Added error explanations for E0308, E0309, and E0310

[cactorium]

For #24407

[rust-highfive]

r? @huonw

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[apasel422]

This should be "can't be contained".

[cactorium]

Ah, sorry, fixed

[tomprogrammer]

You mean "…variable has." (forgot a 'h')

[pnkfelix]

I agree with @cactorium that original is grammatically correct but also difficult to read.

I suggest perhaps:

... 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`

(Or drop the example if you're not into that. :) )

[cactorium]

That's way better worded :)! Thanks!

[cactorium]

Nah, it's correct as is. It's the weird part of English where you have to put preposition after the object of the verb (eg. "Where are you going to?"). I'll see if I can come up with a way to make it less confusing though

[pnkfelix]

@bors r+ 4174aa4 rollup