&a T is covariant in 'a and T, &mut T is invariant in T

All types with generic parameters have a variance with respect to those parameters.

In Rust lifetimes form a subtype relationship: 'static is a subtype of 'a because it is valid for longer.

Key variance rules:

• &'a T is covariant in both 'a and T: you can provide &'static T where &'a T is expected
• &mut T is invariant in T: you must provide exactly T, not a subtype or supertype
• function arguments are contravariant — a function that accepts a longer-lived reference can be used where a shorter-lived one is expected

See also

• Variance Describes When a Subtype Can Be Used in Place of a Supertype, Covariant, Invariant, or Contravariant — the general concept this note applies to Rust lifetimes specifically
• A Lifetime Names the Region of Code a Reference Must Be Valid For, Not Necessarily the Full Scope — the subtype relationship between lifetimes is grounded in the definition: a longer lifetime is a subtype because it satisfies every constraint a shorter one would
• ‘static Means Valid Until Program Shutdown — Static Variables Have It, But Not All ‘static References Point to Static Memory — 'static is the bottom of the lifetime subtype hierarchy: it is a subtype of every lifetime, which is why &'static T can be used wherever &'a T is expected
• &mut T Is an Exclusive Reference, No Other Reference Can Coexist With It — &mut T being invariant in T is a direct consequence of its exclusivity guarantee; covariance would allow type-unsafe writes through the exclusive reference
• Reliability means preventing faults from causing failures — invariance of &mut T is a compile-time reliability mechanism: the unsoundness it prevents (writing the wrong type through an exclusive reference) would be a fault that propagates silently into a failure

References

• J. Gjengset, Rust for Rustaceans: Idiomatic Programming for Experienced Developers. No Starch Press, 2021.