Support compiling rustc without LLVM

[bjorn3]

Doesn't yet compile without llvm enabled.

Todo:

• Make crate metadata reading work without LLVM
• Make crate metadata writing work without LLVM

Error log when not using llvm:

$ ../x.py build
    Finished dev [unoptimized] target(s) in 0.0 secs
Building stage0 std artifacts (x86_64-apple-darwin -> x86_64-apple-darwin)
    Finished release [optimized] target(s) in 0.0 secs
Copying stage0 std from stage0 (x86_64-apple-darwin -> x86_64-apple-darwin / x86_64-apple-darwin)
Building stage0 test artifacts (x86_64-apple-darwin -> x86_64-apple-darwin)
    Finished release [optimized] target(s) in 0.0 secs
Copying stage0 test from stage0 (x86_64-apple-darwin -> x86_64-apple-darwin / x86_64-apple-darwin)
Building stage0 compiler artifacts (x86_64-apple-darwin -> x86_64-apple-darwin)
   Compiling rustc_driver v0.0.0 (file:///~/Documents/rust_fork/src/librustc_driver)
error[E0433]: failed to resolve. Use of undeclared type or module `rustc_trans`
   --> ~/Documents/rust_fork/src/librustc_driver/lib.rs:210:54
    |
210 |     let cstore = Rc::new(CStore::new(&dep_graph, box rustc_trans::LlvmMetadataLoader));
    |                                                      ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^ Use of undeclared type or module `rustc_trans`

error[E0433]: failed to resolve. Use of undeclared type or module `rustc_trans`
   --> ~/Documents/rust_fork/src/librustc_driver/lib.rs:417:66
    |
417 |                 let cstore = Rc::new(CStore::new(&dep_graph, box rustc_trans::LlvmMetadataLoader));
    |                                                                  ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^ Use of undeclared type or module `rustc_trans`

error: aborting due to 2 previous errors

error: Could not compile `rustc_driver`.

To learn more, run the command again with --verbose.
thread 'main' panicked at 'command did not execute successfully: "~/Documents/rust_fork/no_llvm_build/build/x86_64-apple-darwin/stage0/bin/cargo" "build" "-j" "8" "--target" "x86_64-apple-darwin" "--release" "--features" " jemalloc" "--manifest-path" "/Users/bjorn/Documents/rust_fork/src/rustc/Cargo.toml" "--message-format" "json"
expected success, got: exit code: 101', /Users/bjorn/Documents/rust_fork/src/bootstrap/compile.rs:555
stack backtrace:
   0: std::sys::imp::backtrace::tracing::imp::unwind_backtrace
   1: std::panicking::default_hook::{{closure}}
   2: std::panicking::default_hook
   3: std::panicking::rust_panic_with_hook
   4: std::panicking::begin_panic
   5: std::panicking::begin_panic_fmt
   6: bootstrap::compile::run_cargo
   7: bootstrap::compile::rustc
   8: bootstrap::step::build_rules::{{closure}}
   9: bootstrap::step::Rules::run
  10: bootstrap::step::run
  11: bootstrap::Build::build
  12: bootstrap::main
  13: __rust_maybe_catch_panic
  14: std::rt::lang_start
  15: main
failed to run: ~/Documents/rust_fork/no_llvm_build/build/bootstrap/debug/bootstrap build
Build completed unsuccessfully in 0:00:04

Edit: remove getting rustdoc working from todo list.

[rust-highfive]

r? @arielb1

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[bjorn3]

This would allow miri to run on platforms without llvm like redox os

[est31]

Note that @steveklabnik is currently working on a rewrite of rustdoc to get rid of most compiler dependencies, not sure if you want to duplicate work here.

[bjorn3]

@est31 thanks for pointing me to that!

Will remove the commit with rustdoc changes later this day.

[bors]

☔ The latest upstream changes (presumably #42664) made this pull request unmergeable. Please resolve the merge conflicts.

[brson]

Neat project.

There's quite a lot of cfg in the driver. Seems like a bit of a smell. And the driver is actually sliced up to completely remove trans. Eventually we would expect this to be more refined, with trans supporting multiple backends, and disabling llvm just disabling one particular backend. It might be more elegant here to sink all the driver cfg into trans, and have trans itself report that there is no backend.

[ids1024]

This would allow miri to run on platforms without llvm like redox os

Actually Rustc and LLVM now work under Redox (as part of my Google Summer of Code project to make Redox OS self-hosting). Of course that doesn't invalidate this PR which has many other potential benefits, but I thought I might as well mention it.

[bjorn3]

@ids1024 didn't know.

[bjorn3]

Closing in favor of #42932.