One negative side-effect of this change is that there might be quite a bit of copying strings out of the codemap, i.e. one copy for every block that gets translated, just for taking a look at the last character of the block. If this turns out to cause a performance problem then `CodeMap::span_to_snippet()` could be changed return `Option<&str>` instead of `Option<String>`.
Fixes#18791
This breaks code like
```
let t = (42i, 42i);
... t.0::<int> ...;
```
Change this code to not contain an unused type parameter. For example:
```
let t = (42i, 42i);
... t.0 ...;
```
Closes https://github.com/rust-lang/rust/issues/19096
[breaking-change]
r? @aturon
Use the expected type to infer the argument/return types of unboxed closures. Also, in `||` expressions, use the expected type to decide if the result should be a boxed or unboxed closure (and if an unboxed closure, what kind).
This supercedes PR #19089, which was already reviewed by @pcwalton.
