syntax: unify all MacResult's into a single trait.

There's now one unified way to return things from a macro, instead of
being able to choose the `AnyMacro` trait or the `MRItem`/`MRExpr`
variants of the `MacResult` enum. This does simplify the logic handling
the expansions, but the biggest value of this is it makes macros in (for
example) type position easier to implement, as there's this single thing
to modify.

By my measurements (using `-Z time-passes` on libstd and librustc etc.),
this appears to have little-to-no impact on expansion speed. There are
presumably larger costs than the small number of extra allocations and
virtual calls this adds (notably, all `macro_rules!`-defined macros have
not changed in behaviour, since they had to use the `AnyMacro` trait
anyway).

src/libfourcc/lib.rs

@@ -57,7 +57,7 @@ use syntax::ast::Name;
 use syntax::attr::contains;
 use syntax::codemap::{Span, mk_sp};
 use syntax::ext::base;
-use syntax::ext::base::{SyntaxExtension, BasicMacroExpander, NormalTT, ExtCtxt, MRExpr};
+use syntax::ext::base::{SyntaxExtension, BasicMacroExpander, NormalTT, ExtCtxt, MacExpr};
 use syntax::ext::build::AstBuilder;
 use syntax::parse;
 use syntax::parse::token;
@@ -73,7 +73,7 @@ pub fn macro_registrar(register: |Name, SyntaxExtension|) {
         None));
 }
 
-pub fn expand_syntax_ext(cx: &mut ExtCtxt, sp: Span, tts: &[ast::TokenTree]) -> base::MacResult {
+pub fn expand_syntax_ext(cx: &mut ExtCtxt, sp: Span, tts: &[ast::TokenTree]) -> ~base::MacResult {
     let (expr, endian) = parse_tts(cx, tts);
 
     let little = match endian {
@@ -101,12 +101,12 @@ pub fn expand_syntax_ext(cx: &mut ExtCtxt, sp: Span, tts: &[ast::TokenTree]) ->
             }
             _ => {
                 cx.span_err(expr.span, "unsupported literal in fourcc!");
-                return MRExpr(cx.expr_lit(sp, ast::LitUint(0u64, ast::TyU32)));
+                return base::DummyResult::expr(sp)
             }
         },
         _ => {
             cx.span_err(expr.span, "non-literal in fourcc!");
-            return MRExpr(cx.expr_lit(sp, ast::LitUint(0u64, ast::TyU32)));
+            return base::DummyResult::expr(sp)
         }
     };
 
@@ -126,7 +126,7 @@ pub fn expand_syntax_ext(cx: &mut ExtCtxt, sp: Span, tts: &[ast::TokenTree]) ->
         };
     }
     let e = cx.expr_lit(sp, ast::LitUint(val as u64, ast::TyU32));
-    MRExpr(e)
+    MacExpr::new(e)
 }
 
 struct Ident {

src/libhexfloat/lib.rs

@@ -53,7 +53,7 @@ use syntax::ast;
 use syntax::ast::Name;
 use syntax::codemap::{Span, mk_sp};
 use syntax::ext::base;
-use syntax::ext::base::{SyntaxExtension, BasicMacroExpander, NormalTT, ExtCtxt, MRExpr};
+use syntax::ext::base::{SyntaxExtension, BasicMacroExpander, NormalTT, ExtCtxt, MacExpr};
 use syntax::ext::build::AstBuilder;
 use syntax::parse;
 use syntax::parse::token;
@@ -97,7 +97,7 @@ fn hex_float_lit_err(s: &str) -> Option<(uint, ~str)> {
     }
 }
 
-pub fn expand_syntax_ext(cx: &mut ExtCtxt, sp: Span, tts: &[ast::TokenTree]) -> base::MacResult {
+pub fn expand_syntax_ext(cx: &mut ExtCtxt, sp: Span, tts: &[ast::TokenTree]) -> ~base::MacResult {
     let (expr, ty_lit) = parse_tts(cx, tts);
 
     let ty = match ty_lit {
@@ -121,12 +121,12 @@ pub fn expand_syntax_ext(cx: &mut ExtCtxt, sp: Span, tts: &[ast::TokenTree]) ->
             }
             _ => {
                 cx.span_err(expr.span, "unsupported literal in hexfloat!");
-                return base::MacResult::dummy_expr(sp);
+                return base::DummyResult::expr(sp);
             }
         },
         _ => {
             cx.span_err(expr.span, "non-literal in hexfloat!");
-            return base::MacResult::dummy_expr(sp);
+            return base::DummyResult::expr(sp);
         }
     };
 
