Auto merge of #2121 - RalfJung:less-ice, r=RalfJung

don't ICE when libcore is missing

Fixes #2120

Author: bors

src/eval.rs

@@ -172,15 +172,18 @@ pub fn create_ecx<'mir, 'tcx: 'mir>(
     Evaluator::late_init(&mut ecx, config)?;
 
     // Make sure we have MIR. We check MIR for some stable monomorphic function in libcore.
-    let sentinel = ecx.resolve_path(&["core", "ascii", "escape_default"]);
-    if !tcx.is_mir_available(sentinel.def.def_id()) {
-        tcx.sess.fatal("the current sysroot was built without `-Zalways-encode-mir`. Use `cargo miri setup` to prepare a sysroot that is suitable for Miri.");
+    let sentinel = ecx.try_resolve_path(&["core", "ascii", "escape_default"]);
+    if !matches!(sentinel, Some(s) if tcx.is_mir_available(s.def.def_id())) {
+        tcx.sess.fatal(
+            "the current sysroot was built without `-Zalways-encode-mir`, or libcore seems missing. \
+            Use `cargo miri setup` to prepare a sysroot that is suitable for Miri."
+        );
     }
 
-    // Setup first stack-frame
+    // Setup first stack frame.
     let entry_instance = ty::Instance::mono(tcx, entry_id);
 
-    // First argument is constructed later, because its skipped if the entry function uses #[start]
+    // First argument is constructed later, because it's skipped if the entry function uses #[start].
 
     // Second argument (argc): length of `config.args`.
     let argc = Scalar::from_machine_usize(u64::try_from(config.args.len()).unwrap(), &ecx);

src/helpers.rs

@@ -71,11 +71,16 @@ fn try_resolve_did<'tcx>(tcx: TyCtxt<'tcx>, path: &[&str]) -> Option<DefId> {
 }
 
 pub trait EvalContextExt<'mir, 'tcx: 'mir>: crate::MiriEvalContextExt<'mir, 'tcx> {
+    /// Gets an instance for a path; fails gracefully if the path does not exist.
+    fn try_resolve_path(&self, path: &[&str]) -> Option<ty::Instance<'tcx>> {
+        let did = try_resolve_did(self.eval_context_ref().tcx.tcx, path)?;
+        Some(ty::Instance::mono(self.eval_context_ref().tcx.tcx, did))
+    }
+
     /// Gets an instance for a path.
     fn resolve_path(&self, path: &[&str]) -> ty::Instance<'tcx> {
-        let did = try_resolve_did(self.eval_context_ref().tcx.tcx, path)
-            .unwrap_or_else(|| panic!("failed to find required Rust item: {:?}", path));
-        ty::Instance::mono(self.eval_context_ref().tcx.tcx, did)
+        self.try_resolve_path(path)
+            .unwrap_or_else(|| panic!("failed to find required Rust item: {:?}", path))
     }
 
     /// Evaluates the scalar at the specified path. Returns Some(val)