librustc: dispatch and pattern benchmarks fail to build

[matthiaskrgr]

repo @ c318691
./x.py bench

error[E0432]: unresolved import `test`
  --> librustc/benches/pattern.rs:11:5
   |
11 | use test::Bencher;
   |     ^^^^ Did you mean `test::test`?
error[E0658]: syntax for subslices in slice patterns is not yet stabilized (see issue #23121)
  --> librustc/benches/pattern.rs:31:20
   |
31 |             [1,2,3,..] => 10,
   |                    ^^
   |
   = help: add #![feature(slice_patterns)] to the crate attributes to enable
error: aborting due to 2 previous errors
Some errors occurred: E0432, E0658.
For more information about an error, try `rustc --explain E0432`.
error: Could not compile `rustc`.
warning: build failed, waiting for other jobs to finish...
error[E0658]: syntax for subslices in slice patterns is not yet stabilized (see issue #23121)
  --> librustc/benches/pattern.rs:31:20
   |
31 |             [1,2,3,..] => 10,
   |                    ^^
   |
   = help: add #![feature(slice_patterns)] to the crate attributes to enable
error: aborting due to previous error
For more information about this error, try `rustc --explain E0658`.
error: Could not compile `rustc`.
warning: build failed, waiting for other jobs to finish...
error[E0432]: unresolved import `test`
  --> librustc/benches/dispatch.rs:11:5
   |
11 | use test::Bencher;
   |     ^^^^ Did you mean `test::test`?
error: trait objects without an explicit `dyn` are deprecated
  --> librustc/benches/dispatch.rs:32:20
   |
32 |     let t = &s as &Trait;
   |                    ^^^^^ help: use `dyn`: `dyn Trait`
   |
   = note: requested on the command line with `-D bare-trait-objects`
error: aborting due to 2 previous errors
For more information about this error, try `rustc --explain E0432`.
error: Could not compile `rustc`.