rustc: Exit quickly on only --emit dep-info

[alexcrichton]

This commit alters the compiler to exit quickly if the only output being emitted
is dep-info, which doesn't need a lot of other information to generate.

Closes #40328

[rust-highfive]

r? @arielb1

(rust_highfive has picked a reviewer for you, use r? to override)

[alexcrichton]

r? @nrc

This may not be the best method of doing so, but I figured you'd likely know the right direction!

[nrc]

@bors: r+

[bors]

📌 Commit 12dee0e has been approved by nrc

[luser]

@eddyb had suggested that the right place for a fix would be to set control.after_hir_lowering.stop like is done here:

rust/src/librustc_driver/lib.rs

Lines 492 to 500 in b04ebef

	if sess.opts.debugging_opts.no_analysis ||
	sess.opts.debugging_opts.ast_json {
	control.after_hir_lowering.stop = Compilation::Stop;
	}
	if !sess.opts.output_types.keys().any(|&i| i == OutputType::Exe ||
	i == OutputType::Metadata) {
	control.after_llvm.stop = Compilation::Stop;
	}

[eddyb]

Both approaches are valid FWIW, I don't mind this landing.

[eddyb]

@rust-lang/tools Do we want to backport this so it's fixed in the next stable? Or is that too risky?

[alexcrichton]

I'd personally be fine either way, but the next stable is real soon

[bors]

🔒 Merge conflict

[alexcrichton]

@bors: retry

[alexcrichton]

@bors: r=nrc

[bors]

📌 Commit 5c8aa74 has been approved by nrc