Change the debug! macro ast expansion for temporaries

[alexcrichton]

Currently, expanding debug!("{}", foo) looks something like:

static PRECOMPILED_FORMAT_STRING = ...;
let foo_addr = &foo;
let arguments: &fmt::Arguments = unsafe { /* do things with foo_addr */ };
fmt::writeln(logger, arguments)

This is a problem with the new temporary rules because you can't debug!("{}", some_ref_cell.borrow().get()). This is because the borrow() ends after the enclosing statement which in this case is let foo_addr = &foo;

In order to allow debugging these values, we should change the ast expansion to:

static PRECOMPILED_FORMAT_STRING = ...;
match foo {
    ref foo_addr => {
        let arguments: &fmt::Arguments = unsafe { /* do things with foo_addr */ };
        fmt::writeln(logger, arguments)
    }
}

Note that with a match the enclosing statement encompasses the entire formatting call, so debugging foo.borrow().get() should be valid. One other portion of this bug would be ensuring that codegen doesn't suffer in terms of compile-time or size as a result of this change. It is currently unknown how much a match scope would affect codegen.

cc @nikomatsakis

[edwardw]

This may break some code. For example, format!("expected constant: {}", (*err)); won't compile any more because (*err) will end up in the match head and deny(unnecessary_parens) is turned on by default.

[edwardw]

I saw several such compiling errors in librustc itself already.

[nikomatsakis]

Argh. This seems to be more of a bug with the linter, or at least a
very unfortunate interaction! I feel like linters (or at least this
linter) should ignore code generated by macro expansion.