SI-10120 Extra advice on unclosed char literal

Folks from other languages might mistakenly enclose
a string in single quotes. Since this presents as
a symbol literal followed by the unpaired single
quote, we can add a syntax reminder.

Also polish the wording for bad string
interpolation.

Author: som-snytt

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

@@ -532,6 +532,15 @@ trait Scanners extends ScannersCommon {
           }
           fetchDoubleQuote()
         case '\'' =>
+          def unclosedCharLit() = {
+            val msg = "unclosed character literal"
+            // previous token was Symbol contiguous with the orphan single quote at offset
+            if (token == SYMBOLLIT && offset == lastOffset) {
+              syntaxError(s"""$msg (or use " for string literal "$strVal")""")
+            } else {
+              syntaxError(msg)
+            }
+          }
           def fetchSingleQuote() = {
             nextChar()
             if (isIdentifierStart(ch))
@@ -545,16 +554,14 @@ trait Scanners extends ScannersCommon {
             }
             else if (!isAtEnd && (ch != SU && ch != CR && ch != LF || isUnicodeEscape)) {
               getLitChar()
-              if (ch == '\'') {
+              if (ch != '\'') unclosedCharLit()
+              else {
                 nextChar()
                 token = CHARLIT
                 setStrVal()
-              } else {
-                syntaxError("unclosed character literal")
               }
             }
-            else
-              syntaxError("unclosed character literal")
+            else unclosedCharLit()
           }
           fetchSingleQuote()
         case '.' =>
@@ -786,7 +793,7 @@ trait Scanners extends ScannersCommon {
             next.token = kwArray(idx)
           }
         } else {
-          syntaxError("invalid string interpolation: `$$', `$'ident or `$'BlockExpr expected")
+          syntaxError(s"invalid string interpolation $$$ch, expected: $$$$, $$identifier or $${expression}")
         }
       } else {
         val isUnclosedLiteral = !isUnicodeEscape && (ch == SU || (!multiLine && (ch == CR || ch == LF)))

test/files/neg/badtok-1.check

@@ -7,4 +7,7 @@ badtok-1.scala:2: error: unclosed character literal
 badtok-1.scala:6: error: empty character literal (use '\'' for single quote)
 '''
 ^
-three errors found
+badtok-1.scala:9: error: unclosed character literal (or use " for string literal "abc")
+'abc'
+    ^
+four errors found

test/files/neg/badtok-1.scala

@@ -4,3 +4,6 @@
 
 // SI-10133
 '''
+
+// SI-10120
+'abc'

test/files/neg/t5856.check

@@ -1,4 +1,4 @@
-t5856.scala:10: error: invalid string interpolation: `$$', `$'ident or `$'BlockExpr expected
+t5856.scala:10: error: invalid string interpolation $", expected: $$, $identifier or ${expression}
   val s9 = s"$"
              ^
 t5856.scala:10: error: unclosed string literal