This PR solves the following issues (or at least help users to understand the problem):
```Rust
#![crate_name = "b"]
#![crate_type = "rlib"]
pub fn his_function_has_a_very_long_name_and_should_make_cargo_doc_explodes_because_it_will_want_to_make_a_filename_with_it_in_excess_of_the_max_filename_length_for_most_filesystem_this_is_not_yet_long_enough_i_need_moreis_function_has_a_very_long_name_and_should_make_cargo_doc_explodes_because_it_will_want_to_make_a_filename_with_it_in_excess_of_the_max_filename_length_for_most_filesystem_this_is_not_yet_long_enough_i_need_more_() {}
```
```Rust
#![crate_name = "b"]
#![crate_type = "rlib"]
pub struct his_function_has_a_very_long_name_and_should_make_cargo_doc_explodes_because_it_will_want_to_make_a_filename_with_it_in_excess_of_the_max_filename_length_for_most_filesystem_this_is_not_yet_long_enough_i_need_moreis_function_has_a_very_long_name_and_should_make_cargo_doc_explodes_because_it_will_want_to_make_a_filename_with_it_in_excess_of_the_max_filename_length_for_most_filesystem_this_is_not_yet_long_enough_i_need_more_;
```
For the maximum filename length chosen, @gkoz gave me [this link](http://unix.stackexchange.com/a/32834).
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