Prefer `-1` for `None`
Currently we pick "weird" numbers like `1114112` for `None::<char>`. While that's not *wrong*, it's kinda *unnatural* -- a human wouldn't make that choice.
This PR instead picks `-1` for thinge like `None::<char>` -- like [clang's `WEOF`](63ae74b78a/libc/include/llvm-libc-macros/wchar-macros.h (L15)) -- and `None::<bool>` and such.
Any enums with more than one niched value (so not `Result` nor `Option`) remain as they were before. Also we continue to use `0` when that's possible -- `-1` is only preferred when zero *isn't* possible.
---
Inspired when someone in discord posted an example like this <https://rust.godbolt.org/z/W94s9qdYW> and I thought it was odd that we're currently picking `-9223372036854775808` to be the value to store to mark an `Option<Vec<_>>` as `None`. (Especially since that needs an 8-byte immediate on x64, and writing `-1` is only a 4-byte immediate.)
One extension worth noting is the use of revisions as custom prefixes for
FileCheck. If your codegen test has different behavior based on the chosen
target or different compiler flags that you want to exercise, you can use a
revisions annotation, like so:
After specifying those variations, you can write different expected, or
explicitly unexpected output by using <prefix>-SAME: and <prefix>-NOT:,
like so:
d4f7856569