Fix false negative with String and add tests

`String` is currently not a diagnostic item so it needs special
treatment

Author: bluthej

clippy_lints/src/methods/clear_with_drain.rs

@@ -1,16 +1,17 @@
 use clippy_utils::diagnostics::span_lint_and_sugg;
 use clippy_utils::is_range_full;
-use clippy_utils::ty::is_type_diagnostic_item;
+use clippy_utils::ty::{is_type_diagnostic_item, is_type_lang_item};
 use rustc_errors::Applicability;
 use rustc_hir as hir;
-use rustc_hir::{Expr, ExprKind, QPath};
+use rustc_hir::{Expr, ExprKind, LangItem, QPath};
 use rustc_lint::LateContext;
 use rustc_span::symbol::sym;
 use rustc_span::Span;
 
 use super::CLEAR_WITH_DRAIN;
 
-const ACCEPTABLE_TYPES_WITH_ARG: [rustc_span::Symbol; 3] = [sym::String, sym::Vec, sym::VecDeque];
+// Add `String` here when it is added to diagnostic items
+const ACCEPTABLE_TYPES_WITH_ARG: [rustc_span::Symbol; 2] = [sym::Vec, sym::VecDeque];
 
 const ACCEPTABLE_TYPES_WITHOUT_ARG: [rustc_span::Symbol; 3] = [sym::BinaryHeap, sym::HashMap, sym::HashSet];
 
@@ -30,11 +31,14 @@ pub(super) fn check(cx: &LateContext<'_>, expr: &Expr<'_>, recv: &Expr<'_>, span
 fn match_acceptable_type(cx: &LateContext<'_>, expr: &hir::Expr<'_>, types: &[rustc_span::Symbol]) -> bool {
     let expr_ty = cx.typeck_results().expr_ty(expr).peel_refs();
     types.iter().any(|&ty| is_type_diagnostic_item(cx, expr_ty, ty))
+    // String type is a lang item but not a diagnostic item for now so we need a separate check
+        || is_type_lang_item(cx, expr_ty, LangItem::String)
 }
 
 fn suggest(cx: &LateContext<'_>, expr: &Expr<'_>, recv: &Expr<'_>, span: Span) {
     if let Some(adt) = cx.typeck_results().expr_ty(recv).ty_adt_def()
-        && let Some(ty_name) = cx.tcx.get_diagnostic_name(adt.did())
+    // Use `opt_item_name` while `String` is not a diagnostic item
+        && let Some(ty_name) = cx.tcx.opt_item_name(adt.did())
     {
         span_lint_and_sugg(
             cx,