syntax: Add a macro, format_args_method!()

Currently, the format_args!() macro takes as its first argument an expression
which is the callee of an ExprCall. This means that if format_args!() is used
with calling a method a closure must be used. Consider this code, however:

    format_args!(|args| { foo.writer.write_fmt(args) }, "{}", foo.field)

The closure borrows the entire `foo` structure, disallowing the later borrow of
`foo.field`. To preserve the semantics of the `write!` macro, it is also
impossible to borrow specifically the `writer` field of the `foo` structure
because it must be borrowed mutably, but the `foo` structure is not guaranteed
to be mutable itself.

This new macro is invoked like:

    format_args_method!(foo.writer, write_fmt, "{}", foo.field)

This macro will generate an ExprMethodCall which allows the borrow checker to
understand that `writer` and `field` should be borrowed separately.

This macro is not strictly necessary, with DST or possibly UFCS other
workarounds could be used. For now, though, it looks like this is required to
implement the `write!` macro.

Author: alexcrichton

src/libsyntax/ext/base.rs

@@ -281,7 +281,10 @@ pub fn syntax_expander_table() -> SyntaxEnv {
                                 ext::fmt::expand_syntax_ext));
     syntax_expanders.insert(intern("format_args"),
                             builtin_normal_expander(
-                                ext::format::expand_args));
+                                ext::format::expand_format_args));
+    syntax_expanders.insert(intern("format_args_method"),
+                            builtin_normal_expander(
+                                ext::format::expand_format_args_method));
     syntax_expanders.insert(intern("env"),
                             builtin_normal_expander(
                                     ext::env::expand_env));

src/libsyntax/ext/deriving/show.rs

@@ -120,23 +120,18 @@ fn show_substructure(cx: &mut ExtCtxt, span: Span,
     // AST construction!
     // we're basically calling
     //
-    // format_arg!(|__args| ::std::fmt::write(fmt.buf, __args), "<format_string>", exprs...)
+    // format_arg_method!(fmt, write_fmt, "<format_string>", exprs...)
     //
     // but doing it directly via ext::format.
     let formatter = substr.nonself_args[0];
-    let buf = cx.expr_field_access(span, formatter, cx.ident_of("buf"));
-
-    let std_write = vec!(cx.ident_of("std"), cx.ident_of("fmt"), cx.ident_of("write"));
-    let args = cx.ident_of("__args");
-    let write_call = cx.expr_call_global(span, std_write, vec!(buf, cx.expr_ident(span, args)));
-    let format_closure = cx.lambda_expr(span, vec!(args), write_call);
 
+    let meth = cx.ident_of("write_fmt");
     let s = token::intern_and_get_ident(format_string.as_slice());
     let format_string = cx.expr_str(span, s);
 
     // phew, not our responsibility any more!
     format::expand_preparsed_format_args(cx, span,
-                                         format_closure,
+                                         format::MethodCall(formatter, meth),
                                          format_string, exprs, Vec::new(),
                                          HashMap::new())
 }

src/libsyntax/ext/format.rs

@@ -59,6 +59,11 @@ struct Context<'a, 'b> {
     next_arg: uint,
 }
 
+pub enum Invocation {
+    Call(@ast::Expr),
+    MethodCall(@ast::Expr, ast::Ident),
+}
+
 /// Parses the arguments from the given list of tokens, returning None
 /// if there's a parse error so we can continue parsing other format!
 /// expressions.
@@ -67,8 +72,9 @@ struct Context<'a, 'b> {
 ///
 ///     Some((fmtstr, unnamed arguments, ordering of named arguments,
 ///           named arguments))
-fn parse_args(ecx: &mut ExtCtxt, sp: Span, tts: &[ast::TokenTree])
-    -> (@ast::Expr, Option<(@ast::Expr, Vec<@ast::Expr>, Vec<StrBuf>,
+fn parse_args(ecx: &mut ExtCtxt, sp: Span, allow_method: bool,
+              tts: &[ast::TokenTree])
+    -> (Invocation, Option<(@ast::Expr, Vec<@ast::Expr>, Vec<StrBuf>,
                             HashMap<StrBuf, @ast::Expr>)>) {
     let mut args = Vec::new();
     let mut names = HashMap::<StrBuf, @ast::Expr>::new();
@@ -80,22 +86,31 @@ fn parse_args(ecx: &mut ExtCtxt, sp: Span, tts: &[ast::TokenTree])
                                                 .map(|x| (*x).clone())
                                                 .collect());
     // Parse the leading function expression (maybe a block, maybe a path)
-    let extra = p.parse_expr();
+    let invocation = if allow_method {
+        let e = p.parse_expr();
+        if !p.eat(&token::COMMA) {
+            ecx.span_err(sp, "expected token: `,`");
+            return (Call(e), None);
+        }
+        MethodCall(e, p.parse_ident())
+    } else {
+        Call(p.parse_expr())
+    };
     if !p.eat(&token::COMMA) {
         ecx.span_err(sp, "expected token: `,`");
-        return (extra, None);
+        return (invocation, None);
     }
 
