Auto merge of rust-lang#11656 - y21:unnecessary_string_from_utf8, r=Jarcho

[`unnecessary_to_owned`]: catch `to_owned` on byte slice to create temporary `&str`

Closes rust-lang#11648

Detects the pattern `&String::from_utf8(bytes.to_vec()).unwrap()` and suggests `core::str::from_utf8(bytes).unwrap()`, which avoids the unnecessary intermediate allocation.

I decided to put this in the existing `unnecessary_to_owned` lint (rather than creating a new lint) for a few reasons:
- we get to use some of its logic (for example, recognizing any of the functions in the `to_owned` family, e.g. `to_vec`)
- the actual inefficient operation that can be avoided here is the call to `.to_vec()`, so this is in a way similar to the other cases caught by `unnecessary_to_owned`, just through a bunch of type conversions
- we can make this more "generic" later and catch other cases, so imo it's best not to tie this lint specifically to the `String` type

changelog: [`unnecessary_to_owned`]: catch `&String::from_utf8(bytes.to_vec()).unwrap()` and suggest `core::str::from_utf8(bytes).unwrap()`

Author: bors

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 {

clippy_utils/src/paths.rs

@@ -88,6 +88,7 @@ pub const SYMBOL_INTERN: [&str; 4] = ["rustc_span", "symbol", "Symbol", "intern"
 pub const SYMBOL_TO_IDENT_STRING: [&str; 4] = ["rustc_span", "symbol", "Symbol", "to_ident_string"];
 pub const SYM_MODULE: [&str; 3] = ["rustc_span", "symbol", "sym"];
 pub const SYNTAX_CONTEXT: [&str; 3] = ["rustc_span", "hygiene", "SyntaxContext"];
+pub const STRING_FROM_UTF8: [&str; 4] = ["alloc", "string", "String", "from_utf8"];
 #[expect(clippy::invalid_paths)] // internal lints do not know about all external crates
 pub const TOKIO_FILE_OPTIONS: [&str; 5] = ["tokio", "fs", "file", "File", "options"];
 #[expect(clippy::invalid_paths)] // internal lints do not know about all external crates

tests/ui/unnecessary_to_owned.fixed

@@ -157,6 +157,25 @@ fn main() {
     require_path(&std::path::PathBuf::from("x"));
     require_str(&String::from("x"));
     require_slice(&[String::from("x")]);
+
+    let slice = [0u8; 1024];
+    let _ref_str: &str = core::str::from_utf8(&slice).expect("not UTF-8");
+    let _ref_str: &str = core::str::from_utf8(b"foo").unwrap();
+    let _ref_str: &str = core::str::from_utf8(b"foo".as_slice()).unwrap();
+    // Expression is of type `&String`, can't suggest `str::from_utf8` here
+    let _ref_string = &String::from_utf8(b"foo".to_vec()).unwrap();
+    macro_rules! arg_from_macro {
+        () => {
+            b"foo".to_vec()
+        };
+    }
+    macro_rules! string_from_utf8_from_macro {
+        () => {
+            &String::from_utf8(b"foo".to_vec()).unwrap()
+        };
+    }
+    let _ref_str: &str = &String::from_utf8(arg_from_macro!()).unwrap();
+    let _ref_str: &str = string_from_utf8_from_macro!();
 }
 
