&T Is a Shared Reference, Copy but Not Mutable

&T is a pointer that may be shared and it’s called shared reference. Each shared reference is Copy, so it can be duplicated without consuming the original, but it is not mutable and cast to a mutable one is not allowed.

Some edge cases exist to the last rule, see Reference Types and Interior Mutability > Interior mutability

See also

• Rust compiler validates a graph of flows between accesses — a shared reference creates a shared flow; multiple shared flows can coexist, but none can overlap with an exclusive flow (&mut T)
• Reference Types and Interior Mutability — the complete picture: &T is not “immutable” but “shared”; interior mutability is the controlled mechanism for mutation through a shared reference
• Lifetimes — every &T has an associated lifetime; the borrow checker validates that the shared reference does not outlive the value it points to
• ‘static Means Valid Until Program Shutdown — Static Variables Have It, But Not All ‘static References Point to Static Memory — a &'static T is a shared reference with the longest possible lifetime; string literals are the most common example
• False Sharing Forces CPUs to Serialise Access When Two Threads Write to the Same Cache Line — at the hardware level, multiple shared references reading the same cache line is safe and efficient; false sharing only occurs when writes are involved, which &T prevents at the language level

References

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