Also disallow bare generators in interpolation syntax

This should be the only other case which needs to be disallowed - the
reference parser uses parse_eq_star here and that's also what's used
within parse_brackets with `,` `;` and `for` being the only allowed
continuation tokens.

Also refine error recovery a little and add additional tests.

src/parser.jl

@@ -3180,11 +3180,16 @@ function parse_string(ps::ParseState, raw::Bool)
                 # "a $(x + y) b"  ==>  (string "a " (parens (call-i x + y)) " b")
                 # "hi$("ho")"     ==>  (string "hi" (parens (string "ho")))
                 m = position(ps)
-                parse_atom(ps)
-                if peek_behind(ps, skip_parens=false).kind != K"parens"
-                    # "$(x,y)" ==> (string (error (tuple-p x y)))
+                bump(ps, TRIVIA_FLAG)
+                opts = parse_brackets(ps, K")") do had_commas, had_splat, num_semis, num_subexprs
+                    return (needs_parameters=false,
+                            simple_interp=!had_commas && !had_splat && num_semis == 0)
+                end
+                if !opts.simple_interp || peek_behind(ps).kind == K"generator"
+                    # "$(x,y)" ==> (string (parens (error x y)))
                     emit(ps, m, K"error", error="invalid interpolation syntax")
                 end
+                emit(ps, m, K"parens")
             elseif k == K"var"
                 # var identifiers disabled in strings
                 # "$var"  ==>  (string var)

test/diagnostics.jl

@@ -96,6 +96,9 @@ end
         Diagnostic(2, 19, :error, "try without catch or finally")
         Diagnostic(20, 19, :error, "Expected `end`")
     ]
+
+    @test diagnostic("\"\$(x,y)\"") ==
+        Diagnostic(3, 7, :error, "invalid interpolation syntax")
 end
 
 @testset "parser warnings" begin

test/parser.jl

@@ -869,8 +869,9 @@ tests = [
         "\"\"\"\n\$x\n a\"\"\"" => "(string-s x \"\\n\" \" a\")"
         "\"a \$(x + y) b\""  =>  "(string \"a \" (parens (call-i x + y)) \" b\")"
         "\"hi\$(\"ho\")\""   =>  "(string \"hi\" (parens (string \"ho\")))"
-        "\"\$(x,y)\""        =>  "(string (error (tuple-p x y)))"
-        "\"\$(x;y)\""        =>  "(string (error (block-p x y)))"
+        "\"\$(x,y)\""        =>  "(string (parens (error x y)))"
+        "\"\$(x;y)\""        =>  "(string (parens (error x y)))"
+        "\"\$(x for y in z)\"" => "(string (parens (error (generator x (= y z)))))"
         "\"a \$foo b\""  =>  "(string \"a \" foo \" b\")"
         "\"\$var\""      =>  "(string var)"
         "\"\$outer\""    =>  "(string outer)"