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.

7 files changed, 81 insertions, 30 deletions

diff --git a/Cargo.toml b/Cargo.toml
index a29b3830..94ebc202 100644
--- a/Cargo.toml
+++ b/Cargo.toml
@@ -1,4 +1,5 @@
 [workspace]
+resolver = "2"
 members = [
     "autocrypt",
     "buffered-reader",
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"]
diff --git a/ipc/Cargo.toml b/ipc/Cargo.toml
index 038b1a7e..dfe652ae 100644
--- a/ipc/Cargo.toml
+++ b/ipc/Cargo.toml
@@ -55,10 +55,14 @@ tokio = { version = "1", features = [ "macros" ] }
 [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"]
diff --git a/net/Cargo.toml b/net/Cargo.toml
index 845dad79..724130fa 100644
--- a/net/Cargo.toml
+++ b/net/Cargo.toml
@@ -48,10 +48,14 @@ hyper = { version = "0.14", features = [ "server" ] }
 [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"]
diff --git a/openpgp/Cargo.toml b/openpgp/Cargo.toml
index 74f9319b..85874b7b 100644
--- a/openpgp/Cargo.toml
+++ b/openpgp/Cargo.toml
@@ -109,6 +109,7 @@ crypto-rust = [
 ]
 crypto-cng = ["eax", "winapi", "win-crypto-ng", "ed25519-dalek", "num-bigint-dig"]
 crypto-openssl = ["openssl", "openssl-sys", "foreign-types-shared"]
+__implicit-crypto-backend-for-tests = []
 
 # Experimental and variable-time cryptographic backends opt-ins
 allow-experimental-crypto = []
diff --git a/openpgp/build.rs b/openpgp/build.rs
index 9de8fb76..60f07620 100644
--- a/openpgp/build.rs
+++ b/openpgp/build.rs
@@ -63,13 +63,20 @@ fn crypto_backends_sanity_check() {
     }
 
     let backends = vec![
-        (cfg!(feature = "crypto-nettle"),
+        (cfg!(all(feature = "crypto-nettle",
+                  not(all(feature = "__implicit-crypto-backend-for-tests",
+                          any(feature = "crypto-openssl",
+                              feature = "crypto-rust"))))),
          Backend {
              name: "Nettle",
              production_ready: true,
              constant_time: true,
          }),
-        (cfg!(feature = "crypto-cng"),
+        (cfg!(all(feature = "crypto-cng",
+                  not(all(feature = "__implicit-crypto-backend-for-tests",
+                          any(feature = "crypto-nettle",
+                              feature = "crypto-openssl",
+                              feature = "crypto-rust"))))),
          Backend {
              name: "Windows CNG",
              production_ready: true,
diff --git a/openpgp/src/crypto/backend.rs b/openpgp/src/crypto/backend.rs
index 780b0bb1..28fcfe14 100644
--- a/openpgp/src/crypto/backend.rs
+++ b/openpgp/src/crypto/backend.rs
@@ -3,21 +3,51 @@
 
 pub(crate) mod sha1cd;
 
-#[cfg(feature = "crypto-nettle")]
+// Nettle is the default backend, but on Windows targets we instead
+// enable CNG for running the tests in non-leaf crates that depend on
+// sequoia-openpgp.  This creates a conflict, and makes `cargo test`
+// fail.  To mitigate this, only enable the Nettle backend if we are
+// not compiling the tests and have a different backend selected.
+//
+// Note: If you add a new crypto backend, add it to the expression,
+// and also synchronize the expression to `build.rs`.
+#[cfg(all(feature = "crypto-nettle",
+          not(all(feature = "__implicit-crypto-backend-for-tests",
+                  any(feature = "crypto-openssl",
+                      feature = "crypto-rust")))))]
 mod nettle;
-#[cfg(feature = "crypto-nettle")]
+#[cfg(all(feature = "crypto-nettle",
+          not(all(feature = "__implicit-crypto-backend-for-tests",
+                  any(feature = "crypto-openssl",
+                      feature = "crypto-rust")))))]
 pub use self::nettle::*;
 
+// Nettle is the default backend, but on Windows targets we instead
+// enable CNG for running the tests in non-leaf crates that depend on
+// sequoia-openpgp.  This creates a conflict, and makes `cargo test`
+// fail.  To mitigate this, only enable the CNG backend if we are
+// not compiling the tests and have a different backend selected.
+//
+// Note: If you add a new crypto backend, add it to the expression,
+// and also synchronize the expression to `build.rs`.
+#[cfg(all(feature = "crypto-cng",
+          not(all(feature = "__implicit-crypto-backend-for-tests",
+                  any(feature = "crypto-nettle",
+                      feature = "crypto-openssl",
+                      feature = "crypto-rust")))))]
+mod cng;
+#[cfg(all(feature = "crypto-cng",
+          not(all(feature = "__implicit-crypto-backend-for-tests",
+                  any(feature = "crypto-nettle",
+                      feature = "crypto-openssl",
+                      feature = "crypto-rust")))))]
+pub use self::cng::*;
+
 #[cfg(feature = "crypto-rust")]
 mod rust;
 #[cfg(feature = "crypto-rust")]
 pub use self::rust::*;
 
-#[cfg(feature = "crypto-cng")]
-mod cng;
-#[cfg(feature = "crypto-cng")]
-pub use self::cng::*;
-
 #[cfg(feature = "crypto-openssl")]
 mod openssl;
 #[cfg(feature = "crypto-openssl")]