Cargo config reference should more clearly explain mapping complex keys to environment variable names

[b-spencer]

Problem

The cargo config reference page says this:

For each configuration key above of the form foo.bar the environment variable CARGO_FOO_BAR can also be used to define the value. For example the build.jobs key can also be defined by CARGO_BUILD_JOBS.

This is correct, and the simple example is useful. However, the transformation of non-alphanumeric characters is not documented and it isn't obvious how to map a more complex config parameter such as:

[target.aarch64-linux-android]
linker = "/some/linker"

to an environment variable. I tried many combinations unsuccessfully until I eventually found an old issue asking about the same problem:

I tried:

LD
LINKER
CARGO_BUILD_LINKER
CARGO_TARGET_i686-pc-windows-gnu_LINKER

But none had the same effect as a config file containing:

[target.i686-pc-windows-gnu]
linker = "c:/Rtools/mingw_32/bin/gcc"

Possible Solution(s)

I suggest expanding on the docs to give a complex example such as:

Replace any characters . or - in key names with _ and upper case the result. For example,

[target.aarch64-linux-android]
linker = "/some/linker"

can be set with the environment variable CARGO_TARGET_AARCH64_LINUX_ANDROID_LINKER=/some/linker.

I hope this helps the next person who stumbles across the need to set these values via the environment.