This experimental target was never fully completed. The operating system
is not that interesting or popular anyway, and the maintainer is no
longer around.
Not worth the maintenance burden. This code can be resurrected later if
it is worth it. In such case it will be subject to greater scrutiny.
The functions `Compilation.create` and `Compilation.update` previously
returned inferred error sets, which had built up a lot of crap over
time. This meant that certain error conditions -- particularly certain
filesystem errors -- were not being reported properly (at best the CLI
would just print the error name). This was also a problem in
sub-compilations, where at times only the error name -- which might just
be something like `LinkFailed` -- would be visible.
This commit makes the error handling here more disciplined by
introducing concrete error sets to these functions (and a few more as a
consequence). These error sets are small: errors in `update` are almost
all reported via compile errors, and errors in `create` are reported
through a new `Compilation.CreateDiagnostic` type, a tagged union of
possible error cases. This allows for better error reporting.
Sub-compilations also report errors more correctly in several cases,
leading to more informative errors in the case of compiler bugs.
Also fixes some race conditions in library building by replacing calls
to `setMiscFailure` with calls to `lockAndSetMiscFailure`. Compilation
of libraries such as libc happens on the thread pool, so the logic must
synchronize its access to shared `Compilation` state.
This commit replaces the "fuzzer" UI, previously accessed with the
`--fuzz` and `--port` flags, with a more interesting web UI which allows
more interactions with the Zig build system. Most notably, it allows
accessing the data emitted by a new "time report" system, which allows
users to see which parts of Zig programs take the longest to compile.
The option to expose the web UI is `--webui`. By default, it will listen
on `[::1]` on a random port, but any IPv6 or IPv4 address can be
specified with e.g. `--webui=[::1]:8000` or `--webui=127.0.0.1:8000`.
The options `--fuzz` and `--time-report` both imply `--webui` if not
given. Currently, `--webui` is incompatible with `--watch`; specifying
both will cause `zig build` to exit with a fatal error.
When the web UI is enabled, the build runner spawns the web server as
soon as the configure phase completes. The frontend code consists of one
HTML file, one JavaScript file, two CSS files, and a few Zig source
files which are built into a WASM blob on-demand -- this is all very
similar to the old fuzzer UI. Also inherited from the fuzzer UI is that
the build system communicates with web clients over a WebSocket
connection.
When the build finishes, if `--webui` was passed (i.e. if the web server
is running), the build runner does not terminate; it continues running
to serve web requests, allowing interactive control of the build system.
In the web interface is an overall "status" indicating whether a build
is currently running, and also a list of all steps in this build. There
are visual indicators (colors and spinners) for in-progress, succeeded,
and failed steps. There is a "Rebuild" button which will cause the build
system to reset the state of every step (note that this does not affect
caching) and evaluate the step graph again.
If `--time-report` is passed to `zig build`, a new section of the
interface becomes visible, which associates every build step with a
"time report". For most steps, this is just a simple "time taken" value.
However, for `Compile` steps, the compiler communicates with the build
system to provide it with much more interesting information: time taken
for various pipeline phases, with a per-declaration and per-file
breakdown, sorted by slowest declarations/files first. This feature is
still in its early stages: the data can be a little tricky to
understand, and there is no way to, for instance, sort by different
properties, or filter to certain files. However, it has already given us
some interesting statistics, and can be useful for spotting, for
instance, particularly complex and slow compile-time logic.
Additionally, if a compilation uses LLVM, its time report includes the
"LLVM pass timing" information, which was previously accessible with the
(now removed) `-ftime-report` compiler flag.
To make time reports more useful, ZIR and compilation caches are ignored
by the Zig compiler when they are enabled -- in other words, `Compile`
steps *always* run, even if their result should be cached. This means
that the flag can be used to analyze a project's compile time without
having to repeatedly clear cache directory, for instance. However, when
using `-fincremental`, updates other than the first will only show you
the statistics for what changed on that particular update. Notably, this
gives us a fairly nice way to see exactly which declarations were
re-analyzed by an incremental update.
