Add a forbid-output property in cfail tests
Pop the expansion context after expanding a method macro

We were leaving these on the stack, causing spurious backtraces.

I've confirmed that this test fails without the fix.
std: Don't require bitflags! be u32

If you didn't have a trailing comma at the end of the variants, you could use
any type you wanted, but if you used a trailing comma the macro would
erroneously require the bits be a u32.
auto merge of #17319 : kmcallister/rust/method-macro-bt, r=pcwalton

We were leaving these on the stack, causing spurious backtraces.
auto merge of #17404 : alexcrichton/rust/bitflags-u32, r=sfackler

If you didn't have a trailing comma at the end of the variants, you could use
any type you wanted, but if you used a trailing comma the macro would
erroneously require the bits be a u32.
Add detection of unused enum variants
Use more descriptive names in dead code messages
Remove unused enum variants
Remove dead code from librustc