Commits on Jun 4, 2021

1. rustc: Store metadata-in-rlibs in object files …

   This commit updates how rustc compiler metadata is stored in rlibs.
   Previously metadata was stored as a raw file that has the same format as
   `--emit metadata`. After this commit, however, the metadata is encoded
   into a small object file which has one section which is the contents of
   the metadata.
   
   The motivation for this commit is to fix a common case where rust-lang#83730
   arises. The problem is that when rustc crates a `dylib` crate type it
   needs to include entire rlib files into the dylib, so it passes
   `--whole-archive` (or the equivalent) to the linker. The problem with
   this, though, is that the linker will attempt to read all files in the
   archive. If the metadata file were left as-is (today) then the linker
   would generate an error saying it can't read the file. The previous
   solution was to alter the rlib just before linking, creating a new
   archive in a temporary directory which has the metadata file removed.
   
   This problem from before this commit is now removed if the metadata file
   is stored in an object file that the linker can read. The only caveat we
   have to take care of is to ensure that the linker never actually
   includes the contents of the object file into the final output. We apply
   similar tricks as the `.llvmbc` bytecode sections to do this.
   
   This involved changing the metadata loading code a bit, namely updating
   some of the LLVM C APIs used to use non-deprecated ones and fiddling
   with the lifetimes a bit to get everything to work out. Otherwise though
   this isn't intended to be a functional change really, only that metadata
   is stored differently in archives now.
   
   This should end up fixing rust-lang#83730 because by default dylibs will no
   longer have their rlib dependencies "altered" meaning that
   split-debuginfo will continue to have valid paths pointing at the
   original rlibs. (note that we still "alter" rlibs if LTO is enabled to
   remove Rust object files and we also "alter" for the #[link(cfg)]
   feature, but that's rarely used).
   
   Closes rust-lang#83730

   

   alexcrichton committed Jun 4, 2021
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