# Brainstorming Bootstrap

## Goals

- Faster compile times
- Standard workflows (`cargo test`, `cargo check`)
- No mind-bending `#[cfg(bootstrap)]`
- Easier to find workflows (only building libstd, building stage1)

## Approaches

### Today

- `libstd` from master is both:
* linked to the master rustc
* linked to the programs compiled by rustc
- This requires using `cfg(bootstrap)` in `libstd`, but not `rustc`.
In other words, libstd needs to compile with both beta and master.
- `rustc` can use features from `master` libstd.
(but it still restricted to *language* features from beta)
- `rustc` only needs to be compatible with 1 libstd, not 2
- `cd compiler/rustc && cargo build` works
- `cd library/test && cargo build` works
- `cargo build` from the root directory does not work
- `cargo build` from `library/std` or any other lib directory does not work

Good: can `cargo test` stdlib with beta.

Bad: `#[cfg(bootstrap)]` is hard to understand, using beta compiler and master libstd is mind-bending (even some members of the compiler team were not aware this was the case)

Bad: makes rustc fundamentally not a usual crate

Bad: requires building `std` twice to do anything useful (programs without `#![no_core]`)

### As if rustc is written in Go

- `rustc` is built with both the beta compiler and stdlib.
* building with `stage1` compiler & stdlib is a sanity check, not a part of usual workflow.
* important for distros: they want to bootstrap from both N-1 and N.
- stdlib is built only with master `rustc` (we remove all `cfg(bootstrap)` from libstd)
- `rustc` can only depend on `libstd` features from the latest beta, not from features in the master branch
- might require `cfg(bootstrap)` in `rustc` for changes between beta and master (things like [DiscriminantKind](https://github.com/rust-lang/rust/pull/70705/files?file-filters%5B%5D=.rs&file-filters%5B%5D=.stderr#diff-8fbc542a08b9274446b2fbf02bd610b2)).
- Important note: `cfg(bootstrap)` is required for nightly features used by the compiler. It is **not** required in the code generated by the compiler. `cfg(bootstrap)` can be replaced with `autocfg`-based feature detection.

Good: replaces mind-bending `#[cfg(bootstrap)]` from stdlib with conceptually simpler "need to be compatible with two nightlies" in compiler

Good: `rustc` is a bog-standard crate

Bad: using nightly stdlib features in the compiler has 6 weeks lag

Bad: changing nightly stdlib features might break bootstrap, must rewrite compiler's code to be compatible with both beta and nightly stdlib. We can ban unstable library features in the compiler, but dogfooding is helpful.

Bad: testing stdlib requires compiling or downloading master compiler, can't use beta.

Good: libstd is built only once (by the master compiler) for typical workflows.

### Custom std (needs support in Cargo)

- master `libstd` is linked to master rustc and programs compiled with it, the same as today
- `rustc` is built with both the beta compiler and master stdlib
- building the stdlib with `stage1` is required and part of a normal workflow (this is true in all of the alternatives)

Good: In an ideal world, the following work without setup:
- `cargo test` for the compiler itself
- `cargo test` for the standard library

Good: Building the standard library doesn't require building `rustc`

Bad: How does this manage _partial_ builds, like only building stage 1? ([more discussion](https://www.reddit.com/r/rust/comments/irhj4o/blog_post_rust_in_2021/g4zwdzt/&context=3))

Bad: `std` still has `cfg(bootstrap)` so it can be built with both beta and nightly

Bad: requires building `std` twice to do anything useful (programs without `#![no_core]`)