Commits on Feb 24, 2021

1. Remove ENABLE_DOWNLOAD_RUSTC constant …

   This was introduced as part of the MVP for `download-rustc`.
   Unfortunately, it doesn't work very well:
   
   - Steps are ignored by default, which makes it easy to leave out a step
   that should be built. For example, the MVP forgot to enable any tests,
   so it was *only* possible to build locally.
   - It didn't work correctly even when it was enabled: calling
     `builder.ensure()` would completely ignore the constant and rebuild the
     step anyway. This has no obvious fix since `ensure()` has to return a
     `Step::Output`.
   
   Instead, this handles `download-rustc` in `impl Step for Rustc` and
   `impl Step for Std`, which to my knowledge are the only build steps that
   don't first go through `impl Step for Sysroot` (`Rustc` is used for
   the `rustc-dev` component).
   
   See rust-lang#79540 (comment)
   and rust-lang#81930 for further context.
   
   Here are some example runs with these changes and `download-rustc`
   enabled:
   
   ```
   $ x.py build src/tools/clippy
   Building stage1 tool clippy-driver (x86_64-unknown-linux-gnu)
       Finished release [optimized] target(s) in 1m 09s
   Building stage1 tool cargo-clippy (x86_64-unknown-linux-gnu)
       Finished release [optimized] target(s) in 0.11s
   $ x.py test src/tools/clippy
   Updating only changed submodules
   Submodules updated in 0.01 seconds
       Finished dev [unoptimized + debuginfo] target(s) in 0.09s
   Building stage1 tool clippy-driver (x86_64-unknown-linux-gnu)
       Finished release [optimized] target(s) in 0.09s
   Building rustdoc for stage1 (x86_64-unknown-linux-gnu)
       Finished release [optimized] target(s) in 0.28s
       Finished release [optimized] target(s) in 15.26s
        Running build/x86_64-unknown-linux-gnu/stage1-tools/x86_64-unknown-linux-gnu/release/deps/clippy_driver-8b407b140e0aa91c
   test result: ok. 592 passed; 0 failed; 3 ignored; 0 measured; 0 filtered out
   $ x.py build src/tools/rustdoc
   Building rustdoc for stage1 (x86_64-unknown-linux-gnu)
       Finished release [optimized] target(s) in 41.28s
   Build completed successfully in 0:00:41
   $ x.py test src/test/rustdoc-ui
   Building stage0 tool compiletest (x86_64-unknown-linux-gnu)
       Finished release [optimized] target(s) in 0.12s
   Building rustdoc for stage1 (x86_64-unknown-linux-gnu)
       Finished release [optimized] target(s) in 0.10s
   test result: ok. 105 passed; 0 failed; 0 ignored; 0 measured; 0 filtered out; finished in 8.15s
   $ x.py build compiler/rustc
       Finished dev [unoptimized + debuginfo] target(s) in 0.09s
   Build completed successfully in 0:00:00
   ```
   
   Note a few things:
   
   - Clippy depends on stage1 rustc-dev artifacts, but rustc didn't have to
     be recompiled. Instead, the artifacts were copied automatically.
   - All steps are always enabled. There is no danger of forgetting a step,
     since only the entrypoints have to handle `download-rustc`.
   - Building the compiler (`compiler/rustc`) automatically does no work.

   

   jyn514 committed Feb 24, 2021
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