Auto merge of #24003 - rprichard:span-fixes, r=huonw

 * In `noop_fold_expr`, call `new_span` in these cases:
    - `ExprMethodCall`'s identifier
    - `ExprField`'s identifier
    - `ExprTupField`'s integer

   Calling `new_span` for `ExprMethodCall`'s identifier is necessary to print
   an acceptable diagnostic for `write!(&2, "")`. We see this error:
   ```
   <std macros>:2:20: 2:66 error: type `&mut _` does not implement any method in scope named `write_fmt`
   <std macros>:2 ( & mut * $ dst ) . write_fmt ( format_args ! ( $ ( $ arg ) * ) ) )
                                     ^~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
   ```
   With this change, we also see a macro expansion backtrace leading to
   the `write!(&2, "")` call site.

 * After fully expanding a macro, we replace the expansion expression's
   span with the original span. Call `fld.new_span` to add a backtrace to
   this span. (Note that I'm call `new_span` after `bt.pop()`, so the macro
   just expanded isn't on the backtrace.)

   The motivating example for this change is `println!("{}")`. The format
   string literal is `concat!($fmt, "arg")` and is inside the libstd macro.
   We need to see the backtrace to find the `println!` call site.

 * Add a backtrace to the `format_args!` format expression span.

r?  alexcrichton

Addresses #23459

1 files changed, 0 insertions, 2 deletions

diff --git a/src/libsyntax/ext/log_syntax.rs b/src/libsyntax/ext/log_syntax.rs
index 8173dd93f74..9869108952c 100644
--- a/src/libsyntax/ext/log_syntax.rs
+++ b/src/libsyntax/ext/log_syntax.rs
@@ -26,8 +26,6 @@ pub fn expand_syntax_ext<'cx>(cx: &'cx mut base::ExtCtxt,
         return base::DummyResult::any(sp);
     }
 
-    cx.print_backtrace();
-
     println!("{}", print::pprust::tts_to_string(tts));
 
     // any so that `log_syntax` can be invoked as an expression and item.