Variables and mutability

Most languages let variables change freely and offer const as an opt-in. Rust inverts it: immutable by default, changeable only by explicit request. This one inversion prevents a shocking number of bugs.

let: immutable by default

Uncomment camp = 2 and the compiler delivers one of its famous teaching-errors:

error[E0384]: cannot assign twice to immutable variable `camp`
help: consider making this binding mutable: `mut camp`

It names the problem and hands you the fix.

mut: opting into change

let mut announces "this value will change" — to the compiler and to every human reader. Scanning Rust code, the muts show you exactly where the moving parts are.

Shadowing: the third option

Rust lets you re-let a name — even changing its type:

Each let creates a new variable that shadows the old one. Idiomatic for transform pipelines: parse the text, keep the name.