diff --git a/src/etc/monodebug.pl b/src/etc/monodebug.pl
new file mode 100755
index 000000000000..324c576a4bda
--- /dev/null
+++ b/src/etc/monodebug.pl
@@ -0,0 +1,80 @@
+#!/usr/bin/perl
+
+#
+# This is a tool that helps with debugging incorrect monomorphic instance collapse.
+#
+# To use:
+#    $ RUST_LOG=rustc::middle::trans::monomorphize rustc ARGS 2>&1 >log.txt
+#    $ ./monodebug.pl log.txt
+#
+# This will show all generics that got collapsed. You can inspect this list to find the instances
+# that were mistakenly combined into one. Fixes will (most likely) be applied to type_use.rs.
+#
+# Questions about this tool go to pcwalton.
+#
+
+use strict;
+use warnings;
+use Data::Dumper qw(Dumper);
+use Text::Balanced qw(extract_bracketed);
+
+my %funcs;
+while (<>) {
+    chomp;
+    /^rust: ~"monomorphic_fn\((.*)"$/ or next;
+    my $text = $1;
+    $text =~ /fn_id=(\{ crate: \d+, node: \d+ \} \([^)]+\)), real_substs=(.*?), substs=(.*?), hash_id = \@\{ (.*) \}$/ or next;
+    my ($fn_id, $real_substs, $substs, $hash_id) = ($1, $2, $3, $4);
+
+    #print "$hash_id\n";
+    $hash_id =~ /^def: { crate: \d+, node: \d+ }, params: ~\[ (.*) \], impl_did_opt: (?:None|Some\({ crate: \d+, node: \d+ }\))$/ or next;
+    my $params = $1;
+
+    my @real_substs;
+    @real_substs = $real_substs =~ /\\"(.*?)\\"/g;
+
+    my @mono_params;
+    while (1) {
+        $params =~ s/^, //;
+        if ($params =~ s/^mono_precise//) {
+            extract_bracketed($params, '()');
+            push @mono_params, 'precise';
+            next;
+        }
+        if ($params =~ s/^mono_repr//) {
+            my $sub = extract_bracketed($params, '()');
+            push @mono_params, "repr($sub)";
+            next;
+        }
+        if ($params =~ s/^mono_any//) {
+            push @mono_params, "any";
+            next;
+        }
+        last;
+    }
+
+    my @key_params;
+    for (my $i = 0; $i < @mono_params; ++$i) {
+        if ($mono_params[$i] eq 'precise') {
+            push @key_params, 'precise(' . $real_substs[$i] . ')';
+        } else {
+            push @key_params, $mono_params[$i];
+        }
+    }
+
+    my $key = "$fn_id with " . (join ', ', @key_params);
+    $funcs{$key}{$real_substs} = 1;
+}
+
+while (my ($key, $substs) = each %funcs) {
+    my @params = keys %$substs;
+    next if @params == 1;
+
+    print "$key\n";
+    print(('-' x (length $key)), $/);
+    for my $param (@params) {
+        print "$param\n";
+    }
+    print "\n";
+}
+
diff --git a/src/librustc/middle/trans/type_use.rs b/src/librustc/middle/trans/type_use.rs
index dedf667a4d93..33200a2dd828 100644
--- a/src/librustc/middle/trans/type_use.rs
+++ b/src/librustc/middle/trans/type_use.rs
@@ -192,11 +192,11 @@ fn node_type_needs(cx: ctx, use_: uint, id: node_id) {
     type_needs(cx, use_, ty::node_id_to_type(cx.ccx.tcx, id));
 }
 
-fn mark_for_method_call(cx: ctx, e_id: node_id) {
+fn mark_for_method_call(cx: ctx, e_id: node_id, callee_id: node_id) {
     do option::iter(&cx.ccx.maps.method_map.find(e_id)) |mth| {
         match mth.origin {
           typeck::method_static(did) => {
-            do cx.ccx.tcx.node_type_substs.find(e_id).iter |ts| {
+            do cx.ccx.tcx.node_type_substs.find(callee_id).iter |ts| {
                 let type_uses = type_uses_for(cx.ccx, did, ts.len());
                 for vec::each2(type_uses, *ts) |uses, subst| {
                     type_needs(cx, *uses, *subst)
@@ -269,7 +269,7 @@ fn mark_for_expr(cx: ctx, e: @expr) {
         // the chosen field.
         let base_ty = ty::node_id_to_type(cx.ccx.tcx, base.id);
         type_needs(cx, use_repr, ty::type_autoderef(cx.ccx.tcx, base_ty));
-        mark_for_method_call(cx, e.id);
+        mark_for_method_call(cx, e.id, e.callee_id);
       }
       expr_log(_, _, val) => {
         node_type_needs(cx, use_tydesc, val.id);
@@ -299,7 +299,7 @@ fn mark_for_expr(cx: ctx, e: @expr) {
               _ => ()
           }
         }
-        mark_for_method_call(cx, e.id);
+        mark_for_method_call(cx, e.id, e.callee_id);
       }
       expr_paren(e) => mark_for_expr(cx, e),
       expr_match(*) | expr_block(_) | expr_if(*) |