Compiling cargo fails during PGO profiling

[pietroalbini]

We're currently not using lockfiles during PGO profilings, and Cargo just broke with the latest serde version. Error message:

    Checking crates-io v0.31.1 (/tmp/cargo/crates/crates-io)
error[E0283]: type annotations needed
   --> src/cargo/util/config/de.rs:530:63
    |
530 |                 seed.deserialize(Tuple2Deserializer(1i32, env.as_ref()))
    |                                                           |   |
    |                                                           |   |
    |                                                           |   cannot infer type for type parameter `T` declared on the trait `AsRef`
    |                                                           this method call resolves to `&T`
    |
    = note: cannot satisfy `std::string::String: AsRef<_>`
help: use the fully qualified path for the potential candidates
    |
530 |                 seed.deserialize(Tuple2Deserializer(1i32, <std::string::String as AsRef<OsStr>>::as_ref(env)))
    |                                                           ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
530 |                 seed.deserialize(Tuple2Deserializer(1i32, <std::string::String as AsRef<std::path::Path>>::as_ref(env)))
    |                                                           ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
530 |                 seed.deserialize(Tuple2Deserializer(1i32, <std::string::String as AsRef<str>>::as_ref(env)))
    |                                                           ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
530 |                 seed.deserialize(Tuple2Deserializer(1i32, <std::string::String as AsRef<[u8]>>::as_ref(env)))

error: aborting due to previous error

For more information about this error, try `rustc --explain E0283`.

[pietroalbini]

Found the cause: building cargo for PGO profiling doesn't use the monorepo's lockfile, and since Cargo itself doesn't have any lockfile there are no locked dependencies during the PGO profiling build.

[pietroalbini]

cc @rust-lang/cargo

I'm going to fix this on Rust CI's side by using the monorepo's lockfile when building Cargo for PGO profiling, but you probably want to report the serde regression upstream.