Appendices

Appendix A — Compiler-error decoder: the 15 errors you will actually see

Code	Message (short)	What it means	Usual fix	Chapter
E0382	use of moved value	you used a value after giving it away	borrow (`&x`), clone, or restructure	6
E0499	cannot borrow as mutable more than once	two `&mut` alive at once	shorten one borrow, split the borrow	7
E0502	cannot borrow as mutable because also borrowed as immutable	reading and writing overlap	finish reading first; copy the value out	7
E0505	cannot move out because it is borrowed	moved a value while a reference to it is alive	end the borrow first	19
E0506	cannot assign to borrowed value	wrote to something being read	reorder	7
E0597	does not live long enough	a reference outlives its owner	move the owner to an outer scope, or own the data	19
E0515	cannot return reference to local	returned `&` to a value that dies on return	return an owned value	13
E0716	temporary value dropped while borrowed	`&String::new()`-style temporaries	bind the temporary with `let` first	13
E0106	missing lifetime specifier	the output borrows from an ambiguous input	add `<'a>`, or return an owned type	19
E0373	closure may outlive the current function	a thread or task closure borrows a local	`move`, and own the data	20, 28
E0277	trait bound not satisfied	wrong type, or missing `Send`/`Clone`/`Display`...	implement or derive the trait, change the type (`Rc` → `Arc`)	16, 28
E0308	mismatched types	often a stray `;` or a missing `.into()`/`?`	read the expected vs found types	5
E0599	no method found	missing trait import, or the wrong type/state	`use` the trait; check typestate	16, 62
E0004	non-exhaustive patterns	a `match` misses a case	add the arm (do not reach for `_` by reflex)	9
E0384	cannot assign twice to immutable variable	missing `mut`	`let mut`	3

rustc --explain E0502 prints a full explanation with examples for any code.