add nostd test crate

.github/workflows/main.yml

@@ -192,3 +192,17 @@ jobs:
         cargo +nightly-2022-11-17 run --target=${{ matrix.host_target }}
       env:
         RUST_BACKTRACE: 1
+
+    - name: test mustang-no-std as program
+      working-directory: test-crates/origin-as-just-a-library
+      run: |
+        cargo +nightly-2022-11-17 run -Zbuild-std=core,alloc --target=../../mustang/target-specs/${{ matrix.host_target }}.json
+      env:
+        RUST_BACKTRACE: 1
+
+    - name: test mustang-no-std as tests
+      working-directory: test-crates/origin-as-just-a-library
+      run: |
+        cargo +nightly-2022-11-17 test -Zbuild-std=core,alloc,test,std --target=../../mustang/target-specs/${{ matrix.host_target }}.json
+      env:
+        RUST_BACKTRACE: 1

test-crates/mustang-nostd/Cargo.toml

@@ -0,0 +1,12 @@
+[package]
+name = "mustang-nostd"
+version = "0.1.0"
+edition = "2021"
+
+# See more keys and their definitions at https://doc.rust-lang.org/cargo/reference/manifest.html
+
+[dependencies]
+# The mustang crate provides the `can_run_this!()` macro.
+mustang = { path = "../../mustang", default-features = false, features = ["threads", "default-alloc"] }
+
+[workspace]

test-crates/mustang-nostd/README.md

@@ -0,0 +1,21 @@
+This crate demonstrates the use of mustang with no_std. Specifying `-Zbuild-std=core,alloc` lets you skip compiling std as it will not be used:
+
+```
+cargo run --target=x86_64-mustang-linux-gnu -Zbuild-std=core,alloc
+```
+
+This line:
+
+```toml
+mustang = { path = "../../mustang", default-features = false, features = ["threads", "default-alloc"] }
+```
+
+tells cargo to not enable the "std" feature which results in no dependencies on std. c-gull will re-export the no-std version of c-scape. c-scape is still necessary because `dlmalloc` and `unwinding` rely on some libc functionality. `dlmalloc` uses libc for syscalls and needs a pthread implementation for `GlobalAlloc`. `unwinding` uses the libc `dl_iterate_phdr` function, see [comment](https://github.com/sunfishcode/mustang/blob/bf6b53a4c5edd1dec71fa65f468b2c76ff96eb62/mustang/Cargo.toml#L19).
+
+You can use either `#[start]` or replacing the C shim as in the example. See the unstable book for more [details](https://doc.rust-lang.org/unstable-book/language-features/lang-items.html#writing-an-executable-without-stdlib). It may or may not be undefined behavior to call `panic!()` from the function as no catch_unwind has been set, see the start feature [tracking issue](https://github.com/rust-lang/rust/issues/29633) and [following issue](https://github.com/rust-lang/rust/issues/107381).
+
+To use tests, make sure to also compile test and std:
+
+```
+cargo test --target=x86_64-mustang-linux-gnu -Zbuild-std=core,alloc,std,test
+```

test-crates/mustang-nostd/src/main.rs

@@ -0,0 +1,38 @@
+#![feature(lang_items)]
+#![no_std]
+// When testing we do not want to use our main function
+#![cfg_attr(not(test), no_main)]
+
+mustang::can_run_this!();
+
+use core::ffi::c_int;
+
+#[cfg(not(test))]
+#[panic_handler]
+fn panic(_panic: &core::panic::PanicInfo<'_>) -> ! {
+    loop {}
+}
+
+#[cfg(not(test))]
+#[lang = "eh_personality"]
+extern "C" fn eh_personality() {}
+
+// Small hack for rust-analyzer.
+//
+// If we do `#[cfg(not(test))]` then rust-analyzer will say the code is inactive and we
+// lose the ability to use rust-analyzer.
+// By disabling `no_mangle` during test, we do not get the two main functions defined error.
+// This function ends up just behaving like any other (unused) function in the binary during
+// test compilation.
+//
+// Using `#[start]` would not require us to use this hack but then we lose `_envp`
+// function parameter.
+#[cfg_attr(not(test), no_mangle)]
+extern "C" fn main(_argc: c_int, _argv: *mut *mut u8, _envp: *mut *mut u8) -> c_int {
+    0
+}
+
+#[test]
+fn test_tests() {
+    assert_eq!(1, 1)
+}