fix(theme/#1933): Scope matching precedence (#2301)

__Issue:__ Our scope-selection strategy was not correct in some cases - particularly, the precedence in which we would apply the scopes.

For example, in #1933 - with the dracula theme - there'd be a case where a token would have the following textmate scopes:
- `support.function.console.js`
- `meta.function-call.js`
- `source.js`

For the dracula theme, there is a `source` selector that is intended to be a fallback - however, it was taking precedence over some of the other theme selectors that should've been applied, like `meta.function-call`.

This would cause the entire source file to be highlighted with the `source` scope selector, instead of the more specific ones.

__Defect:__ We apply selectors in reverse order (first, we apply any selectors matching `source.js`, and then `meta.function-call.js source.js`, etc...) - this gives us right precedence. However, if we had a match, we were failing to 'fall-back' to a more specific selector.

__Fix:__ Add a test case to reproduce, and fall back to override with more specific scopes.

This looks to fix a couple of related issues:
- #1933 - no syntax highlighting with OneDark / Dracula for some files
- #2006  - incorrect syntax highlighting for tsx files
- #1879  - html syntax highlighting not displayed
- #1878 - nested syntax highlights not shown

Thanks @CrossR for the helpful tool improvements in #2255 and @thismat for the investigation to narrow down the issue in #1933 !

`dracula` theme - __before__:

![image](https://user-images.githubusercontent.com/13532591/90297301-e2a6a000-de42-11ea-9380-178dc6345bf8.png)

`dracula` theme - __after__:

![image](https://user-images.githubusercontent.com/13532591/90297344-feaa4180-de42-11ea-999c-95936b4e3369.png)

src/reason-textmate/ThemeScopes.re

@@ -13,6 +13,8 @@ module Scope = {
 
   let ofString = s => String.split_on_char('.', s);
 
+  let toString = scope => String.concat(".", scope);
+
   let rec matches = (selector: t, v: t) => {
     switch (selector, v) {
     | ([], _) => true
@@ -44,6 +46,9 @@ module Scopes = {
     |> String.split_on_char(' ')
     |> List.map(v => Scope.ofString(String.trim(v)));
 
+  let toString = scopes =>
+    String.concat("\n", scopes |> List.map(Scope.toString));
+
   let rec matches = (selector: t, v: t) => {
     switch (selector, v) {
     | ([], _) => true
@@ -58,6 +63,22 @@ module Scopes = {
   };
 };
 
+module ResolvedStyle = {
+  type t = {
+    foreground: string,
+    background: string,
+    bold: bool,
+    italic: bool,
+  };
+
+  let default = (~foreground, ~background, ()) => {
+    foreground,
+    background,
+    bold: false,
+    italic: false,
+  };
+};
+
 module TokenStyle = {
   [@deriving show({with_path: false})]
   type t = {
@@ -74,6 +95,66 @@ module TokenStyle = {
     };
   };
 
+  let merge = (prev, style) => {
+    let foreground =
+      switch (prev.foreground, style.foreground) {
+      | (Some(v), _) => Some(v)
+      | (_, Some(v)) => Some(v)
+      | _ => None
+      };
+
+    let background =
+      switch (prev.background, style.background) {
+      | (Some(v), _) => Some(v)
+      | (_, Some(v)) => Some(v)
+      | _ => None
+      };
+
+    let bold =
+      switch (prev.bold, style.bold) {
+      | (Some(v), _) => Some(v)
+      | (_, Some(v)) => Some(v)
+      | _ => None
+      };
+
+    let italic =
+      switch (prev.italic, style.italic) {
+      | (Some(v), _) => Some(v)
+      | (_, Some(v)) => Some(v)
+      | _ => None
+      };
+
+    {background, foreground, bold, italic};
+  };
+
+  let resolve = (~default: ResolvedStyle.t, style) => {
+    let foreground =
+      switch (style.foreground) {
+      | Some(v) => v
+      | None => default.foreground
+      };
+
+    let bold =
+      switch (style.bold) {
+      | Some(v) => v
+      | None => default.bold
+      };
+
+    let italic =
+      switch (style.italic) {
+      | Some(v) => v
+      | None => default.italic
+      };
+
+    let background =
+      switch (style.background) {
+      | Some(v) => v
+      | None => default.background
+      };
+
+    ResolvedStyle.{bold, italic, foreground, background};
+  };
+
   let create =
       (
         ~foreground: option(string)=None,
@@ -96,22 +177,6 @@ module TokenStyle = {
   };
 };
 
