Don't select a cryptograhic backend in non-leaf crates.

  - This way, only the leaf package has to concern itself with the
    selection of a cryptographic backend for Sequoia.  Notably, we
    don't have to repeat all of sequoia-openpgp's features in all
    crates that use sequoia-openpgp.

  - Enable the new feature resolver which allows for this method.

  - A complication arises because we want to make `cargo test` work by
    default for the intermediate crates without developers having to
    select a cryptographic backend.  To make that work, we implicitly
    select a backend in the dev dependencies which are enabled when
    compiling the tests.  To make it even more convenient, we select
    the most convenient backend, which is CNG for Windows and Nettle,
    our default, for every other platform.

  - Now that we have implicitly selected CNG on Windows for running
    the tests, when the user wants to use Nettle on Windows, and does
    `cargo test --features sequoia-openpgp/crypto-nettle`, then two
    backends are selected: the implicitly selected CNG and the
    explicitly selected Nettle.  In this case, we detect that an
    implicit selection has been made, and ignore the implicitly
    selected backend.  Now, this has already been compiled by
    cargo (remember that we cannot influence the set of dependencies
    at the time the build script is run), but we can still ignore the
    implicit backend using conditional compilation (i.e. it will not
    be included in the resulting binary).  The same happens on
    non-Windows platforms where Nettle is the implicit default for
    tests when the user explicitly requests a different backend.  In
    both cases, Nettle and CNG are slim wrappers around native
    libraries, so the wasted compilation time is low.

1 files changed, 11 insertions, 7 deletions

diff --git a/autocrypt/Cargo.toml b/autocrypt/Cargo.toml
index b0e2fc0a..cca5041a 100644
--- a/autocrypt/Cargo.toml
+++ b/autocrypt/Cargo.toml
@@ -28,10 +28,14 @@ base64 = ">=0.12"
 [lib]
 bench = false
 
-[features]
-default = ["sequoia-openpgp/default"]
-crypto-nettle = ["sequoia-openpgp/crypto-nettle"]
-crypto-cng = ["sequoia-openpgp/crypto-cng"]
-compression = ["sequoia-openpgp/compression"]
-compression-deflate = ["sequoia-openpgp/compression-deflate"]
-compression-bzip2 = ["sequoia-openpgp/compression-bzip2"]
+# Enables a crypto backend for the tests:
+[target.'cfg(not(windows))'.dev-dependencies]
+sequoia-openpgp = { path = "../openpgp", version = "1", default-features = false, features = ["crypto-nettle", "__implicit-crypto-backend-for-tests"] }
+
+# Enables a crypto backend for the tests:
+[target.'cfg(windows)'.dev-dependencies]
+sequoia-openpgp = { path = "../openpgp", version = "1", default-features = false, features = ["crypto-cng", "__implicit-crypto-backend-for-tests"] }
+
+# Enables a crypto backend for the docs.rs generation:
+[package.metadata.docs.rs]
+features = ["sequoia-openpgp/default"]