Ignore symbol shim clash when symbol is provided by compiler_builtins

When this happens, we ignore the symbol from `compiler_builtins`
in favor of Miri's builtin support.

This allows Miri to target platforms like wasm32-unknown-unknown,
where functions like `memcmp` are provided by `compiler_builtins`.

Author: Aaron1011

src/helpers.rs

@@ -943,7 +943,16 @@ pub trait EvalContextExt<'mir, 'tcx: 'mir>: crate::MiriInterpCxExt<'mir, 'tcx> {
         link_name: Symbol,
     ) -> InterpResult<'tcx, ()> {
         self.check_abi(abi, exp_abi)?;
-        if let Some((body, _)) = self.eval_context_mut().lookup_exported_symbol(link_name)? {
+        if let Some((body, instance)) = self.eval_context_mut().lookup_exported_symbol(link_name)? {
+            // If compiler-builtins is providing the symbol, then don't treat it as a clash.
+            // We'll use our built-in implementation in `emulate_foreign_item_by_name` for increased
+            // performance. Note that this means we won't catch any undefined behavior in
+            // compiler-builtins when running other crates, but Miri can still be run on
+            // compiler-builtins itself (or any crate that uses it as a normal dependency)
+            if self.eval_context_ref().tcx.is_compiler_builtins(instance.def_id().krate) {
+                return Ok(());
+            }
+
             throw_machine_stop!(TerminationInfo::SymbolShimClashing {
                 link_name,
                 span: body.span.data(),