auto merge of #3747 : mrobinson/servo/validate-display-list, r=pcwalton

When this option is enabled, the layout task will print an error when
display list items draw outside their owning Flow's position rect. This
will make it easier to detect layout errors before they break rendering.

This is a command-line option for the moment, because we violate this
rule quite a bit still. Once all bugs causing this are fixed, we can be
more aggressive about enabling the option.

1 files changed, 30 insertions, 0 deletions

diff --git a/components/layout/flow.rs b/components/layout/flow.rs
index b8f9a2b55e0..269aac1b81d 100644
--- a/components/layout/flow.rs
+++ b/components/layout/flow.rs
@@ -54,6 +54,7 @@ use servo_msg::compositor_msg::LayerId;
 use servo_util::geometry::Au;
 use servo_util::logical_geometry::WritingMode;
 use servo_util::logical_geometry::{LogicalRect, LogicalSize};
+use servo_util::opts;
 use std::mem;
 use std::num::Zero;
 use std::fmt;
@@ -405,6 +406,9 @@ pub trait ImmutableFlowUtils {
 
     /// Dumps the flow tree for debugging, with a prefix to indicate that we're at the given level.
     fn dump_with_level(self, level: uint);
+
+    /// Print an error when this Flow's display list items are not within its boundaries.
+    fn validate_display_list_geometry(self);
 }
 
 pub trait MutableFlowUtils {
@@ -1024,6 +1028,28 @@ impl<'a> ImmutableFlowUtils for &'a Flow + 'a {
             kid.dump_with_level(level + 1)
         }
     }
+
+    fn validate_display_list_geometry(self) {
+        let position_with_overflow = base(self).position.union(&base(self).overflow);
+        let bounds = Rect(base(self).abs_position,
+                          Size2D(position_with_overflow.size.inline,
+                                 position_with_overflow.size.block));
+
+        for item in base(self).display_list.iter() {
+            let paint_bounds = match item.base().bounds.intersection(&item.base().clip_rect) {
+                None => continue,
+                Some(rect) => rect,
+            };
+
+            if paint_bounds.is_empty() {
+                continue;
+            }
+
+            if bounds.union(&paint_bounds) != bounds {
+                error!("DisplayList item {} outside of Flow overflow ({})", item, paint_bounds);
+            }
+        }
+    }
 }
 
 impl<'a> MutableFlowUtils for &'a mut Flow + 'a {
@@ -1116,6 +1142,10 @@ impl<'a> MutableFlowUtils for &'a mut Flow + 'a {
                 // Nothing to do here, as column groups don't render.
             }
         }
+
+        if opts::get().validate_display_list_geometry {
+            self.validate_display_list_geometry();
+        }
     }
 
     /// Collect and update static y-offsets bubbled up by kids.