rustdoc: very unhelpful span info

[est31]

Do the following:

git clone https://github.com/rust-onig/rust-onig
cd rust-onig
git checkout 2381ea7fdc4b2dfef0905181cf26aa21b5a2ab17
git submodule update --init
cargo doc

Will yield the following output:

$ cargo doc
   Compiling onig_sys v66.1.1 (file://~/src/rust-onig/onig_sys)
 Documenting onig_sys v66.1.1 (file://~/src/rust-onig/onig_sys)
error: unknown start of token: `
 --> <stdin>:1:1
  |
1 | ```c
  | ^
  |
help: unicode character '`' (Grave Accent) looks like ''' (Single Quote), but it's not
 --> <stdin>:1:1
  |
1 | ```c
  | ^

error: Could not document `onig_sys`.

Caused by:
  process didn't exit successfully: `rustdoc --crate-name onig_sys onig_sys/src/lib.rs -o ~/src/rust-onig/target/doc -L dependency=~/src/rust-onig/target/debug/deps --extern libc=~/src/rust-onig/target/debug/deps/liblibc-db55a80d54675fa5.rlib` (exit code: 101)

This is very hard to debug! Which "```" section does Rust error on? Rustdoc should provide good and clear span info on this, with line numbers relative to the file, not relative to something else.

I could only debug this through removing each single ``` section and checking whether the error persists. This way I've found out that the following section is causing this:

    ///   Iterate function call for all names.
    ///
    ///  `int onig_foreach_name(regex_t* reg,
    ///                         int (*func)(const UChar*, const UChar*, int,int*,regex_t*,void*),
    ///                         void* arg)`
    ///
    /// # Returns
    ///
    ///  * normal return: 0
    ///  * error:         func's return value.
    ///
    /// # Arguments
    ///
    ///   1. reg:     regex object.
    ///   2. func:    callback function.
    ///
    ///     ```c
    ///     func(name, name_end, <number of groups>, <group number's list>,
    ///          reg, arg);
    ///     ```
    ///
    ///     if func does not return 0, then iteration is stopped.
    ///   3. arg:     argument for func.
    pub fn onig_foreach_name(reg: OnigRegex,