Building with old library causes ICE

[klutzy]

If I use extern mod a; where the library is built by old rustc, new rustc fails:

task 'rustc' failed at 'index out of bounds: the len is 4399208 but the index is 4399208', /home/lime/src/rust-vanilla/src/libextra/ebml.rs:111
error: internal compiler error: unexpected failure

There was similar reports before (e.g. #8882, #9189). It would be better to print some helpful errors (e.g. version mismatch).

[reem]

Triage: This now fails with a helpful error message about different versions of builtin crates, indicating you may want to rebuild. Should be closed.

[klutzy]

yay!