This commit also reclaims +2 ZIR instruction tags by moving the
following to `extended`:
* func_var_args
* func_extra
* func_extra_var_args
The following ZIR instruction tag is added:
* func_inferred
These are currently incorrect according to the gitattributes(5) and
gitignore(5) man pages. However, it seems github ended up treating them
as we intended due to a bug until recently when that bug was fixed.
I've run into this footgun enough times, nearly every time I want
`ensureUnusedCapacity`, not `ensureCapacity`. This commit deprecates
`ensureCapacity` in favor of `ensureTotalCapacity` and introduces
`ensureUnusedCapacity`.
* AstGen: represent compile errors in ZIR rather than returning
`error.AnalysisFail`.
* ZIR: remove decl_ref and decl_val instructions. These are replaced by
`decl_ref_named` and `decl_val_named`, respectively, which will
probably get renamed in the future to the instructions that were just
deleted.
* AstGen: implement `@This()`, `@fence()`, `@returnAddress()`, and
`@src()`.
* AstGen: struct_decl improved to support fields_len=0 but have decls.
* AstGen: fix missing null bytes after compile error messages.
* SrcLoc: no longer depend on `Decl`. Instead have an explicit field
`parent_decl_node` which is an absolute AST Node index.
* Module: `failed_files` table can have null value, in which case the
key, which is a `*Scope.File`, will have ZIR errors in it.
* ZIR: implement text rendering of struct decls.
* CLI: introduce debug_usage and `zig astgen` command which is enabled
when the compiler is built in debug mode.
This was also an experiment to see if it were easier to implement a new
feature when using the instruction encoder.
Verdict: It's not that much easier, but I think it's certainly much more
readable, because the description of the Instruction annotates what each
field means. Right now, precise knowledge of x86_64 instructions is
still required because things like when to set the 64-bit flag, how to
read x86_64 instruction references, etc. are still not automatically
done for you.
In the future, this interface might make it sligtly easier to write an
assembler for x86_64, by abstracting the bit-fiddling aspects of
instruction encoding.