-module ResolvedStyle = {
-  type t = {
-    foreground: string,
-    background: string,
-    bold: bool,
-    italic: bool,
-  };
-
-  let default = (~foreground, ~background, ()) => {
-    foreground,
-    background,
-    bold: false,
-    italic: false,
-  };
-};
-
 module Selector = {
   type t = {
     scopes: Scopes.t,

src/reason-textmate/TokenTheme.re

@@ -47,7 +47,7 @@ let create =
 
   let trie =
     List.fold_left(
-      (prev, curr) => {
+      (prev: Trie.t(selectorWithParents), curr) => {
         open Selector;
         let {scopes, style} = curr;
 
@@ -206,150 +206,112 @@ let show = (v: t) => {
   );
 };
 
-let _applyStyle: (TokenStyle.t, TokenStyle.t) => TokenStyle.t =
-  (prev: TokenStyle.t, style: TokenStyle.t) => {
-    let foreground =
-      switch (prev.foreground, style.foreground) {
-      | (Some(v), _) => Some(v)
-      | (_, Some(v)) => Some(v)
-      | _ => None
-      };
-
-    let background =
-      switch (prev.background, style.background) {
-      | (Some(v), _) => Some(v)
-      | (_, Some(v)) => Some(v)
-      | _ => None
-      };
+let match = (theme: t, scopes: string) => {
+  let scopes = Scopes.ofString(scopes) |> List.rev;
 
-    let bold =
-      switch (prev.bold, style.bold) {
-      | (Some(v), _) => Some(v)
-      | (_, Some(v)) => Some(v)
-      | _ => None
+  let rec calculateStyle = (~parentScopes, ~acc: list(TokenStyle.t), scopes) => {
+    switch (scopes) {
+    | [] => acc
+    | [scope, ...nextScope] =>
+      // Get the matching path from the Trie
+      let matchingPath = Trie.matches(theme.trie, scope);
+
+      if (matchingPath == []) {
+        // No matches - let's try the next scope!
+        calculateStyle(
+          ~parentScopes=[scope, ...parentScopes],
+          ~acc,
+          nextScope,
+        );
+      } else {
+        let maybeTokenStyle: option(TokenStyle.t) =
+          matchingPath
+          |> List.fold_left(
+               (maybePrev: option(TokenStyle.t), curr) => {
+                 let (_name, selector: option(selectorWithParents)) = curr;
+
+                 switch (selector) {
+                 // No selector at this node. This can happen when a node is on the
+                 // path to a node with a style. Nothing to do here; continue on.
+                 | None => maybePrev
+                 // We have a selector at this node. Let's check it out.
+                 | Some({style, parents}) =>
+                   let prevStyle =
+                     maybePrev |> Option.value(~default=TokenStyle.default);
+
+                   // Get the list of matching parent selectors to apply
+                   let parentsScopesToApply =
+                     parents
+                     |> List.filter(selector =>
+                          Selector.matches(selector, parentScopes)
+                        );
+
+                   switch (parentsScopesToApply, style) {
+                   // Case 1: No parent selectors match AND there is no style. We should continue on.
+                   | ([], None) => None
+
+                   // Case 2: No parent selectors match, but there is a style at the Node. We should apply the style.
+                   | ([], Some(style)) =>
+                     Some(TokenStyle.merge(prevStyle, style))
+
+                   // Case 3: We have parent selectors that match, and may or may not have a style at the node.
+                   // Apply the parent styles, and the node style, if applicable.
+                   | (_, maybeStyle) =>
+                     let newStyle =
+                       maybeStyle
+                       |> Option.map(TokenStyle.merge(prevStyle))
+                       |> Option.value(~default=TokenStyle.default);
+
+                     // Apply any parent selectors that match...
+                     // we should be sorting this by score!
+                     Some(
+                       List.fold_left(
+                         (prev, curr: Selector.t) => {
+                           open Selector;
+                           let {style, _} = curr;
+                           // Reversing the order because the parent style
+                           // should take precedence over previous style
+                           TokenStyle.merge(style, prev);
+                         },
+                         newStyle,
+                         parentsScopesToApply,
+                       ),
+                     );
+                   };
+                 };
+               },
+               None,
+             );
+
+        let acc =
+          maybeTokenStyle
+          |> Option.map(tokenStyle => [tokenStyle, ...acc])
+          |> Option.value(~default=acc);
+
+        calculateStyle(
+          ~parentScopes=[scope, ...parentScopes],
+          ~acc,
+          nextScope,
+        );
       };
+    };
+  };
 
