Rust

Add `Context::request_discard` for multi-pass layouts #12639

Job	Run time
Check wasm32 + wasm-bindgen	42s
android	25s
Format + check + test	33s
cargo-deny aarch64-apple-darwin	38s
cargo-deny aarch64-linux-android	38s
ios	20s
cargo-deny i686-pc-windows-gnu	40s
Check Windows	1m 58s
cargo-deny i686-pc-windows-msvc	43s
cargo-deny i686-unknown-linux-gnu	40s
cargo-deny wasm32-unknown-unknown	42s
cargo-deny x86_64-apple-darwin	36s
cargo-deny x86_64-pc-windows-gnu	42s
cargo-deny x86_64-pc-windows-msvc	40s
cargo-deny x86_64-unknown-linux-gnu	40s
cargo-deny x86_64-unknown-linux-musl	42s
cargo-deny x86_64-unknown-redox	40s
	11m 59s

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