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:
```compile_fail,E0308
let x: i32 = "I am not a number!";
// ~~~ ~~~~~~~~~~~~~~~~~~~~
// | |
// | initializing expression;
// | compiler infers type `&str`
// |
// type `i32` assigned to variable `x`
```