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Introduce an unsafe HasBaseFlow trait for base()/base_mut() casts.

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This commit is contained in:
Simon Sapin 2017-10-14 00:03:57 +02:00
parent 4c5d6fd834
commit b505c9e948
13 changed files with 63 additions and 3 deletions
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14
components/layout/flow.rs
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@ -65,11 +65,19 @@ use table_rowgroup::TableRowGroupFlow;
use table_wrapper::TableWrapperFlow;
use webrender_api::ClipAndScrollInfo;
/// This marker trait indicates that a type is a struct with `#[repr(C)]` whose first field
/// is of type `BaseFlow` or some type that also implements this trait.
///
/// In other words, the memory representation of `BaseFlow` must be a prefix
/// of the memory representation of types implementing `HasBaseFlow`.
#[allow(unsafe_code)]
pub unsafe trait HasBaseFlow {}
/// Virtual methods that make up a float context.
///
/// Note that virtual methods have a cost; we should not overuse them in Servo. Consider adding
/// methods to `ImmutableFlowUtils` or `MutableFlowUtils` before adding more methods here.
pub trait Flow: fmt::Debug + Sync + Send + 'static {
pub trait Flow: HasBaseFlow + fmt::Debug + Sync + Send + 'static {
// RTTI
//
// TODO(pcwalton): Use Rust's RTTI, once that works.
@ -451,7 +459,7 @@ pub trait Flow: fmt::Debug + Sync + Send + 'static {
#[inline(always)]
#[allow(unsafe_code)]
pub fn base<T: ?Sized + Flow>(this: &T) -> &BaseFlow {
pub fn base<T: ?Sized + HasBaseFlow>(this: &T) -> &BaseFlow {
let ptr: *const T = this;
let ptr = ptr as *const BaseFlow;
unsafe { &*ptr }
@ -464,7 +472,7 @@ pub fn child_iter<'a>(flow: &'a Flow) -> FlowListIterator {
#[inline(always)]
#[allow(unsafe_code)]
pub fn mut_base<T: ?Sized + Flow>(this: &mut T) -> &mut BaseFlow {
pub fn mut_base<T: ?Sized + HasBaseFlow>(this: &mut T) -> &mut BaseFlow {
let ptr: *mut T = this;
let ptr = ptr as *mut BaseFlow;
unsafe { &mut *ptr }