@@ -137,7 +137,7 @@ pub fn expand_syntax_ext(cx: &mut ExtCtxt, sp: Span, tts: &[ast::TokenTree]) ->
                 let pos = expr.span.lo + syntax::codemap::Pos::from_uint(err_pos + 1);
                 let span = syntax::codemap::mk_sp(pos,pos);
                 cx.span_err(span, format!("invalid hex float literal in hexfloat!: {}", err_str));
-                return base::MacResult::dummy_expr(sp);
+                return base::DummyResult::expr(sp);
             }
             _ => ()
         }
@@ -147,7 +147,7 @@ pub fn expand_syntax_ext(cx: &mut ExtCtxt, sp: Span, tts: &[ast::TokenTree]) ->
         None => ast::LitFloatUnsuffixed(s),
         Some (ty) => ast::LitFloat(s, ty)
     };
-    MRExpr(cx.expr_lit(sp, lit))
+    MacExpr::new(cx.expr_lit(sp, lit))
 }
 
 struct Ident {

src/libsyntax/ext/asm.rs

@@ -45,7 +45,7 @@ impl State {
 static OPTIONS: &'static [&'static str] = &["volatile", "alignstack", "intel"];
 
 pub fn expand_asm(cx: &mut ExtCtxt, sp: Span, tts: &[ast::TokenTree])
-               -> base::MacResult {
+               -> ~base::MacResult {
     let mut p = parse::new_parser_from_tts(cx.parse_sess(),
                                            cx.cfg(),
                                            tts.iter()
@@ -72,7 +72,7 @@ pub fn expand_asm(cx: &mut ExtCtxt, sp: Span, tts: &[ast::TokenTree])
                                                    "inline assembly must be a string literal.") {
                     Some((s, st)) => (s, st),
                     // let compilation continue
-                    None => return MacResult::dummy_expr(sp),
+                    None => return DummyResult::expr(sp),
                 };
                 asm = s;
                 asm_str_style = Some(style);
@@ -210,7 +210,7 @@ pub fn expand_asm(cx: &mut ExtCtxt, sp: Span, tts: &[ast::TokenTree])
         inputs.push((token::intern_and_get_ident(i.to_str()), out));
     }
 
-    MRExpr(@ast::Expr {
+    MacExpr::new(@ast::Expr {
         id: ast::DUMMY_NODE_ID,
         node: ast::ExprInlineAsm(ast::InlineAsm {
             asm: token::intern_and_get_ident(asm.get()),

src/libsyntax/ext/base.rs

@@ -50,19 +50,19 @@ pub trait MacroExpander {
               ecx: &mut ExtCtxt,
               span: Span,
               token_tree: &[ast::TokenTree])
-              -> MacResult;
+              -> ~MacResult;
 }
 
 pub type MacroExpanderFn =
     fn(ecx: &mut ExtCtxt, span: codemap::Span, token_tree: &[ast::TokenTree])
-       -> MacResult;
+       -> ~MacResult;
 
 impl MacroExpander for BasicMacroExpander {
     fn expand(&self,
               ecx: &mut ExtCtxt,
               span: Span,
               token_tree: &[ast::TokenTree])
-              -> MacResult {
+              -> ~MacResult {
         (self.expander)(ecx, span, token_tree)
     }
 }
@@ -78,7 +78,7 @@ pub trait IdentMacroExpander {
               sp: Span,
               ident: ast::Ident,
               token_tree: Vec<ast::TokenTree> )
-              -> MacResult;
+              -> ~MacResult;
 }
 
 impl IdentMacroExpander for BasicIdentMacroExpander {
@@ -87,62 +87,130 @@ impl IdentMacroExpander for BasicIdentMacroExpander {
               sp: Span,
               ident: ast::Ident,
               token_tree: Vec<ast::TokenTree> )
-              -> MacResult {
+              -> ~MacResult {
         (self.expander)(cx, sp, ident, token_tree)
     }
 }
 
 pub type IdentMacroExpanderFn =
-    fn(&mut ExtCtxt, Span, ast::Ident, Vec<ast::TokenTree> ) -> MacResult;
+    fn(&mut ExtCtxt, Span, ast::Ident, Vec<ast::TokenTree> ) -> ~MacResult;
 
 pub type MacroCrateRegistrationFun =
     fn(|ast::Name, SyntaxExtension|);
 
