Quick fix for single-expression raw string interpolation (#84)

Raw string interpolation has proven to be a bit trickier to
implement, since we don’t want to treat `#` as a delimiter
(since it’s used in so many other ways in Swift, for example
for `#if`, `#available`, etc.).

This patch contains a “quick fix” for supporting interpolated
raw strings that contain a single expression per interpolation.
A proper fix should be developed as soon as possible, but this
unblocks using Splash to highlight most raw string interpolated
code samples, for now.

Sources/Splash/Grammar/SwiftGrammar.swift

@@ -124,6 +124,10 @@ private extension SwiftGrammar {
         var tokenType: TokenType { return .string }
 
         func matches(_ segment: Segment) -> Bool {
+            guard !segment.isWithinRawStringInterpolation else {
+                return false
+            }
+
             if segment.isWithinStringLiteral(withStart: "#\"", end: "\"#") {
                 return true
             }
@@ -154,7 +158,8 @@ private extension SwiftGrammar {
                 return false
             }
 
-            return !segment.isWithinStringInterpolation
+            return !segment.isWithinStringInterpolation &&
+                !segment.isWithinRawStringInterpolation
         }
     }
 
@@ -534,6 +539,25 @@ private extension Segment {
         return true
     }
 
+    var isWithinRawStringInterpolation: Bool {
+        // Quick fix for supporting single expressions within raw string
+        // interpolation, a proper fix should be developed ASAP.
+        switch tokens.current {
+        case "\\":
+            return tokens.previous != "\\" && tokens.next == "#"
+        case "#":
+            return tokens.previous == "\\" && tokens.next == "("
+        case "(":
+            return tokens.onSameLine.suffix(2) == ["\\", "#"]
+        case ")":
+            let suffix = tokens.onSameLine.suffix(4)
+            return suffix.prefix(3) == ["\\", "#", "("]
+        default:
+            let suffix = tokens.onSameLine.suffix(3)
+            return suffix == ["\\", "#", "("] && tokens.next == ")"
+        }
+    }
+
     var prefixedByDotAccess: Bool {
         return tokens.previous == "(." || prefix.hasSuffix(" .")
     }

Tests/SplashTests/Tests/LiteralTests.swift

@@ -207,6 +207,18 @@ final class LiteralTests: SyntaxHighlighterTestCase {
         ])
     }
 
+    func testRawStringWithInterpolation() {
+        let components = highlighter.highlight("#\"Hello \\#(variable) world\"#")
+
+        XCTAssertEqual(components, [
+            .token("#\"Hello", .string),
+            .whitespace(" "),
+            .plainText("\\#(variable)"),
+            .whitespace(" "),
+            .token("world\"#", .string)
+        ])
+    }
+
     func testDoubleLiteral() {
         let components = highlighter.highlight("let double = 1.13")
 
@@ -291,6 +303,7 @@ extension LiteralTests {
             ("testMultiLineStringLiteral", testMultiLineStringLiteral),
             ("testSingleLineRawStringLiteral", testSingleLineRawStringLiteral),
             ("testMultiLineRawStringLiteral", testMultiLineRawStringLiteral),
+            ("testRawStringWithInterpolation", testRawStringWithInterpolation),
             ("testDoubleLiteral", testDoubleLiteral),
             ("testIntegerLiteralWithSeparators", testIntegerLiteralWithSeparators),
             ("testKeyPathLiteral", testKeyPathLiteral),