[WIP]: Do not store tag in uninhabited enum variants, or in the single inhabited variant.#145337

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zachs18 wants to merge 7 commits intorust-lang:masterfrom

zachs18:enum-uninhabited-or-single-variant-no-tag

Commits on Sep 24, 2025

Prepare offset_of for non-absent uninhabited variants in using tagless enum representations
zachs18
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Add/extend tests for offset_of! for enums with uninhabited variants
zachs18
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Add tests for Result<u32, (u32, !)> layout. The comments will be accurate after the commits which add the relevant layout optimizations.
zachs18
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Add more enum layout tests. The comments will be accurate after the commits which add the relevant layout optimizations.
zachs18
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Don't encode enum tag for uninhabited repr(Rust) enum variants
zachs18
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Do not store tag for single-inhabited-variant enums (if this results in a better layout)
zachs18
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[WIP] do not hold space for niche in uninhabited variants when using niche-filling layout.
zachs18
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