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23 lines
628 B
Rust
23 lines
628 B
Rust
use rustc_middle::ty;
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pub fn glb(v1: ty::Variance, v2: ty::Variance) -> ty::Variance {
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// Greatest lower bound of the variance lattice as
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// defined in The Paper:
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//
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// *
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// - +
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// o
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match (v1, v2) {
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(ty::Invariant, _) | (_, ty::Invariant) => ty::Invariant,
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(ty::Covariant, ty::Contravariant) => ty::Invariant,
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(ty::Contravariant, ty::Covariant) => ty::Invariant,
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(ty::Covariant, ty::Covariant) => ty::Covariant,
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(ty::Contravariant, ty::Contravariant) => ty::Contravariant,
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(x, ty::Bivariant) | (ty::Bivariant, x) => x,
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}
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}
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