DioxusLabs / rustd

# rustd
rustd is a Rust compiler daemon. One process per user stays running, keeps rust-analyzer's
analysis of your workspaces in memory and, when given a directory, compiled functions in a
store on disk, and answers builds from that state. After an edit it compiles the functions
that changed, with Cranelift or LLVM, and either links them in front of what rustc built or
splices them into the executable that is already there.
cargo and rustc start from the file system on every build: they parse, resolve, type-check
and generate code for a whole crate again when one function in it changed, and a second
checkout of the same repository starts from nothing. rustd keeps that work. Git worktrees of
one repository share one database, and a function compiled once is found again by the hash of
what it was compiled from, in any workspace on the machine or, through an S3 bucket, on
another machine.
It is meant for people and coding agents who build the same code many times, often in several
worktrees at once.
## Status
This is a prototype, published for the first time. Nobody outside the project has used it.
Read this section before relying on it.
- It was written largely with AI assistance under human direction. What stands behind it
 is the testing: every change is judged by comparing rustd's output with rustc's
 (docs/harness.md).
- rustc is still required. It builds the crates the workspace depends on, through cargo,
 so the usual caches of those (sccache, a CI cache) still apply. The workspace's own
 crates are rustd's from the first build on, except build scripts, proc-macro crates and
 what those depend on, which run inside cargo and rustc. rustd does not produce `.rlib`
 or `.rmeta` files.
- Once the daemon has a workspace, rustc does not run on an edit at all: no `cargo check`
 beside the build, so nothing competes with it for the CPU. The price is diagnostics.
 rustd refuses source in which rust-analyzer finds a type or name error, and says each
 as `path:line:column: message [E0308]`; the wording is rust-analyzer's, not rustc's. It has no borrow checker: a use after a move, a missing
 `mut` or a reference that outlives its value is built and run. `cargo rustd check` is
 `cargo check` and gives rustc's verdict.
- It never falls back. A function the daemon cannot compile fails the build with the
 function and the reason, and so does a command it does not build, such as
 `cargo rustd test` today: see What is not supported. No build
 of the workspace is handed to cargo.
- It is pinned to one toolchain, Rust 1.99.0. It links against rustc's output, so it has to
 agree with that rustc on symbol names, type layout and the unstable Rust ABI.
- `cargo rustd` builds programs on Linux (ELF) and on macOS arm64 (Mach-O), in any profile
 and with `RUSTFLAGS`; other targets fail the build. On macOS it has been run on ripgrep, fd and hyperfine only. The command has not
 been built or run on Windows; the daemon underneath it has, through the harnesses.
- The code