Print full char literal on error if any are non-printing

compiler/rustc_parse/src/lexer/unescape_error_reporting.rs

@@ -82,6 +82,16 @@ pub(crate) fn emit_unescape_error(
                         Applicability::MachineApplicable,
                     );
                 }
+            } else {
+                if lit.chars().filter(|x| x.is_whitespace() || x.is_control()).count() >= 1 {
+                    handler.span_note(
+                        span,
+                        &format!(
+                            "there are non-printing characters, the full sequence is `{}`",
+                            lit.escape_default(),
+                        ),
+                    );
+                }
             }
 
             if !has_help {

src/test/ui/parser/char/whitespace-character-literal.rs

@@ -0,0 +1,9 @@
+// This tests that the error generated when a character literal has multiple
+// characters in it contains a note about non-printing characters.
+
+fn main() {
+    // <hair space>x<zero width space>
+    let _hair_space_around = ' x​';
+    //~^ ERROR: character literal may only contain one codepoint
+    //~| NOTE: there are non-printing characters, the full sequence is `\u{200a}x\u{200b}`
+}

src/test/ui/parser/char/whitespace-character-literal.stderr

@@ -0,0 +1,18 @@
+error: character literal may only contain one codepoint
+  --> $DIR/whitespace-character-literal.rs:6:30
+   |
+LL |     let _hair_space_around = ' x​';
+   |                              ^^^^
+   |
+note: there are non-printing characters, the full sequence is `\u{200a}x\u{200b}`
+  --> $DIR/whitespace-character-literal.rs:6:31
+   |
+LL |     let _hair_space_around = ' x​';
+   |                               ^^
+help: if you meant to write a `str` literal, use double quotes
+   |
+LL |     let _hair_space_around = " x​";
+   |                              ~~~~
+
+error: aborting due to previous error
+