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Constants in Rust are different from immutable variables in several ways:

1. Constants are always immutable and cannot be changed at runtime, whereas immutable variables can be reassigned at runtime.

2. Constants are always evaluated at compile-time, whereas immutable variables are evaluated at runtime.

3. Constants can be used in a wider range of contexts, including as array sizes, function arguments, and as part of pattern matching. Immutable variables cannot be used in these contexts.

4. Constants have a stricter type-checking compared to immutable variables. For example, constants of different types cannot be added together, whereas immutable variables of different types can.

Therefore, constants provide a more robust and efficient way of defining values that are known and fixed at compile-time, while immutable variables are more flexible and can be modified at runtime.