-pub trait AnyMacro {
-    fn make_expr(&self) -> @ast::Expr;
-    fn make_items(&self) -> SmallVector<@ast::Item>;
-    fn make_stmt(&self) -> @ast::Stmt;
+/// 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`).
+pub trait MacResult {
+    /// Define a new macro.
+    fn make_def(&self) -> Option<MacroDef> {
+        None
+    }
+    /// Create an expression.
+    fn make_expr(&self) -> Option<@ast::Expr> {
+        None
+    }
+    /// Create zero or more items.
+    fn make_items(&self) -> Option<SmallVector<@ast::Item>> {
+        None
+    }
+
+    /// Create a statement.
+    ///
+    /// By default this attempts to create an expression statement,
+    /// returning None if that fails.
+    fn make_stmt(&self) -> Option<@ast::Stmt> {
+        self.make_expr()
+            .map(|e| @codemap::respan(e.span, ast::StmtExpr(e, ast::DUMMY_NODE_ID)))
+    }
 }
 
+/// A convenience type for macros that return a single expression.
+pub struct MacExpr {
+    e: @ast::Expr
+}
+impl MacExpr {
+    pub fn new(e: @ast::Expr) -> ~MacResult {
+        ~MacExpr { e: e } as ~MacResult
+    }
+}
+impl MacResult for MacExpr {
+    fn make_expr(&self) -> Option<@ast::Expr> {
+        Some(self.e)
+    }
+}
+/// A convenience type for macros that return a single item.
+pub struct MacItem {
+    i: @ast::Item
+}
+impl MacItem {
+    pub fn new(i: @ast::Item) -> ~MacResult {
+        ~MacItem { i: i } as ~MacResult
+    }
+}
+impl MacResult for MacItem {
+    fn make_items(&self) -> Option<SmallVector<@ast::Item>> {
+        Some(SmallVector::one(self.i))
+    }
+    fn make_stmt(&self) -> Option<@ast::Stmt> {
+        Some(@codemap::respan(
+            self.i.span,
+            ast::StmtDecl(
+                @codemap::respan(self.i.span, ast::DeclItem(self.i)),
+                ast::DUMMY_NODE_ID)))
+    }
+}
 
-pub enum MacResult {
-    MRExpr(@ast::Expr),
-    MRItem(@ast::Item),
-    MRAny(~AnyMacro:),
-    MRDef(MacroDef),
+/// Fill-in macro expansion result, to allow compilation to continue
+/// after hitting errors.
+pub struct DummyResult {
+    expr_only: bool,
+    span: Span
 }
-impl MacResult {
-    /// Create an empty expression MacResult; useful for satisfying
-    /// type signatures after emitting a non-fatal error (which stop
-    /// compilation well before the validity (or otherwise)) of the
-    /// expression are checked.
-    pub fn raw_dummy_expr(sp: codemap::Span) -> @ast::Expr {
+
+impl DummyResult {
+    /// Create a default MacResult that can be anything.
+    ///
+    /// Use this as a return value after hitting any errors and
+    /// calling `span_err`.
+    pub fn any(sp: Span) -> ~MacResult {
+        ~DummyResult { expr_only: false, span: sp } as ~MacResult
+    }
+
+    /// Create a default MacResult that can only be an expression.
+    ///
+    /// Use this for macros that must expand to an expression, so even
+    /// if an error is encountered internally, the user will recieve
+    /// an error that they also used it in the wrong place.
+    pub fn expr(sp: Span) -> ~MacResult {
+        ~DummyResult { expr_only: true, span: sp } as ~MacResult
+    }
+
+    /// A plain dummy expression.
+    pub fn raw_expr(sp: Span) -> @ast::Expr {
         @ast::Expr {
             id: ast::DUMMY_NODE_ID,
             node: ast::ExprLit(@codemap::respan(sp, ast::LitNil)),
             span: sp,
         }
     }
-    pub fn dummy_expr(sp: codemap::Span) -> MacResult {
-        MRExpr(MacResult::raw_dummy_expr(sp))
-    }
-    pub fn dummy_any(sp: codemap::Span) -> MacResult {
-        MRAny(~DummyMacResult { sp: sp })
-    }
-}
-struct DummyMacResult {
-    sp: codemap::Span
 }
-impl AnyMacro for DummyMacResult {
-    fn make_expr(&self) -> @ast::Expr {
-        MacResult::raw_dummy_expr(self.sp)
+
+impl MacResult for DummyResult {
+    fn make_expr(&self) -> Option<@ast::Expr> {
+        Some(DummyResult::raw_expr(self.span))
     }
-    fn make_items(&self) -> SmallVector<@ast::Item> {
-        SmallVector::zero()
+    fn make_items(&self) -> Option<SmallVector<@ast::Item>> {
+        if self.expr_only {
+            None
+        } else {
+            Some(SmallVector::zero())
+        }
     }
-    fn make_stmt(&self) -> @ast::Stmt {
-        @codemap::respan(self.sp,
-                         ast::StmtExpr(MacResult::raw_dummy_expr(self.sp), ast::DUMMY_NODE_ID))
+    fn make_stmt(&self) -> Option<@ast::Stmt> {
+        Some(@codemap::respan(self.span,
+                              ast::StmtExpr(DummyResult::raw_expr(self.span),
+                                            ast::DUMMY_NODE_ID)))
     }
 }
 

