What is the difference between a mutable and immutable variable in Rust?

In Rust, variables can be classified as either "mutable" or "immutable".

An "immutable variable" is a variable that cannot be modified after it is created. It is declared using the keyword "let". Once an immutable variable is assigned a value, it cannot be reassigned. The value it holds is read-only.

On the other hand, a "mutable variable" is a variable that can be modified after it is created. It is declared using the keyword "let mut" or "var". The value it holds can be changed at any time.

The main difference between mutable and immutable variables is that immutable variables cannot be reassigned, while mutable variables can be reassigned any number of times. The immutable variables are the default in Rust, and can be passed around without fear of them being modified by mistake. While mutable variables are used in case we want to change their value in a specific scope.

It's important to note that even though an immutable variable can't be reassigned, it can still contain a mutable object, like a struct, and the fields of that struct can still be modified.

In summary, the main difference is that a mutable variable can be changed after it is created, while an immutable variable can't. This allows for more predictable behavior, and the ability to catch potential bugs early.