allow $ escaping double quotes in interpolations

spec/01-lexical-syntax.md

@@ -503,9 +503,10 @@ not processed, except for Unicode escapes.
 #### Interpolated string
 
 ```ebnf
-interpolatedString ::= alphaid ‘"’ {printableChar \ (‘"’ | ‘$’) | escape} ‘"’ 
+interpolatedString ::= alphaid ‘"’ {printableChar \ (‘"’ | ‘$’) | escape} ‘"’
                          |  alphaid ‘"""’ {[‘"’] [‘"’] char \ (‘"’ | ‘$’) | escape} {‘"’} ‘"""’
-escape                 ::= ‘$$’ 
+escape                 ::= ‘$$’
+                         | ‘$"’
                          | ‘$’ id
                          | ‘$’ BlockExpr
 alphaid                ::= upper idrest
@@ -522,13 +523,14 @@ or multi-line (triple quote).
 Inside a interpolated string none of the usual escape characters are interpreted 
 (except for unicode escapes) no matter whether the string literal is normal 
 (enclosed in single quotes) or multi-line (enclosed in triple quotes). 
-Instead, there are two new forms of dollar sign escape. 
+Instead, there are three new forms of dollar sign escape. 
 The most general form encloses an expression in `${` and `}`, i.e. `${expr}`. 
 The expression enclosed in the braces that follow the leading `$` character is of 
 syntactical category BlockExpr. Hence, it can contain multiple statements, 
 and newlines are significant. Single ‘$’-signs are not permitted in isolation 
 in a interpolated string. A single ‘$’-sign can still be obtained by doubling the ‘$’ 
-character: ‘$$’.
+character: ‘$$’. A single ‘"’-sign in a single quoted interpolation would end the
+interpolation. A single ‘"’-sign can be obtained by the sequence ‘\$"’.
 
 The simpler form consists of a ‘$’-sign followed by an identifier starting with 
 a letter and followed only by letters, digits, and underscore characters, 

spec/13-syntax-summary.md

@@ -63,8 +63,9 @@ multiLineChars   ::=  {[‘"’] [‘"’] charNoDoubleQuote} {‘"’}
 interpolatedString 
                  ::=  alphaid ‘"’ {printableChar \ (‘"’ | ‘\$’) | escape} ‘"’ 
                    |  alphaid ‘"""’ {[‘"’] [‘"’] char \ (‘"’ | ‘\$’) | escape} {‘"’} ‘"""’
-escape           ::=  ‘\$\$’ 
-                   |  ‘\$’ id 
+escape           ::=  ‘\$\$’
+                   |  ‘\$"’
+                   |  ‘\$’ id
                    |  ‘\$’ BlockExpr
 alphaid          ::=  upper idrest
                    |  varid

src/compiler/scala/tools/nsc/ast/parser/Scanners.scala

@@ -912,7 +912,7 @@ trait Scanners extends ScannersCommon {
         }
       } else if (ch == '$') {
         nextRawChar()
-        if (ch == '$') {
+        if (ch == '$' || ch == '"') {
           putChar(ch)
           nextRawChar()
           getStringPart(multiLine)
@@ -938,7 +938,8 @@ trait Scanners extends ScannersCommon {
             next.token = kwArray(idx)
           }
         } else {
-          syntaxError(s"invalid string interpolation $$$ch, expected: $$$$, $$identifier or $${expression}")
+          val expectations = "$$, $\", $identifier or ${expression}"
+          syntaxError(s"invalid string interpolation $$$ch, expected: $expectations")
         }
       } else {
         val isUnclosedLiteral = (ch == SU || (!multiLine && (ch == CR || ch == LF)))

test/files/neg/t5856.check

@@ -1,9 +1,6 @@
-t5856.scala:10: error: invalid string interpolation $", expected: $$, $identifier or ${expression}
-  val s9 = s"$"
-             ^
 t5856.scala:10: error: unclosed string literal
   val s9 = s"$"
-              ^
+             ^
 t5856.scala:2: error: error in interpolated string: identifier or block expected
   val s1 = s"$null"
               ^
@@ -28,4 +25,4 @@ t5856.scala:8: error: error in interpolated string: identifier or block expected
 t5856.scala:9: error: error in interpolated string: identifier or block expected
   val s8 = s"$super"
               ^
-10 errors
+9 errors

test/files/run/interpolation.check

@@ -30,3 +30,9 @@ Best price: 13.35
 
 0
 00
+"everybody loves escaped quotes" is a common sentiment.
+hi"$"
+hi"$"
+hi"$"
+hi"$"
+hi"$"

test/files/run/interpolation.scala

@@ -29,4 +29,12 @@ object Test extends App {
   println(f"")
   println(f"${0}")
   println(f"${0}${0}")
+
+  println(s"$"everybody loves escaped quotes$" is a common sentiment.")
+  println(f"hi$"$$$"")
+  println(raw"hi$"$$$"")
+
+  println(s"""hi$"$$$"""")
+  println(f"""hi$"$$$"""")
+  println(raw"""hi$"$$$"""")
 }