Bug 1568395 - Add more useful error messages to webrender_api. r=kamidphish

Differential Revision: https://phabricator.services.mozilla.com/D39236

--HG--
extra : moz-landing-system : lando

gfx/wr/peek-poke/peek-poke-derive/src/lib.rs

@@ -85,7 +85,8 @@ fn derive_peek_from_for_enum(s: &mut Structure) -> TokenStream {
     assert!(!is_struct(s));
     s.bind_with(|_| BindStyle::Move);
 
-    let discriminant_size_type = get_discriminant_size_type(s.variants().len());
+    let num_variants = s.variants().len();
+    let discriminant_size_type = get_discriminant_size_type(num_variants);
     let body = s
         .variants()
         .iter()
@@ -119,13 +120,19 @@ fn derive_peek_from_for_enum(s: &mut Structure) -> TokenStream {
             }
         });
 
+    let type_name = s.ast().ident.to_string();
+    let max_tag_value = num_variants - 1;
+
     quote! {
         #[inline(always)]
         unsafe fn peek_from(bytes: *const u8, output: *mut Self) -> *const u8 {
             let (variant, bytes) = peek_poke::peek_from_default::<#discriminant_size_type>(bytes);
             match variant {
                 #body
-                _ => unreachable!()
+                out_of_range_tag => {
+                    panic!("WRDL: memory corruption detected while parsing {} - enum tag should be <= {}, but was {}",
+                        #type_name, #max_tag_value, out_of_range_tag);
+                }
             }
         }
     }

gfx/wr/peek-poke/src/lib.rs

@@ -53,19 +53,19 @@ pub unsafe fn peek_from_default<T: Default + Peek>(bytes: *const u8) -> (T, *con
 pub fn peek_from_slice<'a, T: Peek>(src: &'a [u8], dst: &mut T) -> &'a [u8] {
     unsafe {
         // If src.len() == T::max_size() then src is at the start of the red-zone.
-        assert!(T::max_size() < src.len());
+        assert!(T::max_size() < src.len(), "WRDL: unexpected end of display list");
         let end_ptr = T::peek_from(src.as_ptr(), dst);
         let len = end_ptr as usize - src.as_ptr() as usize;
         // Did someone break the T::peek_from() can't read more than T::max_size()
         // bytes contract?
-        assert!(len <= src.len());
+        assert!(len <= src.len(), "WRDL: Peek::max_size was wrong");
         slice::from_raw_parts(end_ptr, src.len() - len)
     }
 }
 
 /// Poke helper to insert a serialized version of `src` at the beginning for `dst`.
 pub fn poke_inplace_slice<T: Poke>(src: &T, dst: &mut [u8]) {
-    assert!(T::max_size() <= dst.len());
+    assert!(T::max_size() <= dst.len(),  "WRDL: buffer too small to write into");
     unsafe {
         src.poke_into(dst.as_mut_ptr());
     }

gfx/wr/webrender_api/src/display_list.rs

@@ -430,7 +430,6 @@ impl<'a> BuiltDisplayListIter<'a> {
                 di::DisplayItem::PopStackingContext => depth -= 1,
                 _ => {}
             }
-            debug_assert!(depth >= 0);
         }
     }
 
@@ -590,12 +589,14 @@ impl Serialize for BuiltDisplayList {
                     item.iter.cur_filters.iter().collect()
                 ),
                 Real::SetFilterData => {
-                    debug_assert!(!item.iter.cur_filter_data.is_empty());
+                    debug_assert!(!item.iter.cur_filter_data.is_empty(),
+                        "next_raw should have populated cur_filter_data");
                     let temp_filter_data = &item.iter.cur_filter_data[item.iter.cur_filter_data.len()-1];
 
                     let func_types: Vec<di::ComponentTransferFuncType> =
                         temp_filter_data.func_types.iter().collect();
-                    debug_assert!(func_types.len() == 4);
+                    debug_assert!(func_types.len() == 4,
+                        "someone changed the number of filter funcs without updating this code");
                     Debug::SetFilterData(di::FilterData {
                         func_r_type: func_types[0],
                         r_values: temp_filter_data.r_values.iter().collect(),
@@ -910,14 +911,15 @@ impl DisplayListBuilder {
         poke_into_vec(&0usize, data);
         poke_into_vec(&len, data);
         let count = poke_extend_vec(iter, data);
-        debug_assert_eq!(len, count);
+        debug_assert_eq!(len, count, "iterator.len() returned two different values");
 
         // Add red zone
         ensure_red_zone::<I::Item>(data);
 
         // Now write the actual byte_size
         let final_offset = data.len();
-        debug_assert!(final_offset >= (byte_size_offset + mem::size_of::<usize>()));
+        debug_assert!(final_offset >= (byte_size_offset + mem::size_of::<usize>()),
+            "space was never allocated for this array's byte_size");
         let byte_size = final_offset - byte_size_offset - mem::size_of::<usize>();
         poke_inplace_slice(&byte_size, &mut data[byte_size_offset..]);
     }