If `--fuzz` is passed to `zig build`, another section of the web
interface becomes visible, this time exposing the fuzzer. This is quite
similar to the fuzzer UI this commit replaces, with only a few cosmetic
tweaks. The interface is closer than before to supporting multiple fuzz
steps at a time (in line with the overall strategy for this build UI,
the goal will be for all of the fuzz steps to be accessible in the same
interface), but still doesn't actually support it. The fuzzer UI looks
quite different under the hood: as a result, various bugs are fixed,
although other bugs remain. For instance, viewing the source code of any
file other than the root of the main module is completely broken (as on
master) due to some bogus file-to-module assignment logic in the fuzzer
UI.
Implementation notes:
* The `lib/build-web/` directory holds the client side of the web UI.
* The general server logic is in `std.Build.WebServer`.
* Fuzzing-specific logic is in `std.Build.Fuzz`.
* `std.Build.abi` is the new home of `std.Build.Fuzz.abi`, since it now
relates to the build system web UI in general.
* The build runner now has an **actual** general-purpose allocator,
because thanks to `--watch` and `--webui`, the process can be
arbitrarily long-lived. The gpa is `std.heap.DebugAllocator`, but the
arena remains backed by `std.heap.page_allocator` for efficiency. I
fixed several crashes caused by conflation of `gpa` and `arena` in the
build runner and `std.Build`, but there may still be some I have
missed.
* The I/O logic in `std.Build.WebServer` is pretty gnarly; there are a
*lot* of threads involved. I anticipate this situation improving
significantly once the `std.Io` interface (with concurrency support)
is introduced.
Alignment and fill options only apply to numbers.
Rework the implementation to mainly branch on the format string rather
than the type information. This is more straightforward to maintain and
more straightforward for comptime evaluation.
Enums support being printed as decimal, hexadecimal, octal, and binary.
`formatInteger` is another possible format method that is
unconditionally called when the value type is struct and one of the
integer-printing format specifiers are used.
I messed up atomic orderings on this variable because they changed in a
local refactor at some point. We need to always release on the store and
acquire on the loads so that a linker thread observing `.ready` sees the
stored MIR.
Without this cap, unlucky scheduling and/or details of what pipeline
stages perform best on the host machine could cause many gigabytes of
MIR to be stuck in the queue. At a certain point, pause the main thread
until some of the functions in flight have been processed.
* "Flush" nodes ("LLVM Emit Object", "ELF Flush") appear under "Linking"
* "Code Generation" disappears when all analysis and codegen is done
* We only show one node under "Semantic Analysis" to accurately convey
that analysis isn't happening in parallel, but rather that we're
pausing one task to do another
Previously, various doc comments heavily disagreed with the
implementation on both what lives where on the filesystem at what time,
and how that was represented in code. Notably, the combination of emit
paths outside the cache and `disable_lld_caching` created a kind of
ad-hoc "cache disable" mechanism -- which didn't actually *work* very
well, 'most everything still ended up in this cache. There was also a
long-standing issue where building using the LLVM backend would put a
random object file in your cwd.
This commit reworks how emit paths are specified in
`Compilation.CreateOptions`, how they are represented internally, and
how the cache usage is specified.
There are now 3 options for `Compilation.CacheMode`:
* `.none`: do not use the cache. The paths we have to emit to are
relative to the compiler cwd (they're either user-specified, or
defaults inferred from the root name). If we create any temporary
files (e.g. the ZCU object when using the LLVM backend) they are
emitted to a directory in `local_cache/tmp/`, which is deleted once
the update finishes.
* `.whole`: cache the compilation based on all inputs, including file
contents. All emit paths are computed by the compiler (and will be
stored as relative to the local cache directory); it is a CLI error to
specify an explicit emit path. Artifacts (including temporary files)
are written to a directory under `local_cache/tmp/`, which is later
renamed to an appropriate `local_cache/o/`. The caller (who is using
`--listen`; e.g. the build system) learns the name of this directory,
and can get the artifacts from it.
* `.incremental`: similar to `.whole`, but Zig source file contents, and
anything else which incremental compilation can handle changes for, is
not included in the cache manifest. We don't need to do the dance
where the output directory is initially in `tmp/`, because our digest
is computed entirely from CLI inputs.
To be clear, the difference between `CacheMode.whole` and
`CacheMode.incremental` is unchanged. `CacheMode.none` is new
(previously it was sort of poorly imitated with `CacheMode.whole`). The
defined behavior for temporary/intermediate files is new.
`.none` is used for direct CLI invocations like `zig build-exe foo.zig`.
The other cache modes are reserved for `--listen`, and the cache mode in
use is currently just based on the presence of the `-fincremental` flag.
