tock-register-interface: make std_unit_tests feature additive

As documented in the Cargo reference, features of a crate should
always be additive due to the way feature unification works across
transitive dependencies. This changes the `no_std_unit_tests` feature
to a `std_unit_tests` feature, which is enabled by default.

Signed-off-by: Leon Schuermann <leon@is.currently.online>

Author: lschuermann

libraries/tock-register-interface/Cargo.toml

@@ -15,11 +15,14 @@ edition = "2018"
 travis-ci = { repository = "tock/tock", branch = "master" }
 
 [features]
-default = [ "register_types" ]
+default = [ "register_types", "std_unit_tests" ]
 
 # Include actual register types (except LocalRegisterCopy). Disabling
 # the feature makes this an interface-only library and removes all
 # usage of unsafe code
 register_types = []
 
-no_std_unit_tests = []
+# Feature flag to enable generation of unit tests for the
+# registers. Enabling this may break compilation in
+# `custom-test-frameworks` environments.
+std_unit_tests = []

libraries/tock-register-interface/README.md

@@ -95,13 +95,15 @@ padding are consistent with the actual fields in the struct, and that alignment
 is correct.
 
 Since those tests would break compilation in `custom-test-frameworks`
-environments, it is possible to opt out of the test generation. To do so, add
-the following cargo feature:
+environments, it is possible to opt out of the test generation. To do
+so, disable the default feature set containing the `std_unit_tests`
+feature:
 
 ```toml
 [dependencies.tock-registers]
 version = "0.4.x"
-features = ["no_std_unit_tests"]
+default-features = false
+features = ["register_types"]
 ```
 
 By default, the visibility of the generated structs and fields is private. You

libraries/tock-register-interface/src/fields.rs

@@ -452,7 +452,7 @@ macro_rules! register_bitfields {
     }
 }
 
-#[cfg(not(feature = "no_std_unit_tests"))]
+#[cfg(feature = "std_unit_tests")]
 #[cfg(test)]
 mod tests {
     #[derive(Debug, PartialEq, Eq)]

libraries/tock-register-interface/src/macros.rs

@@ -195,7 +195,7 @@ macro_rules! test_fields {
     };
 }
 
-#[cfg(not(feature = "no_std_unit_tests"))]
+#[cfg(feature = "std_unit_tests")]
 #[macro_export]
 macro_rules! register_structs {
     {
@@ -224,7 +224,7 @@ macro_rules! register_structs {
     };
 }
 
-#[cfg(feature = "no_std_unit_tests")]
+#[cfg(not(feature = "std_unit_tests"))]
 #[macro_export]
 macro_rules! register_structs {
     {