give tip for unicode chars which might not be visible

Author: chenyukang

compiler/rustc_parse/src/lexer/mod.rs

@@ -322,6 +322,8 @@ impl<'a> StringReader<'a> {
                 let token = unicode_chars::check_for_substitution(self, start, c, &mut err);
                 if c == '\x00' {
                     err.help("source files must contain UTF-8 encoded text, unexpected null bytes might occur when a different encoding is used");
+                } else if let Ok(v) = &err.suggestions && v.len() == 0 && !c.is_ascii() {
+                    err.help(format!("Unicode character {} might not be visible when rendered", escaped_char(c)));
                 }
                 err.emit();
                 token?

src/test/ui/fmt/incorrect-separator.stderr

@@ -3,6 +3,8 @@ error: unknown start of token: \u{326}
    |
 LL |     format!("A number: {}" ̦ iter::once(42).next().unwrap());
    |                            ^
+   |
+   = help: Unicode character \u{326} might not be visible when rendered
 
 error: expected `,`, found `.`
   --> $DIR/incorrect-separator.rs:7:27

src/test/ui/issues/issue-69130.stderr

@@ -3,6 +3,8 @@ error: unknown start of token: \u{a7}
    |
 LL | M (§& u8)}
    |    ^
+   |
+   = help: Unicode character \u{a7} might not be visible when rendered
 
 error[E0106]: missing lifetime specifier
   --> $DIR/issue-69130.rs:4:5

src/test/ui/lexer/lex-bad-token.stderr

@@ -3,6 +3,8 @@ error: unknown start of token: \u{25cf}
    |
 LL | ●
    | ^
+   |
+   = help: Unicode character \u{25cf} might not be visible when rendered
 
 error: aborting due to previous error
 

src/test/ui/lexer/lex-bad-unicode-char.rs

@@ -0,0 +1,4 @@
+fn main() {
+    println!("hello {}"̣, 1);
+    //~^ ERROR: unknown start of token: \u{323}
+}

src/test/ui/lexer/lex-bad-unicode-char.stderr

@@ -0,0 +1,10 @@
+error: unknown start of token: \u{323}
+  --> $DIR/lex-bad-unicode-char.rs:2:24
+   |
+LL |     println!("hello {}"̣, 1);
+   |                        ^
+   |
+   = help: Unicode character \u{323} might not be visible when rendered
+
+error: aborting due to previous error
+