[rust-dev] how is Rust bootstrapped?

[Huon Wilson]

On 09/06/14 20:12, Zoltán Tóth wrote:
> My question is rather theoretical, from the 
> libre-and-open-source-software point of view.
>
> Bootstrapping needs an already existing language to compile the first 
> executable version of Rust.
>
> I read that this was OCaml at some time. I do not have OCaml on my 
> machine, but still managed to build from a cloned Rust repo. The 
> documentation says that building requires a C++ compiler. These 
> suggest that the project moved from OCaml to C++.
>
> But there are also some texts on the web and in the source that 
> suggests that stage0 is actually not compiled from the source 
> repository, but is downloaded as a binary snapshot. If this latter is 
> the case, then can someone compile a suitable stage0 from [C++|OCaml] 
> source himself?
>
>
> _______________________________________________
> Rust-dev mailing list
> Rust-dev at mozilla.org
> https://mail.mozilla.org/listinfo/rust-dev



Yes, those texts are correct, one downloads a stage0 compiler as a 
binary snapshot to compile Rust from source. The stage0 compiler is just 
stored binaries compiled from some commit in the past. Every so often 
someone makes a new stage0 by making the buildbots build a snapshot of a 
more recent commit, allowing the libraries to be completely written in a 
newer iteration of Rust (they no longer have to be able to be compiled 
by the old snapshot).

There's a wiki page about this snapshot process: 
https://github.com/mozilla/rust/wiki/Note-compiler-snapshots


If one was really interested, one could theoretically backtrace through 
history, all the way back to the last version[1] of rustboot (the OCaml 
compiler), and use this to do a "full bootstrap". That is, use the 
rustboot compiler to build the first written-in-Rust compiler as a 
snapshot, and then use this snapshot to build the next one, following 
the chain of snapshotted commits[2] through to eventually get to modern 
Rust.


As others have said, the C++ dependency is just for building LLVM, which 
is linked into rustc as a library, it's not used by the snapshot (that 
is, LLVM is a dependency required when building librustc to get a rustc 
compiler for the next stage; one can use the snapshot to compile 
libraries like libstd etc. without needing LLVM).


Huon


[1]: 
https://github.com/mozilla/rust/tree/ef75860a0a72f79f97216f8aaa5b388d98da6480/src/boot
[2]: https://github.com/mozilla/rust/blob/master/src/snapshots.txt