There are two cases in which `CacheMode.whole` is used despite there
being no `--listen` flag: `zig test` and `zig run`. Unless an explicit
`-femit-bin=xxx` argument is passed on the CLI, these subcommands will
use `CacheMode.whole`, so that they can put the output somewhere without
polluting the cwd (plus, caching is potentially more useful for direct
usage of these subcommands).
Users of `--listen` (such as the build system) can now use
`std.zig.EmitArtifact.cacheName` to find out what an output will be
named. This avoids having to synchronize logic between the compiler and
all users of `--listen`.
Unfortunately, the self-hosted SPIR-V backend is quite tightly coupled
with the self-hosted SPIR-V linker through its `Object` concept (which
is much like `llvm.Object`). Reworking this would be too much work for
this branch. So, for now, I have introduced a special case (similar to
the LLVM backend's special case) to the codegen logic when using this
backend. We will want to delete this special case at some point, but it
need not block this work.
My original goal here was just to get the self-hosted Wasm backend
compiling again after the pipeline change, but it turned out that from
there it was pretty simple to entirely eliminate the shared state
between `codegen.wasm` and `link.Wasm`. As such, this commit not only
fixes the backend, but makes it the second backend (after CBE) to
support the new 1:N:1 threading model.
As of this commit, every backend other than self-hosted Wasm and
self-hosted SPIR-V compiles and (at least somewhat) functions again.
Those two backends are currently disabled with panics.
Note that `Zcu.Feature.separate_thread` is *not* enabled for the fixed
backends. Avoiding linker references from codegen is a non-trivial task,
and can be done after this branch.
The idea here is that instead of the linker calling into codegen,
instead codegen should run before we touch the linker, and after MIR is
produced, it is sent to the linker. Aside from simplifying the call
graph (by preventing N linkers from each calling into M codegen
backends!), this has the huge benefit that it is possible to
parallellize codegen separately from linking. The threading model can
look like this:
* 1 semantic analysis thread, which generates AIR
* N codegen threads, which process AIR into MIR
* 1 linker thread, which emits MIR to the binary
The codegen threads are also responsible for `Air.Legalize` and
`Air.Liveness`; it's more efficient to do this work here instead of
blocking the main thread for this trivially parallel task.
I have repurposed the `Zcu.Feature.separate_thread` backend feature to
indicate support for this 1:N:1 threading pattern. This commit makes the
C backend support this feature, since it was relatively easy to divorce
from `link.C`: it just required eliminating some shared buffers. Other
backends don't currently support this feature. In fact, they don't even
compile -- the next few commits will fix them back up.
Similar to the previous commit, this commit untangles LLD integration
from the self-hosted linkers. Despite the big network of functions which
were involved, it turns out what was going on here is quite simple. The
LLD linking logic is actually very self-contained; it requires a few
flags from the `link.File.OpenOptions`, but that's really about it. We
don't need any of the mutable state on `Elf`/`Coff`/`Wasm`, for
instance. There was some legacy code trying to handle support for using
self-hosted codegen with LLD, but that's not a supported use case, so
I've just stripped it out.
For now, I've just pasted the logic for linking the 3 targets we
currently support using LLD for into this new linker implementation,
`link.Lld`; however, it's almost certainly possible to combine some of
the logic and simplify this file a bit. But to be honest, it's not
actually that bad right now.
This commit ends up eliminating the distinction between `flush` and
`flushZcu` (formerly `flushModule`) in linkers, where the latter
previously meant something along the lines of "flush, but if you're
going to be linking with LLD, just flush the ZCU object file, don't
actually link"?. The distinction here doesn't seem like it was properly
defined, and most linkers seem to treat them as essentially identical
anyway. Regardless, all calls to `flushZcu` are gone now, so it's
deleted -- one `flush` to rule them all!
