Include deduced locations for native libraries in rpath

[catamorphism]

From a FIXME in back::link:

    // FIXME: At some point we want to rpath our guesses as to where
    // native libraries might live, based on the addl_lib_search_paths

[thestinger]

This is really not something that should be done, installed libraries will already be in the search path. Using rpath is very fragile and won't survive changes to the system configuration.

[mzabaluev]

A particular problem here is native libraries that are part of the build. See here and here for an example: not only do I have to use -L and LD_LIBRARY_PATH for a library that the test crate should have no logical visibility for, I have to also explicitly bring it in as an external module.

Of course I could choose to build an install the tiny support library as a separate project, but it's annoying to have to do this. In the C land, we've long had libtool and similar solutions to take care of it.

[graydon]

closing as we have strong feedback that rpath is undesirable; we can approach along other avenues (static linking + use of workspace lib paths)

[jonhoo]

Given that rpath is a configurable parameter these days, and defaults to off, I wonder if this should be re-visited. If the user specifically opts into RPATH with -Crpath=y, it will probably be surprising to discover that only some of the paths that linked libraries were pulled from got written into the RPATH. It certainly surprised me 😅 In my case, I had to pass the paths already passed with -Clink-arg=-L through additional -Wl,rpath arguments through -Clink-arg= in rustflags to get things to work, which seems unfortunate. Though it could also be something the Rust compiler can't easily extract, I'm not sure?

Regardless of the decision ultimately made here, I think the nuances of this behavior should be documented in the -Crpath docs.

Just for reference, the mentioned FIXME now lives here:

rust/compiler/rustc_codegen_ssa/src/back/link.rs

Lines 1614 to 1616 in c5ecc15

	// FIXME (#2397): At some point we want to rpath our guesses as to
	// where extern libraries might live, based on the
	// add_lib_search_paths