adding a build script to re-export the TARGET env var so it can be read in miri

Author: Ellen Arteca

build.rs

@@ -0,0 +1,6 @@
+fn main() {
+    // Re-export the TARGET environment variable so it can
+    // be accessed by miri.
+    let target = std::env::var("TARGET").unwrap();
+    println!("cargo:rustc-env=TARGET={:?}", target);
+}

miri

@@ -105,7 +105,7 @@ esac
 ## Prepare the environment
 # Determine some toolchain properties
 # export the target so its available in miri
-export TARGET=$(rustc +$TOOLCHAIN --version --verbose | grep "^host:" | cut -d ' ' -f 2)
+TARGET=$(rustc +$TOOLCHAIN --version --verbose | grep "^host:" | cut -d ' ' -f 2)
 SYSROOT=$(rustc +$TOOLCHAIN --print sysroot)
 LIBDIR=$SYSROOT/lib/rustlib/$TARGET/lib
 if ! test -d "$LIBDIR"; then

src/shims/ffi_support.rs

@@ -221,6 +221,16 @@ pub trait EvalContextExt<'mir, 'tcx: 'mir>: crate::MiriEvalContextExt<'mir, 'tcx
 
         let this = self.eval_context_mut();
 
+        // Check if host target == the session target.
+        // This check needs to happen _after_ we check if the shared object file does not
+        // export the function: we don't want to throw an error if we are just going to use
+        // one of the shims.
+        if option_env!("TARGET") == Some(&this.tcx.sess.opts.target_triple.to_string()) {
+            throw_ub_format!(
+                "calling external C functions in linked .so file requires target and host to be the same"
+            );
+        }
+
         // Get the function arguments, and convert them to `libffi`-compatible form.
         let mut libffi_args = Vec::<CArg>::with_capacity(args.len());
         for cur_arg in args.iter() {

src/shims/foreign_items.rs

@@ -370,11 +370,6 @@ pub trait EvalContextExt<'mir, 'tcx: 'mir>: crate::MiriEvalContextExt<'mir, 'tcx
         // First deal with any external C functions in linked .so file
         // (if any SO file is specified, and if the host target == the session target)
         if this.machine.external_so_lib.as_ref().is_some() {
-            if option_env!("TARGET") != Some(&this.tcx.sess.opts.target_triple.to_string()) {
-                throw_ub_format!(
-                    "calling external C functions in linked .so file requires target and host to be the same"
-                );
-            }
             // An Ok(false) here means that the function being called was not exported
             // by the specified SO file; we should continue and check if it corresponds to
             // a provided shim.