Improve docs for Formatter
Some improvements to `std::fmt::Formatter` to make it a bit more consistent with other documentation, as well as calling out that you don't ever instantiate one yourself.
This commit is just covering the feature gate itself and the tests
that made direct use of `!` and thus need to opt back into the
feature.
A follow on commit brings back the other change that motivates the
revert: Namely, going back to the old rules for falling back to `()`.
The expression `&s[..i]` in general can panic if `i` is out of bounds or not on
a character boundary for a string, and this caused the codegen for
`Formatter::pad` to be a bit larger than it otherwise needed to be. This commit
replaces this with `s.get(..i).unwrap_or(&s)` which while having different
behavior if `i` is out of bounds has a much smaller code footprint and otherwise
avoids the need for `unsafe` code.
Use an uninitialized buffer in GenericRadix::fmt_int, like in Display::fmt for numeric types
The code using a slice of that buffer is only ever going to use
bytes that are subsequently initialized.
Document format_args! / Arguments<'a> behavior wrt. Display and Debug
This is a follow up PR to #49067 , this documents the behavior of `format_args!` (i.e: `Argument<'a>`) wrt. `Display` and `Debug`.
r? @steveklabnik
Remove core::fmt::num::Decimal
Before ebf9e1aaf6, it was used for Display::fmt, but ebf9e1aaf6 replaced
that with a faster implementation, and nothing else uses it.
Add hexadecimal formatting of integers with fmt::Debug
This can be used for integers within a larger types which implements Debug (possibly through derive) but not fmt::UpperHex or fmt::LowerHex.
```rust
assert!(format!("{:02x?}", b"Foo\0") == "[46, 6f, 6f, 00]");
assert!(format!("{:02X?}", b"Foo\0") == "[46, 6F, 6F, 00]");
```
RFC: https://github.com/rust-lang/rfcs/pull/2226
The new formatting string syntax (`x?` and `X?`) is insta-stable in this PR because I don’t know how to change a built-in proc macro’s behavior based of a feature gate. I can look into adding that, but I also strongly suspect that keeping this feature unstable for a time period would not be useful as possibly no-one would use it during that time.
This PR does not add the new (public) `fmt::Formatter` proposed in the API because:
* There was some skepticism on response to this part of the RFC
* It is not possible to implement as-is without larger changes to `fmt`, because `Formatter` at the moment has no easy way to tell apart for example `Octal` from `Binary`: it only has a function pointer for the relevant `fmt()` method.
If some integer-like type outside of `std` want to implement this behavior, another RFC will likely need to propose a different public API for `Formatter`.
Stabilise feature(never_type). Introduce feature(exhaustive_patterns)
This stabilizes `!`, removing the feature gate as well as the old defaulting-to-`()` behavior. The pattern exhaustiveness checks which were covered by `feature(never_type)` have been moved behind a new `feature(exhaustive_patterns)` gate.
Replace feature(never_type) with feature(exhaustive_patterns).
feature(exhaustive_patterns) only covers the pattern-exhaustives checks
that used to be covered by feature(never_type)
This can be used for integers within a larger types which implements Debug
(possibly through derive) but not fmt::UpperHex or fmt::LowerHex.
```rust
assert!(format!("{:02x?}", b"Foo\0") == "[46, 6f, 6f, 00]");
assert!(format!("{:02X?}", b"Foo\0") == "[46, 6F, 6F, 00]");
```
RFC: https://github.com/rust-lang/rfcs/pull/2226
