Add a feature to print actual static libs linked in, --print=native-static-libs seems not to work

[infinity0]

For Debian we need to keep track of static libs linked into a final binary, for copyright purposes.

It seems that src/librustc_codegen_llvm/back/link.rs performs the actual linking and search of paths, and it refers to a --print=native-static-libs option but this doesn't seem to work.

For example when I link to the system libbacktrace and then build this with RUSTFLAGS="--print=native-static-libs" cargo build --verbose --verbose --verbose I can see the build.rs output which is

cargo:rustc-link-lib=static=backtrace
cargo:rustc-link-search=native=/usr/lib/gcc/x86_64-linux-gnu/7/

But then I don't see any follow-up output that indicates that rustc actually found the file /usr/lib/gcc/x86_64-linux-gnu/7/libbacktrace.a and this was what was actually linked into the final backtrace-sys.rlib.

[infinity0]

@kornelski You were the one that originally implemented --print=native-static-libs in #43067 any comments?

[retep998]

This is not what --print=native-static-libs was intended for, but rather it is for when your crate type is staticlib and it tells you what libraries you must link to when linking to that staticlib.

Certainly though there is room for a feature here to confirm which static libraries the static kind actually ended up pulling in. We can't do the same for the dylib or static-nobundle kinds though because in those cases it is the linker itself that is responsible for choosing what to link to.

[pnkfelix]

(deleted comment that had missed the point of what the original issue filer was concerned about, and thus was forking the discussion into the wrong direction. My comment has now been refiled as #108825.)