Rust

Add `Context::request_discard` for multi-pass layouts #12641

Job	Run time
Check wasm32 + wasm-bindgen	49s
cargo-deny aarch64-apple-darwin	40s
android	23s
cargo-deny aarch64-linux-android	1m 10s
Format + check + test	32s
cargo-deny i686-pc-windows-gnu	39s
ios	52s
cargo-deny i686-pc-windows-msvc	40s
Check Windows	1m 23s
cargo-deny i686-unknown-linux-gnu	38s
cargo-deny wasm32-unknown-unknown	40s
cargo-deny x86_64-apple-darwin	37s
cargo-deny x86_64-pc-windows-gnu	38s
cargo-deny x86_64-pc-windows-msvc	38s
cargo-deny x86_64-unknown-linux-gnu	38s
cargo-deny x86_64-unknown-linux-musl	40s
cargo-deny x86_64-unknown-redox	41s
	12m 18s

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