Improve error reporting: Let as last line in block

[isaacabraham]

What

(From @tpetricek). The following code snippet leads to the resulting error message: -

let sum = 0
for x in 0 .. 10 do
    let sum = sum + x

error FS0588: Block following this 'let' is unfinished. Expect an expression.

Why

The above error message, whilst factually correct, does not explain to the beginner what this is and why it occurs. There are of course two errors here - one that the user is trying to mutate a value (which should be ignored from this issue as it is covered elsewhere) and the second which is that there is no result from the block.

How

Suggest: Every code block is an expression and must have a result. 'let' is not an expression and cannot be the final code element in a block. Consider returning something explicit from this block.

[kurtschelfthout]

Is this saying that ignore is never an option? I'm not sure that is the case but also I can't come up with a really convincing counter-example so perhaps it is. ConcurrentDictionary.TryRemove perhaps? MethodBase.Invoke on a void method?

Ignore me. Thoroughly confused with another error. Time for coffee!

[isaacabraham]

@kurtschelfthout not sure I follow. Ignore should be valid - it would just return unit from the block.

[forki]

PR in #1164