Fails to bootstrap with system Rust 1.33

[jbeich]

After aecf8c2 + cf47a19

$ /usr/local/bin/rustc -vV
rustc 1.33.0-nightly
binary: rustc
commit-hash: 9eac386342c6
commit-date: 2018-12-31
host: x86_64-unknown-freebsd
release: 1.33.0-nightly
LLVM version: 8.0

$ cat config.toml
[build]
...
rustc = "/usr/local/bin/rustc"
cargo = "/usr/local/bin/cargo"
...

$ ./x.py build
[...]
error: use of deprecated item 'core::str::<impl str>::trim_left_matches': superseded by `trim_start_matches`
   --> src/bootstrap/lib.rs:423:30
    |
423 |             .next().unwrap().trim_left_matches("release:").trim();
    |                              ^^^^^^^^^^^^^^^^^
    |
note: lint level defined here
   --> src/bootstrap/lib.rs:107:9
    |
107 | #![deny(warnings)]
    |         ^^^^^^^^
    = note: #[deny(deprecated)] implied by #[deny(warnings)]

error: use of deprecated item 'core::str::<impl str>::trim_left_matches': superseded by `trim_start_matches`
   --> src/bootstrap/dist.rs:227:18
    |
227 |                 .trim_left_matches(trim_chars)
    |                  ^^^^^^^^^^^^^^^^^

error: aborting due to 2 previous errors
error: Could not compile `bootstrap`.

[Keruspe]

src/bootstrap changes from #57765 should probably be backported to beta

[Keruspe]

I think this can be closed now