Avoid needless reexpansions.

src/libsyntax/ext/base.rs

@@ -13,7 +13,7 @@ pub use self::SyntaxExtension::*;
 use ast;
 use ast::{Name, PatKind};
 use attr::HasAttrs;
-use codemap::{self, CodeMap, ExpnInfo};
+use codemap::{self, CodeMap, ExpnInfo, Spanned, respan};
 use syntax_pos::{Span, ExpnId, NO_EXPANSION};
 use errors::DiagnosticBuilder;
 use ext;
@@ -805,8 +805,8 @@ impl<'a> ExtCtxt<'a> {
 /// Extract a string literal from the macro expanded version of `expr`,
 /// emitting `err_msg` if `expr` is not a string literal. This does not stop
 /// compilation on error, merely emits a non-fatal error and returns None.
-pub fn expr_to_string(cx: &mut ExtCtxt, expr: P<ast::Expr>, err_msg: &str)
-                      -> Option<(InternedString, ast::StrStyle)> {
+pub fn expr_to_spanned_string(cx: &mut ExtCtxt, expr: P<ast::Expr>, err_msg: &str)
+                              -> Option<Spanned<(InternedString, ast::StrStyle)>> {
     // Update `expr.span`'s expn_id now in case expr is an `include!` macro invocation.
     let expr = expr.map(|mut expr| {
         expr.span.expn_id = cx.backtrace;
@@ -817,7 +817,7 @@ pub fn expr_to_string(cx: &mut ExtCtxt, expr: P<ast::Expr>, err_msg: &str)
     let expr = cx.expander().fold_expr(expr);
     match expr.node {
         ast::ExprKind::Lit(ref l) => match l.node {
-            ast::LitKind::Str(ref s, style) => return Some(((*s).clone(), style)),
+            ast::LitKind::Str(ref s, style) => return Some(respan(expr.span, (s.clone(), style))),
             _ => cx.span_err(l.span, err_msg)
         },
         _ => cx.span_err(expr.span, err_msg)
@@ -825,6 +825,11 @@ pub fn expr_to_string(cx: &mut ExtCtxt, expr: P<ast::Expr>, err_msg: &str)
     None
 }
 
+pub fn expr_to_string(cx: &mut ExtCtxt, expr: P<ast::Expr>, err_msg: &str)
+                      -> Option<(InternedString, ast::StrStyle)> {
+    expr_to_spanned_string(cx, expr, err_msg).map(|s| s.node)
+}
+
 /// Non-fatally assert that `tts` is empty. Note that this function
 /// returns even when `tts` is non-empty, macros that *need* to stop
 /// compilation should call
@@ -851,7 +856,7 @@ pub fn get_single_str_from_tts(cx: &mut ExtCtxt,
         cx.span_err(sp, &format!("{} takes 1 argument", name));
         return None
     }
-    let ret = cx.expander().fold_expr(panictry!(p.parse_expr()));
+    let ret = panictry!(p.parse_expr());
     if p.token != token::Eof {
         cx.span_err(sp, &format!("{} takes 1 argument", name));
     }

src/libsyntax_ext/format.rs

@@ -17,7 +17,6 @@ use syntax::ast;
 use syntax::ext::base::*;
 use syntax::ext::base;
 use syntax::ext::build::AstBuilder;
-use syntax::fold::Folder;
 use syntax::parse::token::{self, keywords};
 use syntax::ptr::P;
 use syntax_pos::{Span, DUMMY_SP};
@@ -702,10 +701,12 @@ pub fn expand_preparsed_format_args(ecx: &mut ExtCtxt,
     let arg_types: Vec<_> = (0..args.len()).map(|_| Vec::new()).collect();
     let arg_unique_types: Vec<_> = (0..args.len()).map(|_| Vec::new()).collect();
     let macsp = ecx.call_site();
-    // Expand the format literal so that efmt.span will have a backtrace. This
-    // is essential for locating a bug when the format literal is generated in
-    // a macro. (e.g. println!("{}"), which uses concat!($fmt, "\n")).
-    let efmt = ecx.expander().fold_expr(efmt);
+    let msg = "format argument must be a string literal.";
+    let fmt = match expr_to_spanned_string(ecx, efmt, msg) {
+        Some(fmt) => fmt,
+        None => return DummyResult::raw_expr(sp),
+    };
+
     let mut cx = Context {
         ecx: ecx,
         args: args,
@@ -723,14 +724,10 @@ pub fn expand_preparsed_format_args(ecx: &mut ExtCtxt,
         str_pieces: Vec::new(),
         all_pieces_simple: true,
         macsp: macsp,
-        fmtsp: efmt.span,
-    };
-    let fmt = match expr_to_string(cx.ecx, efmt, "format argument must be a string literal.") {
-        Some((fmt, _)) => fmt,
-        None => return DummyResult::raw_expr(sp),
+        fmtsp: fmt.span,
     };
 
-    let mut parser = parse::Parser::new(&fmt);
+    let mut parser = parse::Parser::new(&fmt.node.0);
     let mut pieces = vec![];
 
     loop {