Compile errors for some invalid feature combinations (atsamd-rs#469)

Emit compiler errors if no target chip is specified. Add the `library` Cargo feature to explicitly compile with no specified chip when necessary.

* Compile error if USB is requested but unsupported

* Compile error if no device feature is enabled

* Add `library` feature, reword errors and comments

.github/workflows/build-bsp.yml

@@ -69,4 +69,4 @@ jobs:
       run: |
         set -ex
         cd hal
-        cargo build
+        cargo build --features="library"

hal/Cargo.toml

@@ -146,14 +146,24 @@ optional = true
 
 [features]
 default = ["unproven"]
-# This section lists our feature name to dependency mapping.  This are separated
+
+# This section lists our feature name to dependency mapping.  This is separated
 # out so that the board support crates can reference a single feature name to
 # select the appropriate chip, and to keep that separate from the dependency name
 # for the `rt` feature below, which has to list out all possible optional deps.
 # If we simply used feature names that matched the dependency names, enabling the
 # rt feature would link all possible parts in and cause a linker error due to
 # the high degree of similar symbols present in the various parts.
-device = [] # Should be included if we are building for a specific target device.
+
+# The `device` feature is a dependency of each specific target device.  It
+# should not be enabled directly, as that could lead to cryptic build errors.
+# The `library` feature can be enabled to use atsamd-hal without selecting a
+# specific target device.
+device = []
+
+# Bypasses compile-time checks that a specific device is enabled.
+library = []
+
 samd11 = ["device"] # Convenience feature for shared configuration of samd11 chips.
 samd11c = ["atsamd11c", "samd11"]
 samd11c-rt = ["samd11c", "atsamd11c/rt"]

hal/src/lib.rs

@@ -6,6 +6,13 @@ pub use paste;
 
 pub mod typelevel;
 
+#[cfg(not(any(feature = "library", feature = "device")))]
+compile_error!(
+    "The HAL is usually built for a specific target device, selected using a \
+    feature.  If atsamd-hal is being built as a library, bypass this check by \
+    specifying the `library` feature"
+);
+
 #[cfg(feature = "samd11c")]
 pub use atsamd11c as target_device;
 
@@ -98,6 +105,15 @@ pub mod timer_traits;
 #[cfg(all(feature = "unproven", feature = "dma"))]
 pub mod dmac;
 
+#[cfg(all(feature = "usb", feature = "samd11"))]
+compile_error!("'usb' is enabled, but USB isn't supported on SAMD11");
+
+#[cfg(all(
+    feature = "usb",
+    not(any(feature = "samd21", feature = "min-samd51g", feature = "library"))
+))]
+compile_error!("The 'usb' feature is enabled, but not a chip with USB support");
+
 #[cfg(any(feature = "samd11", feature = "samd21"))]
 pub mod thumbv6m;
 #[cfg(any(feature = "samd11", feature = "samd21"))]