Update useless_conversion and box_collection clippy lints to use String type alias diagnostic item.

Author: zachs18

library/alloc/src/string.rs

@@ -343,6 +343,7 @@ pub mod string;
 /// [`Deref`]: core::ops::Deref "ops::Deref"
 /// [`as_str()`]: String::as_str
 #[stable(feature = "rust1", since = "1.0.0")]
+#[cfg_attr(not(test), rustc_diagnostic_item = "String")]
 pub type String = string::String<Global>;
 
 /// A possible error value when converting a `String` from a UTF-8 byte vector.

src/tools/clippy/clippy_lints/src/types/box_collection.rs

@@ -47,6 +47,7 @@ fn get_std_collection(cx: &LateContext<'_>, qpath: &QPath<'_>) -> Option<Symbol>
                     | sym::BTreeMap
                     | sym::BTreeSet
                     | sym::BinaryHeap
+                    | sym::String
             )
         })
         .or_else(|| {

src/tools/clippy/clippy_lints/src/useless_conversion.rs

@@ -3,7 +3,8 @@ use clippy_utils::source::{snippet, snippet_with_context};
 use clippy_utils::sugg::{DiagExt as _, Sugg};
 use clippy_utils::ty::{is_copy, is_type_diagnostic_item, same_type_and_consts};
 use clippy_utils::{
-    get_parent_expr, is_inherent_method_call, is_trait_item, is_trait_method, is_ty_alias, path_to_local,
+    get_parent_expr, is_inherent_method_call, is_string_ty_alias, is_trait_item, is_trait_method, is_ty_alias,
+    path_to_local,
 };
 use rustc_errors::Applicability;
 use rustc_hir::def_id::DefId;
@@ -333,7 +334,7 @@ impl<'tcx> LateLintPass<'tcx> for UselessConversion {
             ExprKind::Call(path, [arg]) => {
                 if let ExprKind::Path(ref qpath) = path.kind
                     && let Some(def_id) = cx.qpath_res(qpath, path.hir_id).opt_def_id()
-                    && !is_ty_alias(qpath)
+                    && (!is_ty_alias(qpath) || is_string_ty_alias(cx, qpath))
                 {
                     let a = cx.typeck_results().expr_ty(e);
                     let b = cx.typeck_results().expr_ty(arg);

src/tools/clippy/clippy_utils/src/lib.rs

@@ -350,6 +350,18 @@ pub fn is_ty_alias(qpath: &QPath<'_>) -> bool {
     }
 }
 
+/// Checks if the given `QPath` belongs to the specific type alias
+/// `std::string::String`/`alloc::string::String`.
+pub fn is_string_ty_alias(cx: &LateContext<'_>, qpath: &QPath<'_>) -> bool {
+    match *qpath {
+        QPath::Resolved(_, path) => {
+            matches!(path.res, Res::Def(DefKind::TyAlias, id) if cx.tcx.get_diagnostic_name(id) == Some(sym::String))
+        },
+        QPath::TypeRelative(ty, _) if let TyKind::Path(qpath) = ty.kind => is_string_ty_alias(cx, &qpath),
+        _ => false,
+    }
+}
+
 /// Checks if the method call given in `expr` belongs to the given trait.
 /// This is a deprecated function, consider using [`is_trait_method`].
 pub fn match_trait_method(cx: &LateContext<'_>, expr: &Expr<'_>, path: &[&str]) -> bool {