Commits on Mar 29, 2018

1. rustc: Group linked libraries where needed …

   This commit fixes a longstanding issue with the compiler with circular
   dependencies between libcore and libstd. The `core` crate requires at least one
   symbol, the ability to unwind. The `std` crate is the crate which actually
   defines this symbol, but the `std` crate also depends on the `core` crate.
   
   This circular dependency is in general disallowed in Rust as crates cannot have
   cycles amongst them. A special exception is made just for core/std, but this is
   also unfortunately incompatible with how GNU linkers work. GNU linkers will
   process undefined symbols in a left-to-right fashion, only actually linking an
   rlib like libstd if there are any symbols used from it. This strategy is
   incompatible with circular dependencies because if we otherwise don't use
   symbols from libstd we don't discover that we needed it until we're later
   processing libcore's symbols!
   
   To fix this GNU linkers support the `--start-group` and `--end-group` options
   which indicate "libraries between these markers may have circular dependencies
   amongst them. The linker invocation has been updated to automatically pass these
   arguments when we're invoking a GNU linker and automatically calculate where the
   arguments need to go (around libstd and libcore)
   
   Closes rust-lang#18807
   Closes rust-lang#47074

   

   alexcrichton committed Mar 29, 2018
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