By default, variables in Rust are immutable, meaning their value cannot be changed once assigned. To allow a variable to be mutable, the keyword "mut" must be used when declaring the variable.

Immutable example:
```rust
let x = 5;
```
This declares a variable named "x" and assigns it the value of 5. The type of x is inferred by the compiler to be i32.

Mutable example:
```rust
let mut y = 10;
y = 20;
```
This declares a mutable variable "y" with an initial value of 10 and then changes the value to 20.

Variable Mutability - Rust Programming

In Rust, a variable is a container that stores a value of a specific data type. Variables are declared using the keyword "let" and must be assigned a value when they are declared. 

University of Michigan - Ann Arbor

Rust is a compiled, multi-paradigm, general purpose programming language with a focus on reliability and safety. Rust boasts a rich type system and ownership model that guarantees memory and thread safety, eliminating multitudes of bugs at compile time.

Rust Programming Language

rust-lang.org

- rust-lang.org
- The Rust Programming Language Book
- crates.io

Rust Programming Language References

- rustc
- cargo
- rustdoc
- rustup
- rustfmt

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