-    let italic =
-      switch (prev.italic, style.italic) {
-      | (Some(v), _) => Some(v)
-      | (_, Some(v)) => Some(v)
-      | _ => None
-      };
+  let styles = calculateStyle(~parentScopes=[], ~acc=[], scopes);
 
-    {background, foreground, bold, italic};
-  };
+  let result: TokenStyle.t =
+    styles
+    |> List.fold_left(
+         (acc, style) => {TokenStyle.merge(acc, style)},
+         TokenStyle.default,
+       );
 
-let match = (theme: t, scopes: string) => {
-  let scopes = Scopes.ofString(scopes) |> List.rev;
   let default =
     ResolvedStyle.default(
       ~foreground=theme.defaultForeground,
       ~background=theme.defaultBackground,
       (),
     );
 
-  let rec f = scopes => {
-    switch (scopes) {
-    | [] => default
-    | [scope, ...scopeParents] =>
-      // Get the matching path from the Trie
-      let p = Trie.matches(theme.trie, scope);
-
-      switch (p) {
-      // If there were no matches... try the next scope up.
-      | [] => f(scopeParents)
-      // Got matches - we'll apply them in sequence
-      | _ =>
-        let result =
-          List.fold_left(
-            (prev: option(TokenStyle.t), curr) => {
-              let (_name, selector: option(selectorWithParents)) = curr;
-
-              switch (selector) {
-              // No selector at this node. This can happen when a node is on the
-              // path to a node with a style. Nothing to do here; continue on.
-              | None => prev
-              // We have a selector at this node. Let's check it out.
-              | Some({style, parents}) =>
-                let prevStyle =
-                  switch (prev) {
-                  | None => TokenStyle.default
-                  | Some(v) => v
-                  };
-
-                // Get the list of matching parent selectors to apply
-                let parentsScopesToApply =
-                  parents
-                  |> List.filter(selector =>
-                       Selector.matches(selector, scopeParents)
-                     );
-
-                switch (parentsScopesToApply, style) {
-                // Case 1: No parent selectors match AND there is no style. We should continue on.
-                | ([], None) => None
-                // Case 2: No parent selectors match, but there is a style at the Node. We should apply the style.
-                | ([], Some(style)) => Some(_applyStyle(prevStyle, style))
-                // Case 3: We have parent selectors that match, and may or may not have a style at the node.
-                // Apply the parent styles, and the node style, if applicable.
-                | (_, style) =>
-                  let newStyle =
-                    switch (style) {
-                    | Some(v) => _applyStyle(prevStyle, v)
-                    | None => TokenStyle.default
-                    };
-                  // Apply any parent selectors that match...
-                  // we should be sorting this by score!
-                  Some(
-                    List.fold_left(
-                      (prev, curr: Selector.t) => {
-                        open Selector;
-                        let {style, _} = curr;
-                        // Reversing the order because the parent style
-                        // should take precedence over previous style
-                        _applyStyle(style, prev);
-                      },
-                      newStyle,
-                      parentsScopesToApply,
-                    ),
-                  );
-                };
-              };
-            },
-            None,
-            p,
-          );
-
-        switch (result) {
-        | None => f(scopeParents)
-        | Some(result) =>
-          let foreground =
-            switch (result.foreground) {
-            | Some(v) => v
-            | None => default.foreground
-            };
-
-          let bold =
-            switch (result.bold) {
-            | Some(v) => v
-            | None => default.bold
-            };
-
-          let italic =
-            switch (result.italic) {
-            | Some(v) => v
-            | None => default.italic
-            };
-
-          let background =
-            switch (result.background) {
-            | Some(v) => v
-            | None => default.background
-            };
-
-          {background, foreground, bold, italic};
-        };
-      };
-    };
-  };
-  f(scopes);
+  TokenStyle.resolve(~default, result);
 };