Read configuration from environment variables

[alexcrichton]

This commit adds a more principled system to rationalize what ends up being a
configuration value versus an environment variable. This problem is solved by
just saying that they're one and the same! Similar to Bundler, this commit
supports overriding the foo.bar configuration value with the CARGO_FOO_BAR
environment variable.

Currently this is used as part of the get_string and get_i64 methods on
Config. This means, for example, that the following environment variables can
now be used to configure Cargo:

• CARGO_BUILD_JOBS
• CARGO_HTTP_TIMEOUT
• CARGO_HTTP_PROXY

Currently it's not supported to encode a list in an environment variable, so for
example CARGO_PATHS would not be read when reading the global paths
configuration value.

cc #2362
cc #2395 -- intended to close this in tandem with #2397

[rust-highfive]

r? @wycats

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[alexcrichton]

Got a chance to talk about this with @wycats today and we felt good about it. Right now this doesn't support reading lists or tables from environment variables, but that can always come later if we decide to invent an encoding scheme.

r? @brson

[brson]

How does this impact RUSTFLAGS? It would seem that Cargo wants the variable to be called CARGO_RUSTFLAGS. I think I don't mind either way, but doing it with CARGO_ fails to establish a non-cargo convention (and differs from how the rust build itself supports rustflags).

[brson]

@bors r+

[bors]

📌 Commit a40440c has been approved by brson

[bors]

⌛️ Testing commit a40440c with merge 4a8a38c...

[alexcrichton]

Ah yeah I mentioned it in this comment. Basically I think it's fine that we would support RUSTFLAGS in addition to CARGO_BUILD_RUSTFLAGS.

The build.rustflags configuration is also a list of strings, which can't be read from environment variables with this PR, so it's still necessary that RUSTFLAGS exists in any case.

[bors]

☀️ Test successful - cargo-linux-32, cargo-linux-64, cargo-mac-32, cargo-mac-64, cargo-win-gnu-32, cargo-win-gnu-64, cargo-win-msvc-32, cargo-win-msvc-64