
+    #[cfg(sslv2)]
+    /// Only support the SSLv2 protocol
+    Sslv2,
+    /// Only support the SSLv3 protocol
+    Sslv3,
+    /// Only support the TLSv1 protocol
+    Tlsv1,
+    /// Support the SSLv2, SSLv3 and TLSv1 protocols
+    Sslv23,
+}
+```
+
+However, all `match`s can only mention `Sslv2` when the `cfg` is
+active, i.e. the following is invalid:
+
+```rust
+fn name(method: SslMethod) -> &'static str {
+    match method {
+        Sslv2 => "SSLv2",
+        Sslv3 => "SSLv3",
+        _ => "..."
+    }
+}
+```
+
+A valid method would be to have two definitions: `#[cfg(sslv2)] fn
+name(...)` and `#[cfg(not(sslv2)] fn name(...)`. The former has the
+`Sslv2` arm, the latter does not. Clearly, this explodes exponentially
+for each additional `cfg`'d variant in an enum.
+
+Branch weights would allow the careful micro-optimiser to inform the
+compiler that, for example, a certain match arm is rarely taken:
+
+```rust
+match foo {
+    Common => {}
+    #[cold]
+    Rare => {}
+}
+```
+
+The sort of things one could do with other arbitrary annotations are
+
+```rust
+#[allowed_unsafe_actions(ffi)]
+#[audited="2014-04-22"]
+unsafe { ... }
+```
+
+and then have an external tool that checks that that `unsafe` block's
+only unsafe actions are FFI, or a tool that lists blocks that have
+been changed since the last audit or haven't been audited ever.
+
+
+# Detailed design
+
+Normal attribute syntax:
+
+```rust
+fn foo() {
+    #[attr]
+    let x = 1;
+
+    #[attr]
+    foo();
+
+    #[attr]
+    match x {
+        #[attr]
+        Thing => {}
+    }
+
+    #[attr]
+    if foo {
+    } else {
+    }
+}
+```
+
+# Alternatives
+
+There aren't really any general alternatives; one could probably hack
+around the conditional-enum-variants & matches with some macros and
+helper functions to share as much code as possible; but in general
+this won't work.
+
+The other instances could be approximated with macros and helper
+functions, but to an even lesser degree (e.g. how would one annotate a
+general `unsafe` block).
+
+# Unresolved questions
+
+- Should one be able to annotate the `else` branch(es) of an `if`? e.g.
+
+  ```rust
+  if foo {
+  } #[attr] else if bar {
+  } #[attr] else {
+  }
+  ```
+
+  or maybe
+
+  ```rust
+  if foo {
+  } else #[attr] if bar {
+  } else #[attr] {
+  }
+  ```