Cargo concatenates `rustflags` from `.cargo/configs`s in reversed order

[petrochenkov]

Consider this setup:

outer
├── .cargo
│   └── config
└── inner
    ├── .cargo
    │   └── config
    ├── Cargo.toml
    └── src
        └── main.rs

Less specific outer/.cargo/config contains

[build]
rustflags = ["-Ccodegen-units=1"]

More specific outer/inner/.cargo/config contains

[build]
rustflags = ["-Ccodegen-units=2"]

The compiler's command line in this case is

rustc --crate-name inner --edition=2018 'src\main.rs' ... -Ccodegen-units=2 -Ccodegen-units=1

rustc like many other tools prefers the last specified value for the given option to support hierarchical build systems.
With Cargo's flag concatenation behavior this means that less specific flags override more specific flags, which is not a correct behavior.

cc rust-lang/rust#70729 (comment)

[ehuss]

@petrochenkov (or anyone following this issue), we are looking to change the order of how arrays get merged in #12515. Do you have a sense of the risk that might pose in breaking existing environments?