`String` `Debug` implementation prints escaped single quotes

[marcianx]

This is very minor.

println!("{:?}", "'");

(playground)

Expected output: "'"
Actual output: "\'"

Perhaps this was intentional, but I'm not sure why it would be. The closest issue I saw was #3752 which is ancient and was more obviously a bug since escapes were appearing in Display output and not just Debug.

This reproduces in the latest stable (1.50.0) and nightly (1.52.0-nightly) builds.

[osa1]

This is because the same format string method of char (escape_debug_ext) is used for Debug implementation of both str and char (and a few other internal types), so it needs to escake ' otherwise in char context it would generate incorrect Debug string. Adding a is_char argument to this method solves the issue. I tested this and it passes ui tests.

I checked the language spec https://doc.rust-lang.org/stable/reference/tokens.html#string-literals I don't think "\'" is a valid string, right? Am I missing anything?

[marcianx]

Oh, wow. Until you mentioned it, I didn't even notice that this also affects char's Debug output as well so that println!("{:?}", '"'); prints '\"'. Thanks for fixing them both!