Improve 'not all control paths return a value' message

[Wilfred]

Given the code:

fn foo() -> u16 {
    1;
}

Rustc shows the error:

error: not all control paths return a value [E0269]

However, I think this is slightly misleading. There isn't actual a control path here, just a semicolon that shouldn't be there.

I think rustc should say something like "function does not return a value" when the last expression in a function is a simple expression without any control flow.

[nagisa]

Dupe-ish of #21838.

EDIT: it does not print semicolon suggestion in this exact case because the integer is infered to u32, while return value is u16. The hint code could be improved in presence of integers.

EDIT 2: this function does have a control path, albeit only a single one.

[dylanmckay]

This is caused because liveliness analysis will only print the missing semicolon warning if the types of the last statement and the return value of the function are equivalent.

I have a fix incoming.

Note that

fn foo() -> u16 {
    1u16;
}

Will print the "missing semicolon" diagnostic.

[birkenfeld]

Duplicate of #30497.