fix promotion example

[lcnr]

getting the following error after 2:30min was a bit scary 😅

error[E0277]: the trait bound `<<<<<<<<state::State<White, board::Board<board::BoardRank<Filled<piece::ColoredPiece<Rook, White>>, Filled<piece::ColoredPiece<Knight, White>>, Filled<piece::ColoredPiece<Bishop, White>>, Filled<piece::ColoredPiece<piece::Queen, White>>, Filled<piece::ColoredPiece<King, White>>, Filled<piece::ColoredPiece<Bishop, White>>, Filled<piece::ColoredPiece<Knight, White>>, Filled<piece::ColoredPiece<Rook, White>>>, board::BoardRank<Filled<piece::ColoredPiece<Pawn, White>>, Filled<piece::ColoredPiece<Pawn, White>>, Filled<piece::ColoredPiece<Pawn, White>>, Filled<piece::ColoredPiece<Pawn, White>>, Filled<piece::ColoredPiece<Pawn, White>>, Filled<piece::ColoredPiece<Pawn, White>>, Filled<piece::ColoredPiece<Pawn, White>>, Filled<piece::ColoredPiece<Pawn, White>>>, board::BoardRank<board::Empty, board::Empty, board::Empty, board::Empty, board::Empty, board::Empty, board::Empty, board::Empty>, board::BoardRank<board::Empty, board::Empty, board::Empty, board::Empty, board::Empty, board::Empty, board::Empty, board::Empty>, board::BoardRank<board::Empty, board::Empty, board::Empty, board::Empty, board::Empty, board::Empty, board::Empty, board::Empty>, board::BoardRank<board::Empty, board::Empty, board::Empty, board::Empty, board::Empty, board::Empty, board::Empty, board::Empty>, board::BoardRank<Filled<piece::ColoredPiece<Pawn, Black>>, Filled<piece::ColoredPiece<Pawn, Black>>, Filled<piece::ColoredPiece<Pawn, Black>>, Filled<piece::ColoredPiece<Pawn, Black>>, Filled<piece::ColoredPiece<Pawn, Black>>, Filled<piece::ColoredPiece<Pawn, Black>>, Filled<piece::ColoredPiece<Pawn, Black>>, Filled<piece::ColoredPiece<Pawn, Black>>>, board::BoardRank<Filled<piece::ColoredPiece<Pawn, Black>>, Filled<piece::ColoredPiece<Knight, Black>>, Filled<piece::ColoredPiece<Bishop, Black>>, Filled<piece::ColoredPiece<piece::Queen, Black>>, Filled<piece::ColoredPiece<King, Black>>, Filled<piece::ColoredPiece<Bishop, Black>>, Filled<piece::ColoredPiece<Knight, Black>>, Filled<piece::ColoredPiece<Rook, Black>>>>, NoSquare, state::CastleState<True, True, True, True>> as RunMakeEM<square::Square<square::rank::R2, FF>, square::Square<square::rank::R4, FF>, NoPiece>>::Output as RunMakeEM<square::Square<square::rank::R7, FA>, square::Square<square::rank::R6, FA>, NoPiece>>::Output as RunMakeEM<square::Square<square::rank::R4, FF>, square::Square<square::rank::R5, FF>, NoPiece>>::Output as RunMakeEM<square::Square<square::rank::R6, FA>, square::Square<square::rank::R5, FA>, NoPiece>>::Output as RunMakeEM<square::Square<square::rank::R5, FF>, square::Square<square::rank::R6, FF>, NoPiece>>::Output as RunMakeEM<square::Square<square::rank::R5, FA>, square::Square<square::rank::R6, FA>, NoPiece>>::Output as RunMakeEM<square::Square<square::rank::R6, FF>, square::Square<square::rank::R7, FF>, NoPiece>>::Output as RunMakeEM<square::Square<square::rank::R4, FA>, square::Square<square::rank::R3, FA>, NoPiece>>::Output: RunMakeEM<square::Square<square::rank::R7, FG>, square::Square<square::rank::R8, FH>, piece::Queen>` is not satisfied
  --> src/main.rs:37:20
   |
37 |     println!("{}", S10::reify());
   |                    ^^^ the trait `RunMakeEM<square::Square<square::rank::R7, FG>, square::Square<square::rank::R8, FH>, piece::Queen>` is not implemented for `<<<<... as RunMakeEM<..., ..., ...>>::Output as RunMakeEM<..., ..., ...>>::Output as RunMakeEM<..., ..., ...>>::Output as RunMakeEM<..., ..., ...>>::Output`

with that change and rust-lang/rust#119735 cargo +rust1 rustc -- -Znext-solver compiles successfully in 4m26,600s after changing the recursion limit to 1024. Unsure what exactly is responsible for the higher recursion limit requirement, but tbh, I don't care too much about that.

RUSTFLAGS=-Znext-solver time cargo +rust1 test takes nearly 13 minutes, can probably trivially speed this up by deduplicating obligations in the new solver as well 🤷

[Dragon-Hatcher]

What is the difference between those two commands?

[lcnr]

by adding -Znext-solver as a command line flag to rustc, either by using cargo rustc -- -Znext-solver or RUSTFLAGS=-Znext-solver <whatever>, you compile your code using the next generation trait solver for Rust: rust-lang/rust#107374.

cargo rustc is similar to cargo build, except that you can pass arguments to the compiler, rustc, after --.

The +rust1 is to change cargo to use my local toolchain instead of the default. You can also do cargo +nightly build or sth like this.

So cargo +rust1 rustc -- -Znext-solver builds your crate (in this case with the promotion example uncommented) using my local toolchain which includes the changes in rust-lang/rust#119735 and using the next gen trait solver. It does not run tests.

For RUSTFLAGS=-Znext-solver time cargo +rust1 test I changed main.rs to the smaller example again and ran all tests this time, using the time command to get accurate timings.