The reference lacks the specification about the variance of lifetime parameters that have different variances in each position

subtyping.variance.user-composite-types has a wording

If the parameter is used in positions with different variances then the parameter is invariant.

However, it applies only to struct, enum, and union types

subtyping.variance.builtin-composite-types specifies the rules

When used outside of an struct, enum, or union, the variance for parameters is checked at each location separately.

However, it seems to only apply to tuple and function types.

Consider this type

struct A<'a>(&'a ());

type F<'b> = &'b mut A<'b>

The reference lacks the wording to specify the variance of lifetime 'b. That is, in &'b T, 'b is covariant and T is invariant.

It seems that 'b is expected to be invariant in type &'b mut A<'b>.

Start with the linked subtyping.variance.user-composite-types and subtyping.variance.builtin-composite-types sections in the Rust Reference, comparing their stated scope and rules. Clarify how variance is specified for &'b mut A<'b>, including the lifetime and type parameter positions, so the reference fully covers this case.