Shrink wasm stack size

[martijneken]

Benchmarking has determined that the default wasm stack size is 1 MiB. This is quite a lot for small server environments, thus in the C++ SDK we set both stack size and initial heap to 64 KiB: proxy-wasm/proxy-wasm-cpp-sdk#174

In Rust the stack size can be set as follows:

• Bazel rustc_flags: -Clink-args=-zstack-size=65536
• Cargo CLI: RUSTFLAGS="-Clink-args=-zstack-size=65536" cargo build ...
• build.rs: println!("cargo:rustc-cdylib-link-arg=-zstack-size=65536");
• config.toml (not tested): rustflags = ["-C", "link-args=-zstack-size=65536"]

Is there a good way to get the Rust SDK to influence / set this value? It's not ergonomic for every user of proxy-wasm-rust-sdk to define their own build.rs file or config.toml file or use command line build parameters.

There is one way I know to accomplish this, but it comes with caveats:

• Add this line to the SDK's build.rs: println!("cargo:rustc-cdylib-link-arg=-zstack-size=65536");
• This works but is not officially supported: Tracking issue for rustc-cdylib-link-arg transitive warning rust-lang/cargo#9562
• See also discussion on this issue: rustc-link-arg does not propagate transitively rust-lang/cargo#9554
• It also generates a build warning: warning: [email protected]: cargo:rustc-cdylib-link-arg was specified in the build script of proxy-wasm v0.2.3-dev (/usr/local/google/home/mstevenson/git/proxy-wasm-rust-sdk), but that package does not contain a cdylib target

CC @mpwarres @PiotrSikora