Commits on Nov 22, 2016

1. Implement the loop_break_value feature. …

   This implements RFC 1624, tracking issue rust-lang#37339.
   
   - `FnCtxt` (in typeck) gets a stack of `LoopCtxt`s, which store the
     currently deduced type of that loop, the desired type, and a list of
     break expressions currently seen. `loop` loops get a fresh type
     variable as their initial type (this logic is stolen from that for
     arrays). `while` loops get `()`.
   - `break {expr}` looks up the broken loop, and unifies the type of
     `expr` with the type of the loop.
   - `break` with no expr unifies the loop's type with `()`.
   - When building MIR, `loop` loops no longer construct a `()` value at
     termination of the loop; rather, the `break` expression assigns the
     result of the loop. `while` loops are unchanged.
   - `break` respects contexts in which expressions may not end with braced
     blocks. That is, `while break { break-value } { while-body }` is
     illegal; this preserves backwards compatibility.
   - The RFC did not make it clear, but I chose to make `break ()` inside
     of a `while` loop illegal, just in case we wanted to do anything with
     that design space in the future.
   
   This is my first time dealing with this part of rustc so I'm sure
   there's plenty of problems to pick on here ^_^

   

   goffrie committed Nov 22, 2016
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