Add helper methods checking for "#[non_exhaustive] that's active"

A check for `#[non_exhaustive]` is often done in combination with
checking whether the type is local to the crate, in a variety of ways.
Create a helper method and standardize on it as the way to check for
this.

Author: meithecatte

clippy_lints/src/default.rs

@@ -134,7 +134,7 @@ impl<'tcx> LateLintPass<'tcx> for Default {
                 && let ty::Adt(adt, args) = *binding_type.kind()
                 && adt.is_struct()
                 && let variant = adt.non_enum_variant()
-                && (adt.did().is_local() || !variant.is_field_list_non_exhaustive())
+                && !variant.field_list_has_applicable_non_exhaustive()
                 && let module_did = cx.tcx.parent_module(stmt.hir_id)
                 && variant
                     .fields

clippy_lints/src/needless_update.rs

@@ -54,8 +54,9 @@ impl<'tcx> LateLintPass<'tcx> for NeedlessUpdate {
         if let ExprKind::Struct(_, fields, StructTailExpr::Base(base)) = expr.kind {
             let ty = cx.typeck_results().expr_ty(expr);
             if let ty::Adt(def, _) = ty.kind() {
-                if fields.len() == def.non_enum_variant().fields.len()
-                    && !def.variant(0_usize.into()).is_field_list_non_exhaustive()
+                let variant = def.non_enum_variant();
+                if fields.len() == variant.fields.len()
+                    && !variant.is_field_list_non_exhaustive()
                 {
                     span_lint(
                         cx,

clippy_lints/src/unneeded_struct_pattern.rs

@@ -51,7 +51,7 @@ impl LateLintPass<'_> for UnneededStructPattern {
             let variant = cx.tcx.adt_def(enum_did).variant_with_id(did);
 
             let has_only_fields_brackets = variant.ctor.is_some() && variant.fields.is_empty();
-            let non_exhaustive_activated = !variant.def_id.is_local() && variant.is_field_list_non_exhaustive();
+            let non_exhaustive_activated = variant.field_list_has_applicable_non_exhaustive();
             if !has_only_fields_brackets || non_exhaustive_activated {
                 return;
             }