Fix missed embedded-payload niche enum optimization (size enums) - #2
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LusterSourav wants to merge 1 commit into
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Fix missed embedded-payload niche enum optimization (size enums)#2LusterSourav wants to merge 1 commit into
LusterSourav wants to merge 1 commit into
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When the last variant of a niche-encoded enum has exactly one scalar
field at offset 0, its payload can be stored in the otherwise unused
tag bits, shrinking enums such as:
enum Unwind { Terminate(Reason), Cleanup(BasicBlock) }
from 8 bytes to 4 bytes.
`TagEncoding::Niche` gains `embedded_payload: Option<(VariantIdx, u128)>`
recording the embedded variant and how many distinct payload values fit
in the tag. The stable rustc_public mirror carries the same field so
serialized layouts round-trip. All existing consumers only pattern-match
the tag encoding with `..`.
Fixes rust-lang#160054.
LusterSourav
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Fixes rust-lang#160054. An enum whose last niche variant has a single scalar payload field at offset 0 can store that payload in the tag's remaining bits.
embedded_payload: Option<(VariantIdx, u128)>torustc_abi::TagEncoding::Nicheall_variants_fitskips the embedded variant's own layout; tag =niche_start + (i - start) + payloadrustc_publicso serialized layouts stay stableNichematches to..(const_eval, codegen_ssa, cranelift, llvm debuginfo, ty_utils, ty::layout)enum Unwind { Terminate(Reason), Cleanup(BasicBlock) }now 4 bytes (was 8), niche_start 0xFFFF_FF01, embedded_payload Some((2,2))Local: rustc_abi full test suite passes (incl. embedded_payload_niche_layout). LLVM/Cranelift paths covered by CI.