The #line directive in C is limited, at best. Debugging flex/lex/yacc/bison, for instance, is sketchy at best. But of course line numbers are only part of the problem. If someone really wants to create Spiffy New Language as a Rust-front, then SNL structures/objects/collections/whoozits will want to pass symbolic information to the debugger so that users can explore their data structures using meaningful names.
And even so, this assumes that with debug symbols on, you can turn optimization off. Debugging optimized code gets wacky very fast because associating line numbers to source is not always possible any more, and gives non-intuitive results due to code motion and inlining even when it is possible. (You haven't lived until you've seen gdb's source line pointer jump around like a weasel on crack due to code re-arrangement.)
At Rust's current level of maturity, I suspect there are bigger fish to fry than passing debug info from Rust-fronts, so rejecting ambitious debug symbol machinery is probably the right strategic decision for the time being.