Function write_custom_entry 

pub fn write_custom_entry(
    applications: &Path,
    command: &str,
) -> Result<(String, String)>

Writes a desktop entry for a command a person typed, and hands back the entry’s file name.

This is the “Other…” answer in a chooser: none of the applications offered, run this instead. The entry goes in the user’s own applications directory, marked NoDisplay=true so it stays out of menus — it is a record of one person’s one-off choice, not an application anybody installed.

The name follows the convention every other implementation uses (word-usercreated-1.desktop), counting up rather than overwriting, so two different commands beginning with the same word do not quietly become one.

Returns the entry’s file name and the name written inside it. Both, because a caller needs to show one and record the other, and working the name out a second time is how they came to disagree: the entry on disk said zeditor while the caller’s copy said /usr/bin/zeditor.

command is written into Exec= as given. Nothing here parses it — see this module’s doc — which does mean a command that is nonsense produces an entry that fails to launch. That is visible immediately and recoverable by choosing again; guessing at what somebody meant would not be.