Bug 1918673: Make parsing of position-try-fallbacks order-dependent. r=firefox-style-system-reviewers,emilio

Spec currently says canonical order, but this should not be the case (See
bug 1918670).

Differential Revision: https://phabricator.services.mozilla.com/D222296

servo/components/style/values/specified/position.rs

@@ -495,6 +495,7 @@ impl PositionAnchor {
     Clone,
     Copy,
     Debug,
+    Default,
     Eq,
     MallocSizeOf,
     Parse,
@@ -506,19 +507,71 @@ impl PositionAnchor {
     ToResolvedValue,
     ToShmem,
 )]
-#[css(bitflags(mixed = "flip-block,flip-inline,flip-start"))]
+#[repr(u8)]
+/// How to swap values for the automatically-generated position tactic.
+pub enum PositionTryFallbacksTryTacticKeyword {
+    /// Magic value for no change.
+    #[css(skip)]
+    #[default]
+    None,
+    /// Swap the values in the block axis.
+    FlipBlock,
+    /// Swap the values in the inline axis.
+    FlipInline,
+    /// Swap the values in the start properties.
+    FlipStart,
+}
+
+impl PositionTryFallbacksTryTacticKeyword {
+    fn is_none(&self) -> bool {
+        *self == Self::None
+    }
+}
+
+#[derive(
+    Clone,
+    Copy,
+    Debug,
+    Default,
+    Eq,
+    MallocSizeOf,
+    PartialEq,
+    Serialize,
+    SpecifiedValueInfo,
+    ToComputedValue,
+    ToCss,
+    ToResolvedValue,
+    ToShmem,
+)]
 #[repr(C)]
+/// Changes for the automatically-generated position option.
+/// Note that this is order-dependent - e.g. `flip-start flip-inline` != `flip-inline flip-start`.
+///
 /// https://drafts.csswg.org/css-anchor-position-1/#typedef-position-try-fallbacks-try-tactic
-/// <try-tactic>
-pub struct PositionTryFallbacksTryTactic(u8);
-bitflags! {
-    impl PositionTryFallbacksTryTactic: u8 {
-        /// `flip-block`
-        const FLIP_BLOCK = 1 << 0;
-        /// `flip-inline`
-        const FLIP_INLINE = 1 << 1;
-        /// `flip-start`
-        const FLIP_START = 1 << 2;
+pub struct PositionTryFallbacksTryTactic(
+    pub PositionTryFallbacksTryTacticKeyword,
+    pub PositionTryFallbacksTryTacticKeyword,
+    pub PositionTryFallbacksTryTacticKeyword,
+);
+
+impl Parse for PositionTryFallbacksTryTactic {
+    fn parse<'i, 't>(
+        _context: &ParserContext,
+        input: &mut Parser<'i, 't>,
+    ) -> Result<Self, ParseError<'i>> {
+        let first = input.try_parse(PositionTryFallbacksTryTacticKeyword::parse)?;
+        let second = input.try_parse(PositionTryFallbacksTryTacticKeyword::parse).unwrap_or_default();
+        let third = input.try_parse(PositionTryFallbacksTryTacticKeyword::parse).unwrap_or_default();
+        if first == second || first == third || (!second.is_none() && second == third) {
+            return Err(input.new_custom_error(StyleParseErrorKind::UnspecifiedError));
+        }
+        Ok(Self(first, second, third))
+    }
+}
+
+impl PositionTryFallbacksTryTactic {
+    fn is_empty(&self) -> bool {
+        self.0.is_none()
     }
 }
 
@@ -550,7 +603,7 @@ impl Parse for DashedIdentAndOrTryTactic {
     ) -> Result<Self, ParseError<'i>> {
         let mut result = Self {
             ident: DashedIdent::empty(),
-            try_tactic: PositionTryFallbacksTryTactic::empty(),
+            try_tactic: PositionTryFallbacksTryTactic::default(),
         };
 
         loop {

testing/web-platform/meta/css/css-anchor-position/parsing/position-try-fallbacks-computed.html.ini

@@ -1,3 +0,0 @@
-[position-try-fallbacks-computed.html]
-  [Property position-try-fallbacks value 'flip-start flip-inline flip-block']
-    expected: FAIL

testing/web-platform/meta/css/css-anchor-position/parsing/position-try-fallbacks-parsing.html.ini

@@ -1,9 +0,0 @@
-[position-try-fallbacks-parsing.html]
-  [e.style['position-try-fallbacks'\] = "flip-start flip-inline, flip-block" should set the property value]
-    expected: FAIL
-
-  [e.style['position-try-fallbacks'\] = "flip-start flip-inline flip-block" should set the property value]
-    expected: FAIL
-
-  [e.style['position-try-fallbacks'\] = "--bar flip-inline flip-block" should set the property value]
-    expected: FAIL

testing/web-platform/meta/css/css-anchor-position/parsing/position-try-parsing.html.ini

@@ -1,6 +0,0 @@
-[position-try-parsing.html]
-  [e.style['position-try'\] = "most-block-size flip-inline flip-block, --bar, --baz" should set the property value]
-    expected: FAIL
-
-  [e.style['position-try'\] = "most-inline-size flip-inline flip-block, --foo, --bar" should set position-try-fallbacks]
-    expected: FAIL

testing/web-platform/meta/css/css-anchor-position/try-tactic-basic.html.ini

@@ -1,5 +1,4 @@
 [try-tactic-basic.html]
-  expected: ERROR
   [--pf flip-block]
     expected: FAIL
 
@@ -23,3 +22,27 @@
 
   [--pf]
     expected: FAIL
+
+  [--pf flip-inline flip-block]
+    expected: FAIL
+
+  [--pf flip-block flip-start flip-inline]
+    expected: FAIL
+
+  [--pf flip-inline flip-start flip-block]
+    expected: FAIL
+
+  [--pf flip-start flip-block]
+    expected: FAIL
+
+  [--pf flip-start flip-inline]
+    expected: FAIL
+
+  [--pf flip-start flip-block flip-inline]
+    expected: FAIL
+
+  [--pf flip-start flip-inline flip-block]
+    expected: FAIL
+
+  [--pf flip-inline flip-block flip-start]
+    expected: FAIL