Remove ENABLE_DOWNLOAD_RUSTC constant

[jyn514]

ENABLE_DOWNLOAD_RUSTC was introduced as part of the MVP for download-rustc as a way not to rebuild artifacts that have already been downloaded. 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 #79540 (comment) and #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
    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.

Helps with #81930.

r? @Mark-Simulacrum

[jyn514]

In addition to the things I mentioned in the description, I hope this also addresses the concern you had in the MVP:

I am unconvinced that the new Step constant is actually useful in practice and will work well, but it shouldn't affect people not enabling this option and we can iterate on that over time as needed. In particular, I am worried it excludes e.g. clippy or cargo work today; it seems like both of those should in theory support download-rustc as the interaction mode, but with this PR I believe wouldn't.

[Mark-Simulacrum]

Hm not sure I really follow this comment, there's nothing gating this is there? I think it'll always be the case that we skip this compile with download-rustc set?

[jyn514]

download-rustc works for everything besides tools without this block because it's handled in impl Step for Sysroot:

rust/src/bootstrap/compile.rs

Lines 923 to 933 in 8fb272c

	// If we're downloading a compiler from CI, we can use the same compiler for all stages other than 0.
	if builder.config.download_rustc {
	assert_eq!(
	builder.config.build, compiler.host,
	"Cross-compiling is not yet supported with `download-rustc`",
	);
	// Copy the compiler into the correct sysroot.
	let stage0_dir = builder.config.out.join(&*builder.config.build.triple).join("stage0");
	builder.cp_r(&stage0_dir, &sysroot);
	return INTERNER.intern_path(sysroot);
	}

Only building rustc-dev skips the sysroot and builds rustc directly.

[Mark-Simulacrum]

Seems good to me, left a question; with that resolved r=me (happy to take another look or feel free to approve if you think there's a good answer/clarification update)

[jyn514]

Seems good to me, left a question; with that resolved r=me (happy to take another look or feel free to approve if you think there's a good answer/clarification update)

Thanks! I left an explanation which hopefully makes sense. I'm going to approve this in the meantime so people can start using download-rustc for tests, but I'm happy to improve the comment or make changes in a follow-up.

@bors r=Mark-Simulacrum

[bors]

📌 Commit 8fb272c has been approved by Mark-Simulacrum