add examples copied from Roc

Author: lukewilliamboswell

platforms/README.md

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+# Platforms
+
+This folder includes examples for roc platforms and interop with other languages.
+
+*Templates & Embedding/Interop Examples* 🗺
+
+You can also find some platform tempalates that are developed by the community. Here are some examples;
+- [Zig platform template](https://github.com/lukewilliamboswell/roc-platform-template-zig)
+- [Rust platform template](https://github.com/lukewilliamboswell/roc-platform-template-rust)
+- [Go platform template](https://github.com/lukewilliamboswell/roc-platform-template-go)
+- [Swift platform template](https://github.com/lukewilliamboswell/roc-platform-template-swift)
+- [C platform template](https://github.com/lukewilliamboswell/roc-platform-template-c)
+- [Python interop](https://github.com/roc-lang/roc/tree/main/examples/python-interop)
+- [Ruby interop](https://github.com/roc-lang/roc/tree/main/examples/ruby-interop)
+- [NodeJs interop](https://github.com/roc-lang/roc/tree/main/examples/nodejs-interop)
+- [Java interop](https://github.com/roc-lang/roc/tree/main/examples/jvm-interop)

platforms/glue/glue.roc

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+app [main] { pf: platform "rust-platform/main.roc" }
+
+main =
+    msg = "Roc <3 Rust, also on stderr!\n"
+    StdoutWrite "Roc <3 Rust!\n" \{} ->
+        StderrWrite msg \{} ->
+            Done

platforms/glue/rust-platform/.cargo/config.toml

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+[target.x86_64-pc-windows-gnu]
+linker = "zig-cc"

platforms/glue/rust-platform/Cargo.toml

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+[package]
+name = "host"
+version = "0.0.1"
+authors = ["The Roc Contributors"]
+license = "UPL-1.0"
+edition = "2021"
+links = "app"
+
+[lib]
+name = "host"
+path = "src/lib.rs"
+crate-type = ["staticlib", "lib"]
+
+[[bin]]
+name = "host"
+path = "src/main.rs"
+
+[dependencies]
+roc_app = { path = "roc_app" }
+roc_std = { path = "roc_std" }
+libc = "0.2"
+
+[workspace]

platforms/glue/rust-platform/build.rs

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+fn main() {
+    #[cfg(not(windows))]
+    println!("cargo:rustc-link-lib=dylib=app");
+
+    #[cfg(windows)]
+    println!("cargo:rustc-link-lib=dylib=libapp");
+
+    println!("cargo:rustc-link-search=.");
+}

platforms/glue/rust-platform/host.c

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+extern int rust_main();
+
+int main() { return rust_main(); }

platforms/glue/rust-platform/main.roc

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+platform "echo-in-rust"
+    requires {} { main : _ }
+    exposes []
+    packages {}
+    imports []
+    provides [mainForHost]
+
+# mainForHost : [StdoutWrite Str (({} -> Op) as Fx0), StderrWrite Str (({} -> Op) as Fx1), Done] as Op
+mainForHost : [StdoutWrite Str ({} -> Op), StderrWrite Str ({} -> Op), Done] as Op
+mainForHost = main
+
+# mainForHost : { x: Str, y: {} -> Str }
+# mainForHost =
+#     y = "foo"
+#
+#     when main is
+#         _ -> { x: "bar", y: \{} -> y }

platforms/glue/rust-platform/rust-toolchain.toml

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+[toolchain]
+channel = "1.77.2"
+
+profile = "default"
+
+components = [
+    # for usages of rust-analyzer or similar tools inside `nix develop`
+    "rust-src"
+]