new option to note where purity is req'd

src/rustc/driver/session.rs

@@ -37,6 +37,7 @@ enum crate_type { bin_crate, lib_crate, unknown_crate, }
 const no_rt: uint = 256u;
 const coherence: uint = 512u;
 const borrowck_stats: uint = 1024u;
+const borrowck_note_pure: uint = 2048;
 
 fn debugging_opts_map() -> ~[(str, str, uint)] {
     ~[("ppregions", "prettyprint regions with \
@@ -51,7 +52,9 @@ fn debugging_opts_map() -> ~[(str, str, uint)] {
      ("trace", "emit trace logs", trace),
      ("no-rt", "do not link to the runtime", no_rt),
      ("coherence", "perform coherence checking", coherence),
-     ("borrowck-stats", "gather borrowck statistics",  borrowck_stats)
+     ("borrowck-stats", "gather borrowck statistics",  borrowck_stats),
+     ("borrowck-note-pure", "note where purity is req'd",
+      borrowck_note_pure)
     ]
 }
 
@@ -168,6 +171,7 @@ fn no_verify() -> bool { self.debugging_opt(no_verify) }
     fn trace() -> bool { self.debugging_opt(trace) }
     fn coherence() -> bool { self.debugging_opt(coherence) }
     fn borrowck_stats() -> bool { self.debugging_opt(borrowck_stats) }
+    fn borrowck_note_pure() -> bool { self.debugging_opt(borrowck_note_pure) }
 }
 
 /// Some reasonable defaults

src/rustc/middle/borrowck/gather_loans.rs

@@ -246,6 +246,12 @@ fn guarantee_valid(cmt: cmt,
                   some(scope_id) {
                     self.req_maps.pure_map.insert(scope_id, e);
                     self.bccx.req_pure_paths += 1;
+
+                    if self.tcx().sess.borrowck_note_pure() {
+                        self.bccx.span_note(
+                            cmt.span,
+                            #fmt["purity required"]);
+                    }
                   }
                   none {
                     // otherwise, fine, I give up.