cargo doc --open fails with 'already borrowed: BorrowMutError'

[5225225]

Problem

I'm unable to run cargo doc --open without

thread 'main' panicked at 'already borrowed: BorrowMutError', src/tools/cargo/src/cargo/util/config/mod.rs:342:20
stack backtrace:
   0: rust_begin_unwind
             at /rustc/c1e8f3a5857c3c8d095fd212acc6ed442f4cc58b/library/std/src/panicking.rs:515:5
   1: core::panicking::panic_fmt
             at /rustc/c1e8f3a5857c3c8d095fd212acc6ed442f4cc58b/library/core/src/panicking.rs:92:14
   2: core::result::unwrap_failed
             at /rustc/c1e8f3a5857c3c8d095fd212acc6ed442f4cc58b/library/core/src/result.rs:1355:5
   3: cargo::util::config::de::ConfigMapAccess::new_struct
   4: <cargo::util::config::de::Deserializer as serde::de::Deserializer>::deserialize_struct
   5: cargo::util::config::Config::get
   6: cargo::ops::cargo_doc::doc
   7: cargo::commands::doc::exec
   8: cargo::cli::main
   9: cargo::main
note: Some details are omitted, run with `RUST_BACKTRACE=full` for a verbose backtrace.

Steps

1. Go to any cargo project
2. Run cargo doc --open

Possible Solution(s)

I've already implemented a fix for this, to be careful to not keep the result of ws.config() around before we call ws.config() again, will make a pull request for that shortly.

Notes

Output of cargo version:

Happens on cargo 1.54.0-nightly (e931e4796 2021-05-24) as well as the latest git master, being checked out to 0cecbd6.

That being said, I have no idea why the tests didn't catch this. I do have CARGO_TARGET_DIR set to have a global target directory, but even unsetting that and running cargo run -- doc --open in the cargo git repo fails.

[5225225]

Apparently it's because my .cargo/config.toml has

[doc.extern-map]
std = "local"

in it.

Removing this makes cargo doc --open work fine.