src/libsyntax/ext/bytes.rs

@@ -18,10 +18,10 @@ use ext::build::AstBuilder;
 
 use std::char;
 
-pub fn expand_syntax_ext(cx: &mut ExtCtxt, sp: Span, tts: &[ast::TokenTree]) -> base::MacResult {
+pub fn expand_syntax_ext(cx: &mut ExtCtxt, sp: Span, tts: &[ast::TokenTree]) -> ~base::MacResult {
     // Gather all argument expressions
     let exprs = match get_exprs_from_tts(cx, sp, tts) {
-        None => return MacResult::dummy_expr(sp),
+        None => return DummyResult::expr(sp),
         Some(e) => e,
     };
     let mut bytes = Vec::new();
@@ -74,5 +74,5 @@ pub fn expand_syntax_ext(cx: &mut ExtCtxt, sp: Span, tts: &[ast::TokenTree]) ->
     }
 
     let e = cx.expr_vec_slice(sp, bytes);
-    MRExpr(e)
+    MacExpr::new(e)
 }

src/libsyntax/ext/cfg.rs

@@ -26,7 +26,7 @@ use parse::token::InternedString;
 use parse::token;
 use parse;
 
-pub fn expand_cfg(cx: &mut ExtCtxt, sp: Span, tts: &[ast::TokenTree]) -> base::MacResult {
+pub fn expand_cfg(cx: &mut ExtCtxt, sp: Span, tts: &[ast::TokenTree]) -> ~base::MacResult {
     let mut p = parse::new_parser_from_tts(cx.parse_sess(),
                                            cx.cfg(),
                                            tts.iter()
@@ -47,5 +47,5 @@ pub fn expand_cfg(cx: &mut ExtCtxt, sp: Span, tts: &[ast::TokenTree]) -> base::M
     let matches_cfg = attr::test_cfg(cx.cfg().as_slice(),
                                      in_cfg.iter().map(|&x| x));
     let e = cx.expr_bool(sp, matches_cfg);
-    MRExpr(e)
+    MacExpr::new(e)
 }

src/libsyntax/ext/concat.rs

@@ -19,10 +19,10 @@ use std::strbuf::StrBuf;
 
 pub fn expand_syntax_ext(cx: &mut base::ExtCtxt,
                          sp: codemap::Span,
-                         tts: &[ast::TokenTree]) -> base::MacResult {
+                         tts: &[ast::TokenTree]) -> ~base::MacResult {
     let es = match base::get_exprs_from_tts(cx, sp, tts) {
         Some(e) => e,
-        None => return base::MacResult::dummy_expr(sp)
+        None => return base::DummyResult::expr(sp)
     };
     let mut accumulator = StrBuf::new();
     for e in es.move_iter() {
@@ -57,7 +57,7 @@ pub fn expand_syntax_ext(cx: &mut base::ExtCtxt,
             }
         }
     }
-    base::MRExpr(cx.expr_str(
+    base::MacExpr::new(cx.expr_str(
             sp,
             token::intern_and_get_ident(accumulator.into_owned())))
 }