Use custom driver for sysroot building too

This required another custom driver to ensure that build scripts are
built using cg_llvm instead of cg_clif.

After this change only rustdoc still uses -Zcodegen-backend

build_sysroot/build_sysroot.sh

@@ -16,8 +16,8 @@ rm -r sysroot/ 2>/dev/null || true
 
 # Use rustc with cg_clif as hotpluggable backend instead of the custom cg_clif driver so that
 # build scripts are still compiled using cg_llvm.
-export RUSTC=rustc
-export RUSTFLAGS=$RUSTFLAGS" -Ztrim-diagnostic-paths=no -Zcodegen-backend=$(pwd)/../target/"$CHANNEL"/librustc_codegen_cranelift."$dylib_ext" --sysroot $(pwd)/sysroot"
+export RUSTC=$(pwd)/../"target/"$CHANNEL"/cg_clif_build_sysroot"
+export RUSTFLAGS=$RUSTFLAGS" --clif"
 
 # Build libs
 export RUSTFLAGS="$RUSTFLAGS -Zforce-unstable-if-unmarked -Cpanic=abort"

src/bin/cg_clif.rs

@@ -13,11 +13,11 @@ use rustc_session::early_error;
 use rustc_target::spec::PanicStrategy;
 
 #[derive(Default)]
-pub struct TimePassesCallbacks {
+pub struct CraneliftPassesCallbacks {
     time_passes: bool,
 }
 
-impl rustc_driver::Callbacks for TimePassesCallbacks {
+impl rustc_driver::Callbacks for CraneliftPassesCallbacks {
     fn config(&mut self, config: &mut interface::Config) {
         // If a --prints=... option has been given, we don't print the "total"
         // time because it will mess up the --prints output. See #64339.
@@ -47,7 +47,7 @@ impl rustc_driver::Callbacks for TimePassesCallbacks {
 fn main() {
     let start = std::time::Instant::now();
     rustc_driver::init_rustc_env_logger();
-    let mut callbacks = TimePassesCallbacks::default();
+    let mut callbacks = CraneliftPassesCallbacks::default();
     rustc_driver::install_ice_hook();
     let exit_code = rustc_driver::catch_with_exit_code(|| {
         let mut use_jit = false;

src/bin/cg_clif_build_sysroot.rs

@@ -0,0 +1,112 @@
+//! The only difference between this and cg_clif.rs is that this binary defaults to using cg_llvm
+//! instead of cg_clif and requires `--clif` to use cg_clif and that this binary doesn't have JIT
+//! support.
+//! This is necessary as with Cargo `RUSTC` applies to both target crates and host crates. The host
+//! crates must be built with cg_llvm as we are currently building a sysroot for cg_clif.
+//! `RUSTFLAGS` however is only applied to target crates, so `--clif` would only be passed to the
+//! target crates.
+
+#![feature(rustc_private)]
+
+extern crate rustc_data_structures;
+extern crate rustc_driver;
+extern crate rustc_interface;
+extern crate rustc_session;
+extern crate rustc_target;
+
+use std::path::PathBuf;
+
+use rustc_interface::interface;
+use rustc_session::config::ErrorOutputType;
+use rustc_session::early_error;
+use rustc_target::spec::PanicStrategy;
+
+fn find_sysroot() -> String {
+    // Taken from https://github.com/Manishearth/rust-clippy/pull/911.
+    let home = option_env!("RUSTUP_HOME").or(option_env!("MULTIRUST_HOME"));
+    let toolchain = option_env!("RUSTUP_TOOLCHAIN").or(option_env!("MULTIRUST_TOOLCHAIN"));
+    match (home, toolchain) {
+        (Some(home), Some(toolchain)) => format!("{}/toolchains/{}", home, toolchain),
+        _ => option_env!("RUST_SYSROOT")
+            .expect("need to specify RUST_SYSROOT env var or use rustup or multirust")
+            .to_owned(),
+    }
+}
+
+pub struct CraneliftPassesCallbacks {
+    use_clif: bool,
+}
+
+impl rustc_driver::Callbacks for CraneliftPassesCallbacks {
+    fn config(&mut self, config: &mut interface::Config) {
+        if !self.use_clif {
+            config.opts.maybe_sysroot = Some(PathBuf::from(find_sysroot()));
+            return;
+        }
+
+        // FIXME workaround for an ICE
+        config.opts.debugging_opts.trim_diagnostic_paths = false;
+
+        config.opts.cg.panic = Some(PanicStrategy::Abort);
+        config.opts.debugging_opts.panic_abort_tests = true;
+        config.opts.maybe_sysroot = Some(
+            std::env::current_exe()
+                .unwrap()
+                .parent()
+                .unwrap()
+                .parent()
+                .unwrap()
+                .parent()
+                .unwrap()
+                .join("build_sysroot")
+                .join("sysroot"),
+        );
+    }
+}
+
+fn main() {
+    rustc_driver::init_rustc_env_logger();
+    rustc_driver::install_ice_hook();
+    let exit_code = rustc_driver::catch_with_exit_code(|| {
+        let mut use_clif = false;
+
+        let args = std::env::args_os()
+            .enumerate()
+            .map(|(i, arg)| {
+                arg.into_string().unwrap_or_else(|arg| {
+                    early_error(
+                        ErrorOutputType::default(),
+                        &format!("Argument {} is not valid Unicode: {:?}", i, arg),
+                    )
+                })
+            })
+            .filter(|arg| {
+                if arg == "--clif" {
+                    use_clif = true;
+                    false
+                } else {
+                    true
+                }
+            })
+            .collect::<Vec<_>>();
+
+        let mut callbacks = CraneliftPassesCallbacks { use_clif };
+
+        rustc_driver::run_compiler(
+            &args,
+            &mut callbacks,
+            None,
+            None,
+            if use_clif {
+                Some(Box::new(move |_| {
+                    Box::new(rustc_codegen_cranelift::CraneliftCodegenBackend {
+                        config: rustc_codegen_cranelift::BackendConfig { use_jit: false },
+                    })
+                }))
+            } else {
+                None
+            },
+        )
+    });
+    std::process::exit(exit_code)
+}