+impl Variance {
+ /// `a.xform(b)` combines the variance of a context with the
+ /// variance of a type with the following meaning. If we are in a
+ /// context with variance `a`, and we encounter a type argument in
+ /// a position with variance `b`, then `a.xform(b)` is the new
+ /// variance with which the argument appears.
+ ///
+ /// Example 1:
+ ///
+ /// *mut Vec<i32>
+ ///
+ /// Here, the "ambient" variance starts as covariant. `*mut T` is
+ /// invariant with respect to `T`, so the variance in which the
+ /// `Vec<i32>` appears is `Covariant.xform(Invariant)`, which
+ /// yields `Invariant`. Now, the type `Vec<T>` is covariant with
+ /// respect to its type argument `T`, and hence the variance of
+ /// the `i32` here is `Invariant.xform(Covariant)`, which results
+ /// (again) in `Invariant`.
+ ///
+ /// Example 2:
+ ///
+ /// fn(*const Vec<i32>, *mut Vec<i32)
+ ///
+ /// The ambient variance is covariant. A `fn` type is
+ /// contravariant with respect to its parameters, so the variance
+ /// within which both pointer types appear is
+ /// `Covariant.xform(Contravariant)`, or `Contravariant`. `*const
+ /// T` is covariant with respect to `T`, so the variance within
+ /// which the first `Vec<i32>` appears is
+ /// `Contravariant.xform(Covariant)` or `Contravariant`. The same
+ /// is true for its `i32` argument. In the `*mut T` case, the
+ /// variance of `Vec<i32>` is `Contravariant.xform(Invariant)`,
+ /// and hence the outermost type is `Invariant` with respect to
+ /// `Vec<i32>` (and its `i32` argument).
+ ///
+ /// Source: Figure 1 of "Taming the Wildcards:
+ /// Combining Definition- and Use-Site Variance" published in PLDI'11.
+ pub fn xform(self, v: ty::Variance) -> ty::Variance {
+ match (self, v) {
+ // Figure 1, column 1.
+ (ty::Covariant, ty::Covariant) => ty::Covariant,
+ (ty::Covariant, ty::Contravariant) => ty::Contravariant,
+ (ty::Covariant, ty::Invariant) => ty::Invariant,
+ (ty::Covariant, ty::Bivariant) => ty::Bivariant,
+
+ // Figure 1, column 2.
+ (ty::Contravariant, ty::Covariant) => ty::Contravariant,
+ (ty::Contravariant, ty::Contravariant) => ty::Covariant,
+ (ty::Contravariant, ty::Invariant) => ty::Invariant,
+ (ty::Contravariant, ty::Bivariant) => ty::Bivariant,
+
+ // Figure 1, column 3.
+ (ty::Invariant, _) => ty::Invariant,
+
+ // Figure 1, column 4.
+ (ty::Bivariant, _) => ty::Bivariant,
+ }
+ }
+}
+