     if p.token == token::EOF {
         ecx.span_err(sp, "requires at least a format string argument");
-        return (extra, None);
+        return (invocation, None);
     }
     let fmtstr = p.parse_expr();
     let mut named = false;
     while p.token != token::EOF {
         if !p.eat(&token::COMMA) {
             ecx.span_err(sp, "expected token: `,`");
-            return (extra, None);
+            return (invocation, None);
         }
         if p.token == token::EOF { break } // accept trailing commas
         if named || (token::is_ident(&p.token) &&
@@ -110,13 +125,13 @@ fn parse_args(ecx: &mut ExtCtxt, sp: Span, tts: &[ast::TokenTree])
                     ecx.span_err(p.span,
                                  "expected ident, positional arguments \
                                  cannot follow named arguments");
-                    return (extra, None);
+                    return (invocation, None);
                 }
                 _ => {
                     ecx.span_err(p.span,
                                  format!("expected ident for named argument, but found `{}`",
                                          p.this_token_to_str()));
-                    return (extra, None);
+                    return (invocation, None);
                 }
             };
             let interned_name = token::get_ident(ident);
@@ -137,7 +152,7 @@ fn parse_args(ecx: &mut ExtCtxt, sp: Span, tts: &[ast::TokenTree])
             args.push(p.parse_expr());
         }
     }
-    return (extra, Some((fmtstr, args, order, names)));
+    return (invocation, Some((fmtstr, args, order, names)));
 }
 
 impl<'a, 'b> Context<'a, 'b> {
@@ -595,7 +610,7 @@ impl<'a, 'b> Context<'a, 'b> {
 
     /// Actually builds the expression which the iformat! block will be expanded
     /// to
-    fn to_expr(&self, extra: @ast::Expr) -> @ast::Expr {
+    fn to_expr(&self, invocation: Invocation) -> @ast::Expr {
         let mut lets = Vec::new();
         let mut locals = Vec::new();
         let mut names = Vec::from_fn(self.name_positions.len(), |_| None);
@@ -699,8 +714,16 @@ impl<'a, 'b> Context<'a, 'b> {
         let resname = self.ecx.ident_of("__args");
         lets.push(self.ecx.stmt_let(self.fmtsp, false, resname, result));
         let res = self.ecx.expr_ident(self.fmtsp, resname);
-        let result = self.ecx.expr_call(extra.span, extra, vec!(
-                            self.ecx.expr_addr_of(extra.span, res)));
+        let result = match invocation {
+            Call(e) => {
+                self.ecx.expr_call(e.span, e,
+                                   vec!(self.ecx.expr_addr_of(e.span, res)))
+            }
+            MethodCall(e, m) => {
+                self.ecx.expr_method_call(e.span, e, m,
+                                          vec!(self.ecx.expr_addr_of(e.span, res)))
+            }
+        };
         let body = self.ecx.expr_block(self.ecx.block(self.fmtsp, lets,
                                                       Some(result)));
 
@@ -794,13 +817,25 @@ impl<'a, 'b> Context<'a, 'b> {
     }
 }
 
-pub fn expand_args(ecx: &mut ExtCtxt, sp: Span,
-                   tts: &[ast::TokenTree]) -> Box<base::MacResult> {
+pub fn expand_format_args(ecx: &mut ExtCtxt, sp: Span,
+                          tts: &[ast::TokenTree]) -> Box<base::MacResult> {
+
+    match parse_args(ecx, sp, false, tts) {
+        (invocation, Some((efmt, args, order, names))) => {
+            MacExpr::new(expand_preparsed_format_args(ecx, sp, invocation, efmt,
+                                                      args, order, names))
+        }
+        (_, None) => MacExpr::new(ecx.expr_uint(sp, 2))
+    }
+}
+
+pub fn expand_format_args_method(ecx: &mut ExtCtxt, sp: Span,
+                                 tts: &[ast::TokenTree]) -> Box<base::MacResult> {
 
-    match parse_args(ecx, sp, tts) {
-        (extra, Some((efmt, args, order, names))) => {
-            MacExpr::new(expand_preparsed_format_args(ecx, sp, extra, efmt, args,
-                                                order, names))
+    match parse_args(ecx, sp, true, tts) {
+        (invocation, Some((efmt, args, order, names))) => {
+            MacExpr::new(expand_preparsed_format_args(ecx, sp, invocation, efmt,
+                                                      args, order, names))
         }
         (_, None) => MacExpr::new(ecx.expr_uint(sp, 2))
     }
@@ -810,7 +845,7 @@ pub fn expand_args(ecx: &mut ExtCtxt, sp: Span,
 /// name=names...)` and construct the appropriate formatting
 /// expression.
 pub fn expand_preparsed_format_args(ecx: &mut ExtCtxt, sp: Span,
-                                    extra: @ast::Expr,
+                                    invocation: Invocation,
                                     efmt: @ast::Expr, args: Vec<@ast::Expr>,
                                     name_ordering: Vec<StrBuf>,
                                     names: HashMap<StrBuf, @ast::Expr>) -> @ast::Expr {
@@ -869,5 +904,5 @@ pub fn expand_preparsed_format_args(ecx: &mut ExtCtxt, sp: Span,
         }
     }
 
-    cx.to_expr(extra)
+    cx.to_expr(invocation)
 }