Env var configuration errors may contain duplicate error messages

Playing around a bit today, I saw:

$ CARGO_TARGET_X86_64_UNKNOWN_LINUX_GNU_RUNNER=1 cargo +nightly build
error: error in environment variable `CARGO_TARGET_X86_64_UNKNOWN_LINUX_GNU_RUNNER`: could not load config key `target.x86_64-unknown-linux-gnu.runner`: error in environment variable `CARGO_TARGET_X86_64_UNKNOWN_LINUX_GNU_RUNNER`: could not load config key `target.x86_64-unknown-linux-gnu.runner`: failed to deserialize, expected a string or array of strings: data did not match any variant of untagged enum Target

Here the "could not load key..." is duplicated in the error message, but probably shouldn't be! Doing some debugging this is related to ConfigError::with_key_context and it's called twice on the stack both from this location -- https://github.com/rust-lang/cargo/blob/6dd57b2045f37fc8172176e333d0fcd3e07cdea1/src/cargo/util/config/de.rs#L472. The stack trace in the inner call looks like:

      cargo::util::config::ConfigError::with_key_context (self=..., key=0x7ffffffed218, definition=...) at src/cargo/util/config/mod.rs:1239
1 ->  0x00005555562485c3 in <cargo::util::config::de::ValueDeserializer as serde::de::MapAccess>::next_value_seed::{{closure}} (e=...) at src/cargo/util/config/de.rs:472
      0x00005555560edb31 in core::result::Result<T,E>::map_err (self=..., op=...) at /rustc/85976442558bf2d09cec3aa49c9c9ba86fb15c1f/src/libcore/result.rs:606
      0x000055555624426b in <cargo::util::config::de::ValueDeserializer as serde::de::MapAccess>::next_value_seed (self=0x7ffffffed1f0, seed=...) at src/cargo/util/config/de.rs:471
      0x000055555621ab5c in serde::de::MapAccess::next_value (self=0x7ffffffed1f0) at /home/alex/.cargo/registry/src/github.com-1ecc6299db9ec823/serde-1.0.104/src/de/mod.rs:1847
      0x000055555605075a in <<cargo::util::config::value::Value<T> as serde::de::Deserialize>::deserialize::ValueVisitor<T> as serde::de::Visitor>::visit_map (self=..., visitor=...) at src/cargo/util/config/value.rs:140
      0x000055555622cac8 in <cargo::util::config::de::Deserializer as serde::de::Deserializer>::deserialize_struct (self=..., name=..., fields=..., visitor=...) at src/cargo/util/config/de.rs:141
      0x000055555604f30a in <cargo::util::config::value::Value<T> as serde::de::Deserialize>::deserialize (deserializer=...) at src/cargo/util/config/value.rs:151
      0x000055555654854c in <cargo::util::config::path::PathAndArgs as serde::de::Deserialize>::deserialize (deserializer=...) at src/cargo/util/config/path.rs:58
      0x0000555555f7204b in <core::marker::PhantomData<T> as serde::de::DeserializeSeed>::deserialize (self=..., deserializer=...) at /home/alex/.cargo/registry/src/github.com-1ecc6299db9ec823/serde-1.0.104/src/de/mod.rs:786
2 ->  0x000055555624602d in <cargo::util::config::de::ValueDeserializer as serde::de::MapAccess>::next_value_seed (self=0x7ffffffee400, seed=...) at src/cargo/util/config/de.rs:471
      0x000055555621afac in serde::de::MapAccess::next_value (self=0x7ffffffee400) at /home/alex/.cargo/registry/src/github.com-1ecc6299db9ec823/serde-1.0.104/src/de/mod.rs:1847
      0x00005555560522d3 in <<cargo::util::config::value::Value<T> as serde::de::Deserialize>::deserialize::ValueVisitor<T> as serde::de::Visitor>::visit_map (self=..., visitor=...) at src/cargo/util/config/value.rs:140
      0x000055555622ed48 in <cargo::util::config::de::Deserializer as serde::de::Deserializer>::deserialize_struct (self=..., name=..., fields=..., visitor=...) at src/cargo/util/config/de.rs:141
