Make sure that const prop does not produce unsilenceable lints after inlining

compiler/rustc_mir/src/transform/const_prop.rs

@@ -440,7 +440,15 @@ fn remove_const(ecx: &mut InterpCx<'mir, 'tcx, ConstPropMachine<'mir, 'tcx>>, lo
     }
 
     fn lint_root(&self, source_info: SourceInfo) -> Option<HirId> {
-        match &self.source_scopes[source_info.scope].local_data {
+        let mut data = &self.source_scopes[source_info.scope];
+        // FIXME(oli-obk): we should be able to just walk the `inlined_parent_scope`, but it
+        // does not work as I thought it would. Needs more investigation and documentation.
+        while data.inlined.is_some() {
+            trace!(?data);
+            data = &self.source_scopes[data.parent_scope.unwrap()];
+        }
+        trace!(?data);
+        match &data.local_data {
             ClearCrossCrate::Set(data) => Some(data.lint_root),
             ClearCrossCrate::Clear => None,
         }

src/test/ui/const_prop/inline_spans_lint_attribute.rs

@@ -0,0 +1,15 @@
+// Must be build-pass, because check-pass will not run const prop and thus not emit the lint anyway.
+// build-pass
+// compile-flags: -Zmir-opt-level=2
+
+#![deny(warnings)]
+
+fn main() {
+    #[allow(arithmetic_overflow)]
+    let _ = add(u8::MAX, 1);
+}
+
+#[inline(always)]
+fn add(x: u8, y: u8) -> u8 {
+    x + y
+}