Use the grammar to highlight when to/downto are used as labels.

Labels are not handled by semantic highlighting (because there is no precise location information in the AST).
But the grammar defines to/downto as keywords, in order to handle for loops.

There are 3 cases where to/downto can be labels:
1) function definition `(~to as x) =>...`
2) function application `foo(~to=...)`
3) JSX props `<foo to=...`
(punned cases are not problematic as they are also variables, and variables are handled via semantic highlighting).

The grammar now recognises patterns of the form `to = ` and `to as` and highlights `to` as a variable.

The correctness of this relies on an observation: in `to =` and `to as`, it's not possible that `to` is the keyword of a syntactically valid for loop, as for that to be the case it must be followed by an expression.

Author: cristianoc

analysis/tests/src/Parser.res

@@ -84,4 +84,22 @@ module SomeComponent = {
 
 let _ = <SomeComponent.Nested> <div /> </SomeComponent.Nested>
 
-let _ = true
+// true/false
+let _ = true || false
+
+// to/downto as label
+let toAs = (~to as x) => x
+let _toEquals = toAs(~to=10)
+
+let to = 1
+for _ in to + to to to + to {
+  ()
+}
+
+module ToAsProp = {
+  @react.component
+  let make = (~to) => {
+    <> {React.int(to)} </>
+  }
+}
+let _ = <ToAsProp to=3 />

analysis/tests/src/expected/Parser.res.txt

@@ -1,5 +1,5 @@
 Parse tests/src/Parser.res
-structure items:23 diagnostics:0 
+structure items:29 diagnostics:0 
 Lident: M (0,7) Namespace
 Lident: C (1,9) Namespace
 Lident: Component (1,13) Namespace
@@ -103,4 +103,22 @@ JsxTag >: (84,29)
 JsxTag >: (84,61)
 JsxTag <: (84,31)
 Lident: div (84,32) JsxLowercase
+Lident: x (90,25) Variable
+Variable: x (90,19)->(90,20)
+Variable: toAs (90,4)->(90,8)
+Lident: toAs (91,16) Variable
+Variable: _toEquals (91,4)->(91,13)
+Variable: to (93,4)->(93,6)
+Lident: to (94,20) Variable
+Lident: to (94,25) Variable
+Lident: to (94,9) Variable
+Lident: to (94,14) Variable
+Lident: ToAsProp (98,7) Namespace
+Lident: to (101,18) Variable
+Ldot: React (101,8) Namespace
+Lident: int (101,14) Variable
+Variable: to (100,14)->(100,17)
+Variable: make (100,6)->(100,10)
+JsxTag <: (104,8)
+Lident: ToAsProp (104,9) Namespace
 

grammars/rescript.tmLanguage.json

@@ -17,6 +17,32 @@
       "name": "keyword.control",
       "match": "\\b(and|as|assert|constraint|downto|else|exception|external|for|if|in|include|lazy|module|mutable|of|open|rec|switch|to|try|type|when|while|with)\\b"
     },
+    "RE_TO_DOWNTO_AS_LABELS": {
+      "patterns": [
+        {
+          "match": "(to|downto)\\s*(=)",
+          "captures": {
+            "1": {
+              "name": "variable"
+            },
+            "2": {
+              "name": "keyword.operator keyword"
+            }
+          }
+        },
+        {
+          "match": "(to|downto)\\s*(as)",
+          "captures": {
+            "1": {
+              "name": "variable"
+            },
+            "2": {
+              "name": "keyword.control"
+            }
+          }
+        }
+      ]
+    },
     "RE_CONSTANTS_BOOL": {
       "name": "constant.language.boolean",
       "match": "\\b(false|true)\\b"
@@ -93,6 +119,9 @@
     },
     "keyword": {
       "patterns": [
+        {
+          "include": "#RE_TO_DOWNTO_AS_LABELS"
+        },
         {
           "include": "#RE_KEYWORDS"
         },