Commits on Jan 28, 2021

1. rustc: Stabilize -Zrun-dsymutil as -Csplit-debuginfo …

   This commit adds a new stable codegen option to rustc,
   `-Csplit-debuginfo`. The old `-Zrun-dsymutil` flag is deleted and now
   subsumed by this stable flag. Additionally `-Zsplit-dwarf` is also
   subsumed by this flag but still requires `-Zunstable-options` to
   actually activate. The `-Csplit-debuginfo` flag takes one of
   three values:
   
   * `off` - This indicates that split-debuginfo from the final artifact is
     not desired. This is not supported on Windows and is the default on
     Unix platforms except macOS. On macOS this means that `dsymutil` is
     not executed.
   
   * `packed` - This means that debuginfo is desired in one location
     separate from the main executable. This is the default on Windows
     (`*.pdb`) and macOS (`*.dSYM`). On other Unix platforms this subsumes
     `-Zsplit-dwarf=single` and produces a `*.dwp` file.
   
   * `unpacked` - This means that debuginfo will be roughly equivalent to
     object files, meaning that it's throughout the build directory
     rather than in one location (often the fastest for local development).
     This is not the default on any platform and is not supported on Windows.
   
   Each target can indicate its own default preference for how debuginfo is
   handled. Almost all platforms default to `off` except for Windows and
   macOS which default to `packed` for historical reasons.
   
   Some equivalencies for previous unstable flags with the new flags are:
   
   * `-Zrun-dsymutil=yes` -> `-Csplit-debuginfo=packed`
   * `-Zrun-dsymutil=no` -> `-Csplit-debuginfo=unpacked`
   * `-Zsplit-dwarf=single` -> `-Csplit-debuginfo=packed`
   * `-Zsplit-dwarf=split` -> `-Csplit-debuginfo=unpacked`
   
   Note that `-Csplit-debuginfo` still requires `-Zunstable-options` for
   non-macOS platforms since split-dwarf support was *just* implemented in
   rustc.
   
   There's some more rationale listed on rust-lang#79361, but the main gist of the
   motivation for this commit is that `dsymutil` can take quite a long time
   to execute in debug builds and provides little benefit. This means that
   incremental compile times appear that much worse on macOS because the
   compiler is constantly running `dsymutil` over every single binary it
   produces during `cargo build` (even build scripts!). Ideally rustc would
   switch to not running `dsymutil` by default, but that's a problem left
   to get tackled another day.
   
   Closes rust-lang#79361

   

   alexcrichton committed Jan 28, 2021
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