scanner: fix and resotre string interpolation tests (#16242)

vlib/v/checker/tests/str_interpol_invalid_err.out

@@ -4,31 +4,52 @@ vlib/v/checker/tests/str_interpol_invalid_err.vv:8:13: error: illegal format spe
     8 |     _ = '${[1]:x}'
       |                ^
     9 |     _ = '${[1]!:x}'
-   10 |     //_ = '${Foo{}:x}'
+   10 |     _ = '${Foo{}:x}'
 vlib/v/checker/tests/str_interpol_invalid_err.vv:9:14: error: illegal format specifier `x` for type `[1]int`
     7 | fn main() {
     8 |     _ = '${[1]:x}'
     9 |     _ = '${[1]!:x}'
       |                 ^
-   10 |     //_ = '${Foo{}:x}'
+   10 |     _ = '${Foo{}:x}'
    11 |     _ = '${[1]:f}'
+vlib/v/checker/tests/str_interpol_invalid_err.vv:10:15: error: illegal format specifier `x` for type `Foo`
+    8 |     _ = '${[1]:x}'
+    9 |     _ = '${[1]!:x}'
+   10 |     _ = '${Foo{}:x}'
+      |                  ^
+   11 |     _ = '${[1]:f}'
+   12 |     _ := '${none:F}'
 vlib/v/checker/tests/str_interpol_invalid_err.vv:11:13: error: illegal format specifier `f` for type `[]int`
     9 |     _ = '${[1]!:x}'
-   10 |     //_ = '${Foo{}:x}'
+   10 |     _ = '${Foo{}:x}'
    11 |     _ = '${[1]:f}'
       |                ^
    12 |     _ := '${none:F}'
-   13 |     //_ = '${{"a": "b"}:x}'
+   13 |     _ = '${{"a": "b"}:x}'
 vlib/v/checker/tests/str_interpol_invalid_err.vv:12:15: error: illegal format specifier `F` for type `none`
-   10 |     //_ = '${Foo{}:x}'
+   10 |     _ = '${Foo{}:x}'
    11 |     _ = '${[1]:f}'
    12 |     _ := '${none:F}'
       |                  ^
-   13 |     //_ = '${{"a": "b"}:x}'
+   13 |     _ = '${{"a": "b"}:x}'
-   14 |     //_ = '${Alias(Foo{}):x}'
+   14 |     _ = '${Alias(Foo{}):x}'
+vlib/v/checker/tests/str_interpol_invalid_err.vv:13:20: error: illegal format specifier `x` for type `map[string]string`
+   11 |     _ = '${[1]:f}'
+   12 |     _ := '${none:F}'
+   13 |     _ = '${{"a": "b"}:x}'
+      |                       ^
+   14 |     _ = '${Alias(Foo{}):x}'
+   15 |     _ = '${SumType(int(5)):o}'
+vlib/v/checker/tests/str_interpol_invalid_err.vv:14:22: error: illegal format specifier `x` for type `Alias`
+   12 |     _ := '${none:F}'
+   13 |     _ = '${{"a": "b"}:x}'
+   14 |     _ = '${Alias(Foo{}):x}'
+      |                         ^
+   15 |     _ = '${SumType(int(5)):o}'
+   16 | }
 vlib/v/checker/tests/str_interpol_invalid_err.vv:15:25: error: illegal format specifier `o` for type `SumType`
-   13 |     //_ = '${{"a": "b"}:x}'
+   13 |     _ = '${{"a": "b"}:x}'
-   14 |     //_ = '${Alias(Foo{}):x}'
+   14 |     _ = '${Alias(Foo{}):x}'
    15 |     _ = '${SumType(int(5)):o}'
       |                            ^
    16 | }

vlib/v/checker/tests/str_interpol_invalid_err.vv

@@ -7,10 +7,10 @@ type SumType = Alias | int
 fn main() {
 	_ = '${[1]:x}'
 	_ = '${[1]!:x}'
-	//_ = '${Foo{}:x}'
+	_ = '${Foo{}:x}'
 	_ = '${[1]:f}'
 	_ := '${none:F}'
-	//_ = '${{"a": "b"}:x}'
+	_ = '${{"a": "b"}:x}'
-	//_ = '${Alias(Foo{}):x}'
+	_ = '${Alias(Foo{}):x}'
 	_ = '${SumType(int(5)):o}'
 }

vlib/v/scanner/scanner.v

@@ -803,11 +803,13 @@ fn (mut s Scanner) text_scan() token.Token {
 			}
 			`{` {
 				if s.is_inside_string {
+					prev_char := s.text[s.pos - 1]
+					next_char := s.text[s.pos + 1]
 					// Handle new `hello {name}` string interpolation
-					if !s.text[s.pos + 1].is_space() && s.text[s.pos - 1] != `$` {
+					if !next_char.is_space() && next_char != `}` && prev_char !in [`$`, `{`] {
 						return s.new_token(.str_dollar, '', 1)
 					}
-					if s.text[s.pos - 1] == `$` {
+					if prev_char == `$` {
 						// Skip { in `${` in strings
 						continue
 					} else {

vlib/v/tests/string_interpolation_inner_cbr_test.v

@@ -9,12 +9,9 @@ fn test_string_interpolation_inner_cbr() {
 	println(s1)
 	assert s1 == '22'
 
-	/*
-	XTODO
 	s2 := '${St{}}'
 	println(s2)
 	assert s2 == 'St{}'
-	*/
 
 	s3 := '${{
 		'a': 1