Do not suggest char literal for zero-length strings

compiler/rustc_infer/src/infer/error_reporting/mod.rs

@@ -2068,7 +2068,7 @@ impl<'a, 'tcx> InferCtxt<'a, 'tcx> {
                                 if let Some(code) =
                                     code.strip_prefix('"').and_then(|s| s.strip_suffix('"'))
                                 {
-                                    if code.chars().nth(1).is_none() {
+                                    if code.chars().count() == 1 {
                                         err.span_suggestion(
                                             span,
                                             "if you meant to write a `char` literal, use single quotes",

src/test/ui/inference/char-as-str-multi.rs

@@ -1,6 +1,7 @@
-// When a MULTI-character string literal is used where a char should be,
+// When a MULTI/NO-character string literal is used where a char should be,
 // DO NOT suggest changing to single quotes.
 
 fn main() {
     let _: char = "foo"; //~ ERROR mismatched types
+    let _: char = ""; //~ ERROR mismatched types
 }

src/test/ui/inference/char-as-str-multi.stderr

@@ -6,6 +6,14 @@ LL |     let _: char = "foo";
    |            |
    |            expected due to this
 
-error: aborting due to previous error
+error[E0308]: mismatched types
+  --> $DIR/char-as-str-multi.rs:6:19
+   |
+LL |     let _: char = "";
+   |            ----   ^^ expected `char`, found `&str`
+   |            |
+   |            expected due to this
+
+error: aborting due to 2 previous errors
 
 For more information about this error, try `rustc --explain E0308`.