Set the proper sysroot for clippy

Clippy does its own runtime detection of the sysroot, which was
incorrect in this case (it used the beta sysroot). This overrides the
sysroot to use `stage0-sysroot` instead.

- Get `x.py clippy` to work on nightly
- Give a nice error message if nightly clippy isn't installed

Author: jyn514

src/bootstrap/builder.rs

@@ -850,7 +850,40 @@ impl<'a> Builder<'a> {
                 cargo.args(s.split_whitespace());
             }
             rustflags.env("RUSTFLAGS_BOOTSTRAP");
-            rustflags.arg("--cfg=bootstrap");
+            if cmd == "clippy" {
+                // clippy overwrites any sysroot we pass on the command line.
+                // Tell it to use the appropriate sysroot instead.
+                // NOTE: this can't be fixed in clippy because we explicitly don't set `RUSTC`,
+                // so it has no way of knowing the sysroot.
+                rustflags.arg("--sysroot");
+                rustflags.arg(
+                    self.sysroot(compiler)
+                        .as_os_str()
+                        .to_str()
+                        .expect("sysroot must be valid UTF-8"),
+                );
+                // Only run clippy on a very limited subset of crates (in particular, not build scripts).
+                cargo.arg("-Zunstable-options");
+                // Explicitly does *not* set `--cfg=bootstrap`, since we're using a nightly clippy.
+                let host_version = Command::new("rustc").arg("--version").output().map_err(|_| ());
+                if let Err(_) = host_version.and_then(|output| {
+                    if output.status.success()
+                        && t!(std::str::from_utf8(&output.stdout)).contains("nightly")
+                    {
+                        Ok(output)
+                    } else {
+                        Err(())
+                    }
+                }) {
+                    eprintln!(
+                        "error: `x.py clippy` requires a nightly host `rustc` toolchain with the `clippy` component"
+                    );
+                    eprintln!("help: try `rustup default nightly`");
+                    std::process::exit(1);
+                }
+            } else {
+                rustflags.arg("--cfg=bootstrap");
+            }
         }
 
         if self.config.rust_new_symbol_mangling {
@@ -975,7 +1008,6 @@ impl<'a> Builder<'a> {
         // src/bootstrap/bin/{rustc.rs,rustdoc.rs}
         cargo
             .env("RUSTBUILD_NATIVE_DIR", self.native_dir(target))
-            .env("RUSTC", self.out.join("bootstrap/debug/rustc"))
             .env("RUSTC_REAL", self.rustc(compiler))
             .env("RUSTC_STAGE", stage.to_string())
             .env("RUSTC_SYSROOT", &sysroot)
@@ -991,6 +1023,11 @@ impl<'a> Builder<'a> {
             )
             .env("RUSTC_ERROR_METADATA_DST", self.extended_error_dir())
             .env("RUSTC_BREAK_ON_ICE", "1");
+        // Clippy support is a hack and uses the default `cargo-clippy` in path.
+        // Don't override RUSTC so that the `cargo-clippy` in path will be run.
+        if cmd != "clippy" {
+            cargo.env("RUSTC", self.out.join("bootstrap/debug/rustc"));
+        }
 
         // Dealing with rpath here is a little special, so let's go into some
         // detail. First off, `-rpath` is a linker option on Unix platforms