Improve error message for char literals

[steveklabnik]

If you try to put something that's bigger than a char into a char
literal, you get an error:

fn main() {
let c = 'ஶ்ரீ';
}
error: unterminated character constant:

This is a very compiler-centric message. Yes, it's technically
'unterminated', but that's not what you, the user did wrong.

Instead, this commit changes it to

error: character literal that's larger than a char:

As this actually tells you what went wrong.

Fixes#28851

[rust-highfive]

r? @sfackler

(rust_highfive has picked a reviewer for you, use r? to override)

[alexcrichton]

This may want to also consider something like 'aa', for example:

foo.rs:2:8: 2:10 error: unterminated character constant: ';
foo.rs:2 'aa';
^~

Printing instead "character literal that's larger than a char" may also be a little misleading perhaps as a is "smaller than a char" technically. Perhaps something like "multiple code points found" or something like that?

[steveklabnik]

Ahhh yes. I tried to think of a few different things, but you're right that this message has that weakness. I like "multiple code points found", if I change it to that, is this okay?

[nagisa]

I’d change the error message to “char(acter) may only contain/represent 1 codepoint”.

Both the previous and suggested error are pretty nasty in case of combining characters like 'ō' or 'ぱ'. The current makes no sense in any case, and the new doesn’t really point out the reason… “'ō'/'ぱ' cannot fit into char even if they’re 1 codepoint? Oh well; I guess I’ll use a string.”

If we want to get fancy, we could even add a note pointing existence of combining codepoints in characters.

[steveklabnik]

If we want to get fancy, we could even add a note pointing existence of combining codepoints in characters.

I think some notes would be really good, actually.

[steveklabnik]

I've updated this with a better message. What do you think?

[bors]

☔ The latest upstream changes (presumably #29285) made this pull request unmergeable. Please resolve the merge conflicts.

[steveklabnik]

Rebased.

[alexcrichton]

accidental update?

[alexcrichton]

r=me

[steveklabnik]

@bors: r=alexcrichton

[bors]

📌 Commit 00e9ad1 has been approved by alexcrichton

[bors]

⌛ Testing commit 00e9ad1 with merge 792a9f1...

[bors]

☀️ Test successful - auto-linux-32-nopt-t, auto-linux-32-opt, auto-linux-64-debug-opt, auto-linux-64-nopt-t, auto-linux-64-opt, auto-linux-64-x-android-t, auto-linux-cross-opt, auto-linux-musl-64-opt, auto-mac-32-opt, auto-mac-64-nopt-t, auto-mac-64-opt, auto-win-gnu-32-nopt-t, auto-win-gnu-32-opt, auto-win-gnu-64-nopt-t, auto-win-gnu-64-opt, auto-win-msvc-32-opt, auto-win-msvc-64-opt