The end result of this commit and the preceding one is that LLVM and LLD
fit into the pipeline much more sanely:
* If we're using LLVM for the ZCU, that state is on `zcu.llvm_object`
* If we're using LLD to link, then the `link.File` is a `link.Lld`
* Calls to "ZCU link functions" (e.g. `updateNav`) lower to calls to the
LLVM object if it's available, or otherwise to the `link.File` if it's
available (neither is available under `-fno-emit-bin`)
* After everything is done, linking is finalized by calling `flush` on
the `link.File`; for `link.Lld` this invokes LLD, for other linkers it
flushes self-hosted linker state
There's one messy thing remaining, and that's how self-hosted function
codegen in a ZCU works; right now, we process AIR with a call sequence
something like this:
* `link.doTask`
* `Zcu.PerThread.linkerUpdateFunc`
* `link.File.updateFunc`
* `link.Elf.updateFunc`
* `link.Elf.ZigObject.updateFunc`
* `codegen.generateFunction`
* `arch.x86_64.CodeGen.generate`
So, we start in the linker, take a scenic detour through `Zcu`, go back
to the linker, into its implementation, and then... right back out, into
code which is generic over the linker implementation, and then dispatch
on the *backend* instead! Of course, within `arch.x86_64.CodeGen`, there
are some more places which switch on the `link` implementation being
used. This is all pretty silly... so it shall be my next target.
The main goal of this commit is to make it easier to decouple codegen
from the linkers by being able to do LLVM codegen without going through
the `link.File`; however, this ended up being a nice refactor anyway.
Previously, every linker stored an optional `llvm.Object`, which was
populated when using LLVM for the ZCU *and* linking an output binary;
and `Zcu` also stored an optional `llvm.Object`, which was used only
when we needed LLVM for the ZCU (e.g. for `-femit-llvm-bc`) but were not
emitting a binary.
This situation was incredibly silly. It meant there were N+1 places the
LLVM object might be instead of just 1, and it meant that every linker
had to start a bunch of methods by checking for an LLVM object, and just
dispatching to the corresponding method on *it* instead if it was not
`null`.
Instead, we now always store the LLVM object on the `Zcu` -- which makes
sense, because it corresponds to the object emitted by, well, the Zig
Compilation Unit! The linkers now mostly don't make reference to LLVM.
`Compilation` makes sure to emit the LLVM object if necessary before
calling `flush`, so it is ready for the linker. Also, all of the
`link.File` methods which act on the ZCU -- like `updateNav` -- now
check for the LLVM object in `link.zig` instead of in every single
individual linker implementation. Notably, the change to LLVM emit
improves this rather ludicrous call chain in the `-fllvm -flld` case:
* Compilation.flush
* link.File.flush
* link.Elf.flush
* link.Elf.linkWithLLD
* link.Elf.flushModule
* link.emitLlvmObject
* Compilation.emitLlvmObject
* llvm.Object.emit
Replacing it with this one:
* Compilation.flush
* llvm.Object.emit
...although we do currently still end up in `link.Elf.linkWithLLD` to do
the actual linking. The logic for invoking LLD should probably also be
unified at least somewhat; I haven't done that in this commit.
* The `codegen_nav`, `codegen_func`, `codegen_type` tasks are renamed to
`link_nav`, `link_func`, and `link_type`, to more accurately reflect
their purpose of sending data to the *linker*. Currently, `link_func`
remains responsible for codegen; this will change in an upcoming
commit.
* Don't go on a pointless detour through `PerThread` when linking ZCU
functions/`Nav`s; so, the `linkerUpdateNav` etc logic now lives in
`link.zig`. Currently, `linkerUpdateFunc` is an exception, because it
has broader responsibilities including codegen, but this will be
solved in an upcoming commit.
Note that `openLoadArchive` already has linker script support.
With this change I get a failure parsing a real archive in the self
hosted elf linker, rather than the previous behavior of getting an error
while trying to parse a pseudo archive that is actually a load script.
Each target can opt into different sets of legalize features.
By performing these transformations before liveness, instructions
that become unreferenced will have up-to-date liveness information.
This commit makes some big changes to how we track state for Zig source
files. In particular, it changes:
* How `File` tracks its path on-disk
* How AstGen discovers files
* How file-level errors are tracked
* How `builtin.zig` files and modules are created
The original motivation here was to address incremental compilation bugs
with the handling of files, such as #22696. To fix this, a few changes
are necessary.
Just like declarations may become unreferenced on an incremental update,
meaning we suppress analysis errors associated with them, it is also
possible for all imports of a file to be removed on an incremental
update, in which case file-level errors for that file should be
suppressed. As such, after AstGen, the compiler must traverse files
(starting from analysis roots) and discover the set of "live files" for
this update.
