backticks
auto merge of #12441 : kud1ing/rust/backticks, r=brson

Not all of those messages are covered by tests. I am not sure how to trigger them and where to put those tests.

Also some message patterns in the existing tests are not complete.
For example, i find `error: mismatched types: expected "i32" but found "char" (expected i32 but found char)` a bit repetitive, but as i can see there is no test covering that.