This commit performs a final stabilization pass over the std::fmt module,
marking all necessary APIs as stable. One of the more interesting aspects of
this module is that it exposes a good deal of its runtime representation to the
outside world in order for `format_args!` to be able to construct the format
strings. Instead of hacking the compiler to assume that these items are stable,
this commit instead lays out a story for the stabilization and evolution of
these APIs.
There are three primary details used by the `format_args!` macro:
1. `Arguments` - an opaque package of a "compiled format string". This structure
is passed around and the `write` function is the source of truth for
transforming a compiled format string into a string at runtime. This must be
able to be constructed in stable code.
2. `Argument` - an opaque structure representing an argument to a format string.
This is *almost* a trait object as it's just a pointer/function pair, but due
to the function originating from one of many traits, it's not actually a
trait object. Like `Arguments`, this must be constructed from stable code.
3. `fmt::rt` - this module contains the runtime type definitions primarily for
the `rt::Argument` structure. Whenever an argument is formatted with
nonstandard flags, a corresponding `rt::Argument` is generated describing how
the argument is being formatted. This can be used to construct an
`Arguments`.
The primary interface to `std::fmt` is the `Arguments` structure, and as such
this type name is stabilize as-is today. It is expected for libraries to pass
around an `Arguments` structure to represent a pending formatted computation.
The remaining portions are largely "cruft" which would rather not be stabilized,
but due to the stability checks they must be. As a result, almost all pieces
have been renamed to represent that they are "version 1" of the formatting
representation. The theory is that at a later date if we change the
representation of these types we can add new definitions called "version 2" and
corresponding constructors for `Arguments`.
One of the other remaining large questions about the fmt module were how the
pending I/O reform would affect the signatures of methods in the module. Due to
[RFC 526][rfc], however, the writers of fmt are now incompatible with the
writers of io, so this question has largely been solved. As a result the
interfaces are largely stabilized as-is today.
[rfc]: https://github.com/rust-lang/rfcs/blob/master/text/0526-fmt-text-writer.md
Specifically, the following changes were made:
* The contents of `fmt::rt` were all moved under `fmt::rt::v1`
* `fmt::rt` is stable
* `fmt::rt::v1` is stable
* `Error` is stable
* `Writer` is stable
* `Writer::write_str` is stable
* `Writer::write_fmt` is stable
* `Formatter` is stable
* `Argument` has been renamed to `ArgumentV1` and is stable
* `ArgumentV1::new` is stable
* `ArgumentV1::from_uint` is stable
* `Arguments::new_v1` is stable (renamed from `new`)
* `Arguments::new_v1_formatted` is stable (renamed from `with_placeholders`)
* All formatting traits are now stable, as well as the `fmt` method.
* `fmt::write` is stable
* `fmt::format` is stable
* `Formatter::pad_integral` is stable
* `Formatter::pad` is stable
* `Formatter::write_str` is stable
* `Formatter::write_fmt` is stable
* Some assorted top level items which were only used by `format_args!` were
removed in favor of static functions on `ArgumentV1` as well.
* The formatting-flag-accessing methods remain unstable
Within the contents of the `fmt::rt::v1` module, the following actions were
taken:
* Reexports of all enum variants were removed
* All prefixes on enum variants were removed
* A few miscellaneous enum variants were renamed
* Otherwise all structs, fields, and variants were marked stable.
In addition to these actions in the `std::fmt` module, many implementations of
`Show` and `String` were stabilized as well.
In some other modules:
* `ToString` is now stable
* `ToString::to_string` is now stable
* `Vec` no longer implements `fmt::Writer` (this has moved to `String`)
This is a breaking change due to all of the changes to the `fmt::rt` module, but
this likely will not have much impact on existing programs.
Closes#20661
[breaking-change]
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