Additionally, the compiler's previous handling of retryable file errors
was not very good; the source location the error was reported as was
based only on the first discovered import of that file. This source
location also disappeared on future incremental updates. So, as a part
of the file traversal above, we also need to figure out the source
locations of imports which errors should be reported against.
Another observation I made is that the "file exists in multiple modules"
error was not implemented in a particularly good way (I get to say that
because I wrote it!). It was subject to races, where the order in which
different imports of a file were discovered affects both how errors are
printed, and which module the file is arbitrarily assigned, with the
latter in turn affecting which other files are considered for import.
The thing I realised here is that while the AstGen worker pool is
running, we cannot know for sure which module(s) a file is in; we could
always discover an import later which changes the answer.
So, here's how the AstGen workers have changed. We initially ensure that
`zcu.import_table` contains the root files for all modules in this Zcu,
even if we don't know any imports for them yet. Then, the AstGen
workers do not need to be aware of modules. Instead, they simply ignore
module imports, and only spin off more workers when they see a by-path
import.
During AstGen, we can't use module-root-relative paths, since we don't
know which modules files are in; but we don't want to unnecessarily use
absolute files either, because those are non-portable and can make
`error.NameTooLong` more likely. As such, I have introduced a new
abstraction, `Compilation.Path`. This type is a way of representing a
filesystem path which has a *canonical form*. The path is represented
relative to one of a few special directories: the lib directory, the
global cache directory, or the local cache directory. As a fallback, we
use absolute (or cwd-relative on WASI) paths. This is kind of similar to
`std.Build.Cache.Path` with a pre-defined list of possible
`std.Build.Cache.Directory`, but has stricter canonicalization rules
based on path resolution to make sure deduplicating files works
properly. A `Compilation.Path` can be trivially converted to a
`std.Build.Cache.Path` from a `Compilation`, but is smaller, has a
canonical form, and has a digest which will be consistent across
different compiler processes with the same lib and cache directories
(important when we serialize incremental compilation state in the
future). `Zcu.File` and `Zcu.EmbedFile` both contain a
`Compilation.Path`, which is used to access the file on-disk;
module-relative sub paths are used quite rarely (`EmbedFile` doesn't
even have one now for simplicity).
After the AstGen workers all complete, we know that any file which might
be imported is definitely in `import_table` and up-to-date. So, we
perform a single-threaded graph traversal; similar to what
`resolveReferences` plays for `AnalUnit`s, but for files instead. We
figure out which files are alive, and which module each file is in. If a
file turns out to be in multiple modules, we set a field on `Zcu` to
indicate this error. If a file is in a different module to a prior
update, we set a flag instructing `updateZirRefs` to invalidate all
dependencies on the file. This traversal also discovers "import errors";
these are errors associated with a specific `@import`. With Zig's
current design, there is only one possible error here: "import outside
of module root". This must be identified during this traversal instead
of during AstGen, because it depends on which module the file is in. I
tried also representing "module not found" errors in this same way, but
it turns out to be much more useful to report those in Sema, because of
use cases like optional dependencies where a module import is behind a
comptime-known build option.
For simplicity, `failed_files` now just maps to `?[]u8`, since the
source location is always the whole file. In fact, this allows removing
`LazySrcLoc.Offset.entire_file` completely, slightly simplifying some
error reporting logic. File-level errors are now directly built in the
`std.zig.ErrorBundle.Wip`. If the payload is not `null`, it is the
message for a retryable error (i.e. an error loading the source file),
and will be reported with a "file imported here" note pointing to the
import site discovered during the single-threaded file traversal.
The last piece of fallout here is how `Builtin` works. Rather than
constructing "builtin" modules when creating `Package.Module`s, they are
now constructed on-the-fly by `Zcu`. The map `Zcu.builtin_modules` maps
from digests to `*Package.Module`s. These digests are abstract hashes of
the `Builtin` value; i.e. all of the options which are placed into
"builtin.zig". During the file traversal, we populate `builtin_modules`
as needed, so that when we see this imports in Sema, we just grab the
relevant entry from this map. This eliminates a bunch of awkward state
tracking during construction of the module graph. It's also now clearer
exactly what options the builtin module has, since previously it
inherited some options arbitrarily from the first-created module with
that "builtin" module!
