add nostd test crate

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

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+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).

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

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+#![feature(lang_items)]
+#![no_std]
+#![no_main]
+
+mustang::can_run_this!();
+
+use core::{ffi::c_int, panic::PanicInfo};
+
+#[panic_handler]
+fn panic(_panic: &PanicInfo<'_>) -> ! {
+    loop {}
+}
+
+#[lang = "eh_personality"]
+extern "C" fn eh_personality() {}
+
+#[no_mangle]
+extern "C" fn main(_argc: c_int, _argv: *mut *mut u8, _envp: *mut *mut u8) -> c_int {
+    0
+}