[unnecessary_to_owned]: catch to_owned from byte slice to string

clippy_lints/src/methods/unnecessary_to_owned.rs

@@ -1,13 +1,15 @@
 use super::implicit_clone::is_clone_like;
 use super::unnecessary_iter_cloned::{self, is_into_iter};
 use clippy_config::msrvs::{self, Msrv};
-use clippy_utils::diagnostics::span_lint_and_sugg;
-use clippy_utils::source::snippet_opt;
+use clippy_utils::diagnostics::{span_lint_and_sugg, span_lint_and_then};
+use clippy_utils::source::{snippet, snippet_opt};
 use clippy_utils::ty::{
     get_iterator_item_ty, implements_trait, is_copy, is_type_diagnostic_item, is_type_lang_item, peel_mid_ty_refs,
 };
 use clippy_utils::visitors::find_all_ret_expressions;
-use clippy_utils::{fn_def_id, get_parent_expr, is_diag_item_method, is_diag_trait_item, return_ty};
+use clippy_utils::{
+    fn_def_id, get_parent_expr, is_diag_item_method, is_diag_trait_item, match_def_path, paths, return_ty,
+};
 use rustc_errors::Applicability;
 use rustc_hir::def::{DefKind, Res};
 use rustc_hir::def_id::DefId;
@@ -52,6 +54,9 @@ pub fn check<'tcx>(
             if check_into_iter_call_arg(cx, expr, method_name, receiver, msrv) {
                 return;
             }
+            if check_string_from_utf8(cx, expr, receiver) {
+                return;
+            }
             check_other_call_arg(cx, expr, method_name, receiver);
         }
     } else {
@@ -240,6 +245,65 @@ fn check_into_iter_call_arg(
     false
 }
 
+/// Checks for `&String::from_utf8(bytes.{to_vec,to_owned,...}()).unwrap()` coercing to `&str`,
+/// which can be written as just `std::str::from_utf8(bytes).unwrap()`.
+fn check_string_from_utf8<'tcx>(cx: &LateContext<'tcx>, expr: &'tcx Expr<'tcx>, receiver: &'tcx Expr<'tcx>) -> bool {
+    if let Some((call, arg)) = skip_addr_of_ancestors(cx, expr)
+        && !arg.span.from_expansion()
+        && let ExprKind::Call(callee, _) = call.kind
+        && fn_def_id(cx, call).is_some_and(|did| match_def_path(cx, did, &paths::STRING_FROM_UTF8))
+        && let Some(unwrap_call) = get_parent_expr(cx, call)
+        && let ExprKind::MethodCall(unwrap_method_name, ..) = unwrap_call.kind
+        && matches!(unwrap_method_name.ident.name, sym::unwrap | sym::expect)
+        && let Some(ref_string) = get_parent_expr(cx, unwrap_call)
+        && let ExprKind::AddrOf(BorrowKind::Ref, Mutability::Not, _) = ref_string.kind
+        && let adjusted_ty = cx.typeck_results().expr_ty_adjusted(ref_string)
+        // `&...` creates a `&String`, so only actually lint if this coerces to a `&str`
+        && matches!(adjusted_ty.kind(), ty::Ref(_, ty, _) if ty.is_str())
+    {
+        span_lint_and_then(
+            cx,
+            UNNECESSARY_TO_OWNED,
+            ref_string.span,
+            "allocating a new `String` only to create a temporary `&str` from it",
+            |diag| {
+                let arg_suggestion = format!(
+                    "{borrow}{recv_snippet}",
+                    recv_snippet = snippet(cx, receiver.span.source_callsite(), ".."),
+                    borrow = if cx.typeck_results().expr_ty(receiver).is_ref() {
+                        ""
+                    } else {
+                        // If not already a reference, prefix with a borrow so that it can coerce to one
+                        "&"
+                    }
+                );
+
+                diag.multipart_suggestion(
+                    "convert from `&[u8]` to `&str` directly",
+                    vec![
+                        // `&String::from_utf8(bytes.to_vec()).unwrap()`
+                        //   ^^^^^^^^^^^^^^^^^
+                        (callee.span, "core::str::from_utf8".into()),
+                        // `&String::from_utf8(bytes.to_vec()).unwrap()`
+                        //  ^
+                        (
+                            ref_string.span.shrink_to_lo().to(unwrap_call.span.shrink_to_lo()),
+                            String::new(),
+                        ),
+                        // `&String::from_utf8(bytes.to_vec()).unwrap()`
+                        //                     ^^^^^^^^^^^^^^
+                        (arg.span, arg_suggestion),
+                    ],
+                    Applicability::MachineApplicable,
+                );
+            },
+        );
+        true
+    } else {
+        false
+    }
+}
+
 /// Checks whether `expr` is an argument in an `into_iter` call and, if so, determines whether its
 /// call of a `to_owned`-like function is unnecessary.
 fn check_split_call_arg(cx: &LateContext<'_>, expr: &Expr<'_>, method_name: Symbol, receiver: &Expr<'_>) -> bool {