Fixes#2379
= Overlong (non-shortest) sequences
UTF-8's unique encoding scheme allows for some Unicode codepoints
to be represented in multiple ways. For any of these characters,
the spec forbids all but the shortest form. These disallowed longer
sequences are called "overlong". As an interesting side effect of
this rule, the bytes C0 and C1 never appear in valid UTF-8.
= Codepoint range
UTF-8 disallows representation of codepoints beyond U+10FFFF,
which is the highest character which can be encoded in UTF-16.
Because a 4-byte sequence is capable of resulting in such characters,
they must be explicitly rejected. This rule also has an interesting
side effect, which is that bytes F5 to FF never appear.
= References
Detecting an overlong version of a codepoint could get gnarly, but
luckily The Unicode Consortium did the hard work by creating this
handy table of valid byte sequences:
https://unicode.org/versions/corrigendum1.html
I thought this mapped nicely to the parser's state machine, so I
rearranged the relevant states to make use of it.
* add reserved field to match musl struct definition so that
it will work with musl libc.
* add libc getrusage so that it will work with libc
what's not done in this branch is:
* test coverage. See #1629, which should also aim to provide
general test coverage for the std lib.
* rusage struct bits for non-linux operating systems
* `RunStep` moved to lib/std/build/run.zig and gains ability to compare
output and exit code against expected values. Multiple redundant
locations in the test harness code are replaced to use `RunStep`.
* `WriteFileStep` moved to lib/std/build/write_file.zig and gains
ability to write more than one file into the cache directory, for
when the files need to be relative to each other. This makes
usage of `WriteFileStep` no longer problematic when parallelizing
zig build.
* Added `CheckFileStep`, which can be used to validate that the output
of another step produced a valid file. Multiple redundant locations
in the test harness code are replaced to use `CheckFileStep`.
* Added `TranslateCStep`. This exposes `zig translate-c` to the build
system, which is likely to be rarely useful by most Zig users;
however Zig's own test suite uses it both for translate-c tests and
for run-translated-c tests.
* Refactored ad-hoc code to handle source files coming from multiple
kinds of sources, into `std.build.FileSource`.
* Added `std.build.Builder.addExecutableFromWriteFileStep`.
* Added `std.build.Builder.addExecutableSource`.
* Added `std.build.Builder.addWriteFiles`.
* Added `std.build.Builder.addTranslateC`.
* Added `std.build.LibExeObjStep.addCSourceFileSource`.
* Added `std.build.LibExeObjStep.addAssemblyFileFromWriteFileStep`.
* Added `std.build.LibExeObjStep.addAssemblyFileSource`.
* Exposed `std.fs.base64_encoder`.
* `std.mem.Compare` is now `std.math.Order` and the enum tags
renamed to follow new style convention.
* `std.mem.compare` is renamed to `std.mem.order`.
* new function `std.math.order`
