Book 1 · Lesson 1.7

Borrow checker deep dive

Fix common borrow checker errors
Explain mutable vs shared borrows
Connect compile failures to safer services

Prerequisites: 1.6

The borrow checker is Rust’s static proof that references never outlive data or alias illegally.

Borrow	Syntax	Rule
Shared	`&T`	Many readers OR one writer
Mutable	`&mut T`	Exclusive — no other borrows

Scenario 1 of 3

let mut v = vec![1]; let r = &mut v; v.push(2);

Lifetimes (preview)

When references appear in structs or return types, lifetime annotations ('a) tell the compiler how long borrows remain valid. Analyzer SQLx queries often return rows tied to connection lifetime.

EM angle

Borrow errors in CI = cheaper than data races in prod.

Teach-back prompt

Why is &mut incompatible with another active & to the same value?

Annotate lifetimes (preview)

Read analyzer SQLx query that returns rows.
Identify what owns the database connection.
Why can't a row reference outlive the connection?

Lesson 1.7 check

1. &mut T allows… Many writers One exclusive writer No compile checks Only WASM