Replace MacExpr / MacPat / MacItems with MacEager

MacEager is a MacResult implementation for the common case where you've already
built each form of AST that you might return.

Fixes #17637. Based on #18814.

This is a [breaking-change] for syntax extensions:

  * MacExpr::new becomes MacEager::expr.

  * MacPat::new becomes MacEager::pat.

  * MacItems::new becomes MacEager::items. It takes a SmallVector directly,
    not an iterator.

src/libsyntax/ext/base.rs

@@ -28,6 +28,7 @@ use fold::Folder;
 
 use std::collections::HashMap;
 use std::rc::Rc;
+use std::default::Default;
 
 pub trait ItemDecorator {
     fn expand(&self,
@@ -226,9 +227,17 @@ impl<F> IdentMacroExpander for F
     }
 }
 
+// Use a macro because forwarding to a simple function has type system issues
+macro_rules! make_stmt_default {
+    ($me:expr) => {
+        $me.make_expr().map(|e| {
+            P(codemap::respan(e.span, ast::StmtExpr(e, ast::DUMMY_NODE_ID)))
+        })
+    }
+}
+
 /// The result of a macro expansion. The return values of the various
-/// methods are spliced into the AST at the callsite of the macro (or
-/// just into the compiler's internal macro table, for `make_def`).
+/// methods are spliced into the AST at the callsite of the macro.
 pub trait MacResult {
     /// Create an expression.
     fn make_expr(self: Box<Self>) -> Option<P<ast::Expr>> {
@@ -254,63 +263,76 @@ pub trait MacResult {
     /// By default this attempts to create an expression statement,
     /// returning None if that fails.
     fn make_stmt(self: Box<Self>) -> Option<P<ast::Stmt>> {
-        self.make_expr()
-            .map(|e| P(codemap::respan(e.span, ast::StmtExpr(e, ast::DUMMY_NODE_ID))))
+        make_stmt_default!(self)
     }
 }
 
-/// A convenience type for macros that return a single expression.
-pub struct MacExpr {
-    e: P<ast::Expr>
-}
-impl MacExpr {
-    pub fn new(e: P<ast::Expr>) -> Box<MacResult+'static> {
-        box MacExpr { e: e } as Box<MacResult+'static>
-    }
-}
-impl MacResult for MacExpr {
-    fn make_expr(self: Box<MacExpr>) -> Option<P<ast::Expr>> {
-        Some(self.e)
-    }
-    fn make_pat(self: Box<MacExpr>) -> Option<P<ast::Pat>> {
-        match self.e.node {
-            ast::ExprLit(_) => Some(P(ast::Pat {
-                id: ast::DUMMY_NODE_ID,
-                span: self.e.span,
-                node: ast::PatLit(self.e)
-            })),
-            _ => None
+macro_rules! make_MacEager {
+    ( $( $fld:ident: $t:ty, )* ) => {
+        /// `MacResult` implementation for the common case where you've already
+        /// built each form of AST that you might return.
+        #[derive(Default)]
+        pub struct MacEager {
+            $(
+                pub $fld: Option<$t>,
+            )*
+        }
+
+        impl MacEager {
+            $(
+                pub fn $fld(v: $t) -> Box<MacResult> {
+                    box MacEager {
+                        $fld: Some(v),
+                        ..Default::default()
+                    } as Box<MacResult>
+                }
+            )*
         }
     }
 }
-/// A convenience type for macros that return a single pattern.
-pub struct MacPat {
-    p: P<ast::Pat>
-}
-impl MacPat {
-    pub fn new(p: P<ast::Pat>) -> Box<MacResult+'static> {
-        box MacPat { p: p } as Box<MacResult+'static>
-    }
-}
-impl MacResult for MacPat {
-    fn make_pat(self: Box<MacPat>) -> Option<P<ast::Pat>> {
-        Some(self.p)
-    }
-}
-/// A type for macros that return multiple items.
-pub struct MacItems {
-    items: SmallVector<P<ast::Item>>
+
+make_MacEager! {
+    expr: P<ast::Expr>,
+    pat: P<ast::Pat>,
+    items: SmallVector<P<ast::Item>>,
+    methods: SmallVector<P<ast::Method>>,
+    stmt: P<ast::Stmt>,
 }
 
-impl MacItems {
-    pub fn new<I: Iterator<Item=P<ast::Item>>>(it: I) -> Box<MacResult+'static> {
-        box MacItems { items: it.collect() } as Box<MacResult+'static>
+impl MacResult for MacEager {
+    fn make_expr(self: Box<Self>) -> Option<P<ast::Expr>> {
+        self.expr
     }
-}
 
-impl MacResult for MacItems {
-    fn make_items(self: Box<MacItems>) -> Option<SmallVector<P<ast::Item>>> {
-        Some(self.items)
+    fn make_items(self: Box<Self>) -> Option<SmallVector<P<ast::Item>>> {
+        self.items
+    }
+
+    fn make_methods(self: Box<Self>) -> Option<SmallVector<P<ast::Method>>> {
+        self.methods
+    }
+
+    fn make_stmt(self: Box<Self>) -> Option<P<ast::Stmt>> {
+        match self.stmt {
+            None => make_stmt_default!(self),
+            s => s,
+        }
+    }
+
+    fn make_pat(self: Box<Self>) -> Option<P<ast::Pat>> {
+        if let Some(p) = self.pat {
+            return Some(p);
+        }
+        if let Some(e) = self.expr {
+            if let ast::ExprLit(_) = e.node {
+                return Some(P(ast::Pat {
+                    id: ast::DUMMY_NODE_ID,
+                    span: e.span,
+                    node: ast::PatLit(e),
+                }));
+            }
+        }
+        None
     }
 }