Issue 3488

[sanxiyn]

Fix for #3488.
Needs tests.
Needs to fix the pretty printer. (Help appreciated.)

[brson]

Thanks!

There are some notes on how to operate the test suite here: https://github.com/mozilla/rust/wiki/Note-testsuite

I'd suggest that this needs at least one test for 'mut' in normal function arguments, another in closure arguments, and the corresponding negative (compile-fail) tests that non-mutable arguments can't be mutated.

The pretty printer should be fairly straightforward, just find the code that prints function arguments and add mut if necessary.

[catamorphism]

I'm going to take the liberty of finishing this, since it's been sitting for a while.

[sanxiyn]

Thanks, and sorry for not following up. Feel free to take over.

[catamorphism]

I fixed the pretty-printer and added tests. Merged as of 54344c2 -- thanks!