The user-visible effects of this commit are:
* retryable file errors are now consistently reported against the whole
file, with a note pointing to a live import of that file
* some theoretical bugs where imports are wrongly considered distinct
(when the import path moves out of the cwd and then back in) are fixed
* some consistency issues with how file-level errors are reported are
fixed; these errors will now always be printed in the same order
regardless of how the AstGen pass assigns file indices
* incremental updates do not print retryable file errors differently
between updates or depending on file structure/contents
* incremental updates support files changing modules
* incremental updates support files becoming unreferenced
Resolves: #22696
Textual PTX is just assembly language like any other. And if we do ever add
support for emitting PTX object files after reverse engineering the bytecode
format, we'd be emitting ELF files like the CUDA toolchain. So there's really no
need for a special ObjectFormat tag here, nor linker code that treats it as a
distinct format.
This allows emitting object files for s390x-zos (GOFF) and powerpc(64)-aix
(XCOFF).
Note that GOFF emission in LLVM is still being worked on upstream for LLVM 21;
the resulting object files are useless right now. Also, -fstrip is required, or
LLVM will SIGSEGV during DWARF emission.
The old logic only decremented `remaining_prelink_tasks` if `bin_file`
was not `null`. This meant that on `-fno-emit-bin` builds with
registered prelink tasks (e.g. C source files), we exited from
`Compilation.performAllTheWorkInner` early, assuming a prelink error.
Instead, when `bin_file` is `null`, we still decrement
`remaining_prelink_tasks`; we just don't do any actual work.
Resolves: #22682
Windows is a ridiculous operating system designed by toddlers, and so
requires us to close all file handles in the `tmp/xxxxxxx` cache dir
before renaming it into `o/xxxxxxx`. We have a hack in place to handle
this for the main output file, but the MachO linker also outputs a file
with debug symbols, and we weren't closing it! This led to a fuckton of
CI failures when we enabled `.whole` cache mode by default for
self-hosted backends.
thanks jacob for figuring this out while i sat there
This can also be extended to ELF later as it means roughly the same thing there.
This addresses the main issue in #21721 but as I don't have a macOS machine to
do further testing on, I can't confirm whether zig cc is able to pass the entire
cgo test suite after this commit. It can, however, cross-compile a basic program
that uses cgo to x86_64-macos-none which previously failed due to lack of -x
support. Unlike previously, the resulting symbol table does not contain local
symbols (such as C static functions).
I believe this satisfies the related donor bounty: https://ziglang.org/news/second-donor-bounty
Functions like isMinGW() and isGnuLibC() have a good reason to exist: They look
at multiple components of the target. But functions like isWasm(), isDarwin(),
isGnu(), etc only exist to save 4-8 characters. I don't think this is a good
enough reason to keep them, especially given that:
* It's not immediately obvious to a reader whether target.isDarwin() means the
same thing as target.os.tag.isDarwin() precisely because isMinGW() and similar
functions *do* look at multiple components.
* It's not clear where we would draw the line. The logical conclusion before
this commit would be to also wrap Arch.isX86(), Os.Tag.isSolarish(),
Abi.isOpenHarmony(), etc... this obviously quickly gets out of hand.
* It's nice to just have a single correct way of doing something.
Currently zig fails to build while linking the system LLVM/C++ libraries
on my Chimera Linux system due to the fact that libc++.so is a linker
script with the following contents:
INPUT(libc++.so.1 -lc++abi -lunwind)
Prior to this commit, zig would try to convert "ambiguous names" in
linker scripts such as libc++.so.1 in this example into -lfoo style
flags. This fails in this case due to the so version number as zig
checks for exactly the .so suffix.
Furthermore, I do not think that this conversion is semantically correct
since converting libfoo.so to -lfoo could theoretically end up resulting
in libfoo.a getting linked which seems wrong when a different file is
specified in the linker script.
With this patch, this attempted conversion is removed. Instead, zig
always first checks if the exact file/path in the linker script exists
relative to the current working directory.
If the file is classified as a library (including versioned shared
objects such as libfoo.so.1), zig then falls back to checking if
the exact file/path in the linker script exists relative to each
directory in the library search path, selecting the first match or
erroring out if none is found.
This behavior fixes the regression that prevents building zig while
linking the system LLVM/C++ libraries on Chimera Linux.
Instead, `source`, `tree`, and `zir` should all be optional. This is
precisely what we're actually trying to model here; and `File` isn't
optimized for memory consumption or serializability anyway, so it's fine
to use a couple of extra bytes on actual optionals here.
