This error occurs when the compiler was unable to infer the concrete type of a variable. It can occur for several cases, the most common of which is a mismatch in the expected type that the compiler inferred for a variable's initializing expression, and the actual type explicitly assigned to the variable.
For example:
let x: i32 = "I am not a number!"; // ~~~ ~~~~~~~~~~~~~~~~~~~~ // | | // | initializing expression; // | compiler infers type `&str` // | // type `i32` assigned to variable `x`