diff --git a/src/librustc/driver/session.rs b/src/librustc/driver/session.rs
index 4e386fa343d08..6fb7e74947504 100644
--- a/src/librustc/driver/session.rs
+++ b/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),
diff --git a/src/librustc/front/show_span.rs b/src/librustc/front/show_span.rs
index a5bc8e0243bdc..2c076aac095ab 100644
--- a/src/librustc/front/show_span.rs
+++ b/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;