Add comments to span debugger

src/librustc/driver/session.rs

@@ -96,7 +96,7 @@ pub fn debugging_opts_map() -> ~[(&'static str, &'static str, u64)] {
      ("no-landing-pads", "omit landing pads for unwinding",
       NO_LANDING_PADS),
      ("debug-llvm", "enable debug output from LLVM", DEBUG_LLVM),
-     ("show-span", "show spans", SHOW_SPAN),
+     ("show-span", "show spans for compiler debugging", SHOW_SPAN),
      ("count-type-sizes", "count the sizes of aggregate types",
       COUNT_TYPE_SIZES),
      ("meta-stats", "gather metadata statistics", META_STATS),

src/librustc/front/show_span.rs

@@ -8,6 +8,11 @@
 // option. This file may not be copied, modified, or distributed
 // except according to those terms.
 
+//! Span debugger
+//!
+//! This module shows spans for all expressions in the crate
+//! to help with compiler debugging.
+
 use syntax::ast;
 use syntax::visit;
 use syntax::visit::Visitor;