robn@starmoth:~/code/rust/bldump$ cargo run
   Compiling bldump v0.1.0 (file:///home/robn/code/rust/bldump)
src/main.rs:394:26: 394:36 error: unable to infer enough type information about `_`; type annotations or generic parameter binding required [E0282]
src/main.rs:394     let stored_crc = rdr.read_u32().unwrap();
                                         ^~~~~~~~~~
src/main.rs:394:26: 394:36 help: run `rustc --explain E0282` to see a detailed explanation
error: aborting due to previous error
Could not compile `bldump`.

To learn more, run the command again with --verbose.
robn@starmoth:~/code/rust/bldump$ rustc --explain E0282
This error indicates that type inference did not result in one unique possible
type, and extra information is required. In most cases this can be provided
by adding a type annotation. Sometimes you need to specify a generic type
parameter manually.

A common example is the `collect` method on `Iterator`. It has a generic type
parameter with a `FromIterator` bound, which for a `char` iterator is
implemented by `Vec` and `String` among others. Consider the following snippet
that reverses the characters of a string:

```
let x = "hello".chars().rev().collect();
```

In this case, the compiler cannot infer what the type of `x` should be:
`Vec<char>` and `String` are both suitable candidates. To specify which type to
use, you can use a type annotation on `x`:

```
let x: Vec<char> = "hello".chars().rev().collect();
```

It is not necessary to annotate the full type. Once the ambiguity is resolved,
the compiler can infer the rest:

```
let x: Vec<_> = "hello".chars().rev().collect();
```

Another way to provide the compiler with enough information, is to specify the
generic type parameter:

```
let x = "hello".chars().rev().collect::<Vec<char>>();
```

Again, you need not specify the full type if the compiler can infer it:

```
let x = "hello".chars().rev().collect::<Vec<_>>();
```

Apart from a method or function with a generic type parameter, this error can
occur when a type parameter of a struct or trait cannot be inferred. In that
case it is not always possible to use a type annotation, because all candidates
have the same return type. For instance:

```
struct Foo<T> {
    // Some fields omitted.
}

impl<T> Foo<T> {
    fn bar() -> i32 {
        0
    }

    fn baz() {
        let number = Foo::bar();
    }
}
```

This will fail because the compiler does not know which instance of `Foo` to
call `bar` on. Change `Foo::bar()` to `Foo::<T>::bar()` to resolve the error.