Remove the build script for miri

Setting the TARGET env var can be replaced with using
rustc_session::config::host_tuple at runtime. And the check-cfg can be
replaced with using the lints section in Cargo.toml.

src/tools/miri/Cargo.toml

@@ -67,6 +67,10 @@ default = ["stack-cache"]
 stack-cache = []
 stack-cache-consistency-check = ["stack-cache"]
 
+[lints.rust.unexpected_cfgs]
+level = "warn"
+check-cfg = ['cfg(bootstrap)']
+
 # Be aware that this file is inside a workspace when used via the
 # submodule in the rustc repo. That means there are many cargo features
 # we cannot use, such as profiles.

src/tools/miri/build.rs

@@ -1,10 +0,0 @@
-fn main() {
-    // Don't rebuild miri when nothing changed.
-    println!("cargo:rerun-if-changed=build.rs");
-    // Re-export the TARGET environment variable so it can be accessed by miri. Needed to know the
-    // "host" triple inside Miri.
-    let target = std::env::var("TARGET").unwrap();
-    println!("cargo:rustc-env=TARGET={target}");
-    // Allow some cfgs.
-    println!("cargo::rustc-check-cfg=cfg(bootstrap)");
-}

src/tools/miri/src/machine.rs

@@ -713,12 +713,13 @@ impl<'tcx> MiriMachine<'tcx> {
             clock: Clock::new(config.isolated_op == IsolatedOp::Allow),
             #[cfg(unix)]
             native_lib: config.native_lib.as_ref().map(|lib_file_path| {
+                let host_triple = rustc_session::config::host_tuple();
                 let target_triple = tcx.sess.opts.target_triple.tuple();
                 // Check if host target == the session target.
-                if env!("TARGET") != target_triple {
+                if host_triple != target_triple {
                     panic!(
                         "calling external C functions in linked .so file requires host and target to be the same: host={}, target={}",
-                        env!("TARGET"),
+                        host_triple,
                         target_triple,
                     );
                 }