debuginfo: Don't crash when encountering global variable with unknown source span.
Examples for Integer trait methods.
Main example for bigint usage.

Also use `///` for documentation instead of `/**`. End comments with
`.`.

Show imports for bigint example.

They might be useful.
auto merge of #15716 : treeman/rust/bigint-doc, r=alexcrichton

Also use `///` for documentation instead of `/**`. End comments with `.`.
auto merge of #15718 : treeman/rust/integer-doc, r=alexcrichton

Simple usage examples for Integer methods. Also group `div_rem` and `div_mod_floor` together at the bottom of the trait, to reflect the documentation rendering.
librustc: Remove cross-borrowing of `Box<T>` to `&T` from the language,
except where trait objects are involved.

Part of issue #15349, though I'm leaving it open for trait objects.
Cross borrowing for trait objects remains because it is needed until we
have DST.

This will break code like:

    fn foo(x: &int) { ... }

    let a = box 3i;
    foo(a);

Change this code to:

    fn foo(x: &int) { ... }

    let a = box 3i;
    foo(&*a);

[breaking-change]
auto merge of #15515 : pcwalton/rust/cross-borrowing, r=alexcrichton

except where trait objects are involved.

Part of issue #15349, though I'm leaving it open for trait objects.
Cross borrowing for trait objects remains because it is needed until we
have DST.

This will break code like:

    fn foo(x: &int) { ... }

    let a = box 3i;
    foo(a);

Change this code to:

    fn foo(x: &int) { ... }

    let a = box 3i;
    foo(&*a);

[breaking-change]

r? @alexcrichton
auto merge of #15719 : michaelwoerister/rust/global_var_null_span_fix, r=alexcrichton

This should fix issue #15541. It would be good to have an test case for this would also be nice but I haven't had the time to write one. The change is very small though and it doesn't break anything in the existing test suite, so I guess we can add it without test for now.
Add examples for DList methods rotate_forward, rotate_backward, append, prepend and insert_when