make `for PAT in ITER_EXPR { ... }` a terminating-scope for ITER_EXPR.
In effect, temporary anonymous values created during the evaluation of ITER_EXPR no longer not live for the entirety of the block surrounding the for-loop; instead they only live for the extent of the for-loop itself, and no longer.
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There is one case I know of that this breaks, demonstrated to me by @nikomatsakis (but it is also a corner-case that is useless in practice). Here is that case:
```
fn main() {
let mut foo: Vec<&i8> = Vec::new();
for i in &[1, 2, 3] { foo.push(i) }
}
```
Note that if you add any code following the for-loop above, or even a semicolon to the end of it, then the code will stop compiling (i.e., it gathers a vector of references but the gathered vector cannot actually be used.)
(The above code, despite being useless, did occur in one run-pass test by accident; that test is updated here to accommodate the new striction.)
----
So, technically this is a:
[breaking-change]