Add syntax support for attributes on expressions and all syntax

nodes in statement position.

Extended #[cfg] folder to allow removal of statements, and
of expressions in optional positions like expression lists and trailing
block expressions.

Extended lint checker to recognize lint levels on expressions and
locals.

src/libsyntax/ast.rs

@@ -692,8 +692,21 @@ pub enum Stmt_ {
     /// Expr with trailing semi-colon (may have any type):
     StmtSemi(P<Expr>, NodeId),
 
-    StmtMac(P<Mac>, MacStmtStyle),
+    StmtMac(P<Mac>, MacStmtStyle, ThinAttributes),
 }
+
+impl Stmt_ {
+    pub fn attrs(&self) -> &[Attribute] {
+        match *self {
+            StmtDecl(ref d, _) => d.attrs(),
+            StmtExpr(ref e, _) |
+            StmtSemi(ref e, _) => e.attrs(),
+            StmtMac(_, _, Some(ref b)) => b,
+            StmtMac(_, _, None) => &[],
+        }
+    }
+}
+
 #[derive(Clone, Copy, PartialEq, Eq, RustcEncodable, RustcDecodable, Hash, Debug)]
 pub enum MacStmtStyle {
     /// The macro statement had a trailing semicolon, e.g. `foo! { ... };`
@@ -718,6 +731,16 @@ pub struct Local {
     pub init: Option<P<Expr>>,
     pub id: NodeId,
     pub span: Span,
+    pub attrs: ThinAttributes,
+}
+
+impl Local {
+    pub fn attrs(&self) -> &[Attribute] {
+        match self.attrs {
+            Some(ref b) => b,
+            None => &[],
+        }
+    }
 }
 
 pub type Decl = Spanned<Decl_>;
@@ -730,6 +753,15 @@ pub enum Decl_ {
     DeclItem(P<Item>),
 }
 
+impl Decl {
+    pub fn attrs(&self) -> &[Attribute] {
+        match self.node {
+            DeclLocal(ref l) => l.attrs(),
+            DeclItem(ref i) => i.attrs(),
+        }
+    }
+}
+
 /// represents one arm of a 'match'
 #[derive(Clone, PartialEq, Eq, RustcEncodable, RustcDecodable, Hash, Debug)]
 pub struct Arm {
@@ -766,6 +798,16 @@ pub struct Expr {
     pub id: NodeId,
     pub node: Expr_,
     pub span: Span,
+    pub attrs: ThinAttributes
+}
+
+impl Expr {
+    pub fn attrs(&self) -> &[Attribute] {
+        match self.attrs {
+            Some(ref b) => b,
+            None => &[],
+        }
+    }
 }
 
 impl fmt::Debug for Expr {
@@ -1792,6 +1834,12 @@ pub struct Item {
     pub span: Span,
 }
 
+impl Item {
+    pub fn attrs(&self) -> &[Attribute] {
+        &self.attrs
+    }
+}
+
 #[derive(Clone, PartialEq, Eq, RustcEncodable, RustcDecodable, Hash, Debug)]
 pub enum Item_ {
     /// An`extern crate` item, with optional original crate name,
@@ -1904,6 +1952,98 @@ pub struct MacroDef {
     pub body: Vec<TokenTree>,
 }
 
+/// A list of attributes, behind a optional box as
+/// a space optimization.
+pub type ThinAttributes = Option<Box<Vec<Attribute>>>;
+
+pub trait ThinAttributesExt {
+    fn map_opt_attrs<F>(self, f: F) -> Self
+        where F: FnOnce(Vec<Attribute>) -> Vec<Attribute>;
+    fn prepend_outer(mut self, attrs: Self) -> Self;
+    fn append_inner(mut self, attrs: Self) -> Self;
+    fn update<F>(&mut self, f: F)
+        where Self: Sized,
+              F: FnOnce(Self) -> Self;
+    fn as_attrs(&self) -> &[Attribute];
+    fn into_attrs(self) -> Vec<Attribute>;
+}
+
+// FIXME: Rename inner/outer
+// FIXME: Rename opt_attrs
+
+impl ThinAttributesExt for ThinAttributes {
+    fn map_opt_attrs<F>(self, f: F) -> Self
+        where F: FnOnce(Vec<Attribute>) -> Vec<Attribute> {
+
+        // This is kinda complicated... Ensure the function is
+        // always called, and that None inputs or results are
+        // correctly handled.
+        if let Some(mut b) = self {
+            use std::mem::replace;
+
+            let vec = replace(&mut *b, Vec::new());
+            let vec = f(vec);
+            if vec.len() == 0 {
+                None
+            } else {
+                replace(&mut*b, vec);
+                Some(b)
+            }
+        } else {
+            f(Vec::new()).into_opt_attrs()
+        }
+    }
+
+    fn prepend_outer(self, attrs: ThinAttributes) -> Self {
+        attrs.map_opt_attrs(|mut attrs| {
+            attrs.extend(self.into_attrs());
+            attrs
+        })
+    }
+
+    fn append_inner(self, attrs: ThinAttributes) -> Self {
+        self.map_opt_attrs(|mut self_| {
+            self_.extend(attrs.into_attrs());
+            self_
+        })
+    }
+
+    fn update<F>(&mut self, f: F)
+        where Self: Sized,
+              F: FnOnce(ThinAttributes) -> ThinAttributes
+    {
+        let self_ = f(self.take());
+        *self = self_;
+    }
+
+    fn as_attrs(&self) -> &[Attribute] {
+        match *self {
+            Some(ref b) => b,
+            None => &[],
+        }
+    }
+
+    fn into_attrs(self) -> Vec<Attribute> {
+        match self {
+            Some(b) => *b,
+            None => Vec::new(),
+        }
+    }
+}
+
+pub trait AttributesExt {
+    fn into_opt_attrs(self) -> ThinAttributes;
+}
+impl AttributesExt for Vec<Attribute> {
+    fn into_opt_attrs(self) -> ThinAttributes {
+        if self.len() == 0 {
+            None
+        } else {
+            Some(Box::new(self))
+        }
+    }
+}
+
 #[cfg(test)]
 mod tests {
     use serialize;