Use a SmallVec in CastTarget

[alexcrichton]

This commit switches a fixed-size list of [Option<Reg>; 8] to instead holding SmallVec<[Reg; 8]> in the CastTarget type used when calculating ABIs. This is inspired by discussion on Zulip where I'm hoping to in the near future extend the usage of this to possibly beyond 8 elements for a new WebAssembly ABI taking advantage of multi-value.

[rustbot]

These commits modify the Cargo.lock file. Unintentional changes to Cargo.lock can be introduced when switching branches and rebasing PRs.

If this was unintentional then you should revert the changes before this PR is merged.
Otherwise, you can ignore this comment.

[rustbot]

r? @nnethercote

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[rustbot]

The Cranelift subtree was changed

cc @bjorn3

[rust-log-analyzer]

The job pr-check-2 failed! Check out the build log: (web) (plain enhanced) (plain)

Click to see the possible cause of the failure (guessed by this bot)

    Checking rustc_codegen_gcc v0.1.0 (/checkout/compiler/rustc_codegen_gcc)
error[E0599]: no method named `is_none` found for reference `&rustc_abi::Reg` in the current scope
  --> compiler/rustc_codegen_gcc/src/abi.rs:49:41
   |
49 |         if self.prefix.iter().all(|x| x.is_none()) {
   |                                         ^^^^^^^ method not found in `&rustc_abi::Reg`

error[E0599]: `&rustc_abi::Reg` is not an iterator
  --> compiler/rustc_codegen_gcc/src/abi.rs:65:47
   |
65 |             .flat_map(|option_reg| option_reg.map(|reg| reg.gcc_type(cx)))
   |                                               ^^^ `&rustc_abi::Reg` is not an iterator
   |
  ::: compiler/rustc_abi/src/callconv/reg.rs:21:1
   |
21 | pub struct Reg {
   | -------------- doesn't satisfy `rustc_abi::Reg: std::iter::Iterator`
   |
   = note: the following trait bounds were not satisfied:
           `&rustc_abi::Reg: std::iter::Iterator`
           which is required by `&mut &rustc_abi::Reg: std::iter::Iterator`
           `rustc_abi::Reg: std::iter::Iterator`
           which is required by `&mut rustc_abi::Reg: std::iter::Iterator`

For more information about this error, try `rustc --explain E0599`.
[RUSTC-TIMING] rustc_codegen_gcc test:false 1.176
error: could not compile `rustc_codegen_gcc` (lib) due to 2 previous errors
Bootstrap failed while executing `check`

[nnethercote]

The GCC backend still needs fixing. Probably worth squashing the commits, too; there's no reason for them to be separate.

View changes since this review

[nnethercote]

[Option<Reg>; 8], lol, what an awful representation.

If you wanted a nicer representation while still retaining the "at most 8 elements" constraint you could use ArrayVec<[Reg; 8]> instead. That would be very similar to the code you've written, and get the nicer iterations/pushes/etc. Then the change to allow more than 8 elements (later) would just be a conversion to SmallVec.

[nnethercote]

It would be nice to have a comment explaining the 8 here.

View changes since the review