Use an alias to compile for Flipper Zero instead of build.target

Hard-coding `python3` in the runner means it doesn't work on Windows
because only `python` is provided. Conversely, Debian / Ubuntu don't
provide `python` by default. So it is more reliable to use `cargo`
itself as the runner. However, doing so means we need to be able to
compile for the host inside the `crates/` subdirectory.

It is currently not possible to revert Cargo config options to their
default, so the only way to compile for both host and Flipper Zero is
to set the target additively. We achieve this by defining an alias
`cargo test-flipper` that developers can use instead.

.github/workflows/_build_and_test.yml

@@ -24,7 +24,7 @@ jobs:
       run: cargo +nightly build --examples --release --verbose
     - name: Run tests
       run: |
-        cargo +nightly test --release --verbose 2>&1 | tee stderr.txt
+        cargo +nightly test-flipper --release --verbose 2>&1 | tee stderr.txt
     - name: Check that tests failed for the expected reason
       run: |
         cat stderr.txt | grep -q "Error: unable to find Flipper Zero"

crates/.cargo/config.toml

@@ -1,5 +1,18 @@
+[alias]
+test-flipper = "test --target thumbv7em-none-eabihf"
+
 [target.thumbv7em-none-eabihf]
-runner = "python3 ../cargo-runner.py"
+runner = [
+    "cargo",
+    "run",
+    "--manifest-path",
+    "../../tools/Cargo.toml",
+    "--quiet",
+    "--release",
+    "--bin",
+    "run-fap",
+    "--",
+]
 rustflags = [
     # CPU is Cortex-M4 (STM32WB55)
     "-C", "target-cpu=cortex-m4",
@@ -19,6 +32,3 @@ rustflags = [
     # Required to link with `lld`
     "-Z", "no-unique-section-names=yes",
 ]
-
-[build]
-target = "thumbv7em-none-eabihf"

crates/sys/src/lib.rs

@@ -2,6 +2,18 @@
 
 #![no_std]
 
+// Features that identify thumbv7em-none-eabihf.
+// Until target_abi is stable, this also permits thumbv7em-none-eabi.
+#[cfg(not(all(
+    target_arch = "arm",
+    target_feature = "thumb2",
+    target_feature = "v7",
+    target_feature = "dsp",
+    target_os = "none",
+    //target_abi = "eabihf",
+)))]
+core::compile_error!("This crate requires `--target thumbv7em-none-eabihf`");
+
 pub mod furi;
 mod inlines;