Fix panic on raw string assist

Strings that do not contain two quotation marks
would cause a slice indexing panic because code
was assuming `find_usual_string_range` would return
a string with two quotes, but it would incorrectly
also return text ranges containing only a single quote.

crates/ra_assists/src/assists/raw_string.rs

@@ -159,10 +159,17 @@ fn count_hashes(s: &str) -> usize {
 }
 
 fn find_usual_string_range(s: &str) -> Option<TextRange> {
+    let left_quote = s.find('"')?;
+    let right_quote = s.rfind('"')?;
+    if left_quote == right_quote {
+        // `s` only contains one quote
+        None
+    } else {
         Some(TextRange::from_to(
-        TextUnit::from(s.find('"')? as u32),
+            TextUnit::from(left_quote as u32),
-        TextUnit::from(s.rfind('"')? as u32),
+            TextUnit::from(right_quote as u32),
         ))
+    }
 }
 
 #[cfg(test)]
@@ -271,6 +278,30 @@ string"###;
         )
     }
 
+    #[test]
+    fn make_raw_string_not_works_on_partial_string() {
+        check_assist_not_applicable(
+            make_raw_string,
+            r#"
+            fn f() {
+                let s = "foo<|>
+            }
+            "#,
+        )
+    }
+
+    #[test]
+    fn make_usual_string_not_works_on_partial_string() {
+        check_assist_not_applicable(
+            make_usual_string,
+            r#"
+            fn main() {
+                let s = r#"bar<|>
+            }
+            "#,
+        )
+    }
+
     #[test]
     fn add_hash_target() {
         check_assist_target(