      0x000055555604f20a in <cargo::util::config::value::Value<T> as serde::de::Deserialize>::deserialize (deserializer=...) at src/cargo/util/config/value.rs:151
      0x000055555621473f in <serde::de::impls::OptionVisitor<T> as serde::de::Visitor>::visit_some (self=..., deserializer=...) at /home/alex/.cargo/registry/src/github.com-1ecc6299db9ec823/serde-1.0.104/src/de/impls.rs:621
      0x000055555622b75e in <cargo::util::config::de::Deserializer as serde::de::Deserializer>::deserialize_option (self=..., visitor=...) at src/cargo/util/config/de.rs:120
      0x00005555558bc2d9 in serde::de::impls::<impl serde::de::Deserialize for core::option::Option<T>>::deserialize (deserializer=...) at /home/alex/.cargo/registry/src/github.com-1ecc6299db9ec823/serde-1.0.104/src/de/impls.rs:641
      0x000055555648f746 in cargo::util::config::Config::get (self=0x7fffffffd180, key=...) at src/cargo/util/config/mod.rs:1121
      0x0000555556481ea6 in cargo::util::config::target::load_target_triple (config=0x7fffffffd180, triple=...) at src/cargo/util/config/target.rs:74
      0x000055555640aa0f in cargo::util::config::Config::target_cfg_triple (self=0x7fffffffd180, target=...) at src/cargo/util/config/mod.rs:1087
      0x00005555560190b7 in cargo::core::compiler::build_context::BuildContext::new (ws=0x7fffffff9c68, packages=0x7fffffff3af0, config=0x7fffffffd180, build_config=0x7fffffffa198, profiles=0x5555577833a8, units=0x7fffffff5158, extra_compiler_args=...) at src/cargo/core/compiler/build_context/mod.rs:67
      0x0000555556590834 in cargo::ops::cargo_compile::compile_ws (ws=0x7fffffff9c68, options=0x7fffffffa190, exec=0x7fffffff9b98) at src/cargo/ops/cargo_compile.rs:389
      0x000055555658e7f5 in cargo::ops::cargo_compile::compile_with_exec (ws=0x7fffffff9c68, options=0x7fffffffa190, exec=0x7fffffff9b98) at src/cargo/ops/cargo_compile.rs:259
      0x000055555658e6b0 in cargo::ops::cargo_compile::compile (ws=0x7fffffff9c68, options=0x7fffffffa190) at src/cargo/ops/cargo_compile.rs:248
      0x00005555557a3a1a in cargo::commands::build::exec (config=0x7fffffffd180, args=0x55555769c4a8) at src/bin/cargo/commands/build.rs:72
      0x00005555557d7291 in cargo::cli::execute_subcommand (config=0x7fffffffd180, cmd=..., subcommand_args=0x55555769c4a8) at src/bin/cargo/cli.rs:197
      0x00005555557d5227 in cargo::cli::main (config=0x7fffffffd180) at src/bin/cargo/cli.rs:107
      0x0000555555791da6 in cargo::main () at src/bin/cargo/main.rs:39

(line numbers relative to https://github.com/rust-lang/cargo/tree/6dd57b2045f37fc8172176e333d0fcd3e07cdea1)

I wanted to write this down but I'm honestly not sure there's much we can do about this. This is extremely specific I believe and crops up because there's two layered Value<T> instances being deserialized. One is the outer OptValue type and one is inside the PathAndArgs type. I don't think that the outer type knows that the context was already attached, so it ends up getting attached twice.

This looks like it's pretty local to this one specific key, so it's probably fine if we don't prioritize this too much.

Reproduce the duplicate message with the CARGO_TARGET_X86_64_UNKNOWN_LINUX_GNU_RUNNER=1 cargo +nightly build command, then read src/cargo/util/config/de.rs around line 472 and ConfigError::with_key_context in src/cargo/util/config/mod.rs. Compare the layered deserialization paths in src/cargo/util/config/target.rs and path.rs; done means the configuration error reports the key context only once.