 fn require_c_str(_: &CStr) {}

tests/ui/unnecessary_to_owned.rs

@@ -157,6 +157,25 @@ fn main() {
     require_path(&std::path::PathBuf::from("x").to_path_buf());
     require_str(&String::from("x").to_string());
     require_slice(&[String::from("x")].to_owned());
+
+    let slice = [0u8; 1024];
+    let _ref_str: &str = &String::from_utf8(slice.to_vec()).expect("not UTF-8");
+    let _ref_str: &str = &String::from_utf8(b"foo".to_vec()).unwrap();
+    let _ref_str: &str = &String::from_utf8(b"foo".as_slice().to_owned()).unwrap();
+    // Expression is of type `&String`, can't suggest `str::from_utf8` here
+    let _ref_string = &String::from_utf8(b"foo".to_vec()).unwrap();
+    macro_rules! arg_from_macro {
+        () => {
+            b"foo".to_vec()
+        };
+    }
+    macro_rules! string_from_utf8_from_macro {
+        () => {
+            &String::from_utf8(b"foo".to_vec()).unwrap()
+        };
+    }
+    let _ref_str: &str = &String::from_utf8(arg_from_macro!()).unwrap();
+    let _ref_str: &str = string_from_utf8_from_macro!();
 }
 
 fn require_c_str(_: &CStr) {}

tests/ui/unnecessary_to_owned.stderr

@@ -477,8 +477,44 @@ error: unnecessary use of `to_owned`
 LL |     let _ = IntoIterator::into_iter([std::path::PathBuf::new()][..].to_owned());
    |             ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^ help: use: `[std::path::PathBuf::new()][..].iter().cloned()`
 
+error: allocating a new `String` only to create a temporary `&str` from it
+  --> tests/ui/unnecessary_to_owned.rs:162:26
+   |
+LL |     let _ref_str: &str = &String::from_utf8(slice.to_vec()).expect("not UTF-8");
+   |                          ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
+   |
+help: convert from `&[u8]` to `&str` directly
+   |
+LL -     let _ref_str: &str = &String::from_utf8(slice.to_vec()).expect("not UTF-8");
+LL +     let _ref_str: &str = core::str::from_utf8(&slice).expect("not UTF-8");
+   |
+
+error: allocating a new `String` only to create a temporary `&str` from it
+  --> tests/ui/unnecessary_to_owned.rs:163:26
+   |
+LL |     let _ref_str: &str = &String::from_utf8(b"foo".to_vec()).unwrap();
+   |                          ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
+   |
+help: convert from `&[u8]` to `&str` directly
+   |
+LL -     let _ref_str: &str = &String::from_utf8(b"foo".to_vec()).unwrap();
+LL +     let _ref_str: &str = core::str::from_utf8(b"foo").unwrap();
+   |
+
+error: allocating a new `String` only to create a temporary `&str` from it
+  --> tests/ui/unnecessary_to_owned.rs:164:26
+   |
+LL |     let _ref_str: &str = &String::from_utf8(b"foo".as_slice().to_owned()).unwrap();
+   |                          ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
+   |
+help: convert from `&[u8]` to `&str` directly
+   |
+LL -     let _ref_str: &str = &String::from_utf8(b"foo".as_slice().to_owned()).unwrap();
+LL +     let _ref_str: &str = core::str::from_utf8(b"foo".as_slice()).unwrap();
+   |
+
 error: unnecessary use of `to_vec`
-  --> tests/ui/unnecessary_to_owned.rs:202:14
+  --> tests/ui/unnecessary_to_owned.rs:221:14
    |
 LL |     for t in file_types.to_vec() {
    |              ^^^^^^^^^^^^^^^^^^^
@@ -494,64 +530,64 @@ LL +         let path = match get_file_path(t) {
    |
 
 error: unnecessary use of `to_vec`
-  --> tests/ui/unnecessary_to_owned.rs:225:14
+  --> tests/ui/unnecessary_to_owned.rs:244:14
    |
 LL |     let _ = &["x"][..].to_vec().into_iter();
    |              ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^ help: use: `["x"][..].iter().cloned()`
 
 error: unnecessary use of `to_vec`
-  --> tests/ui/unnecessary_to_owned.rs:230:14
+  --> tests/ui/unnecessary_to_owned.rs:249:14
    |
 LL |     let _ = &["x"][..].to_vec().into_iter();
    |              ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^ help: use: `["x"][..].iter().copied()`
 
 error: unnecessary use of `to_string`
-  --> tests/ui/unnecessary_to_owned.rs:278:24
+  --> tests/ui/unnecessary_to_owned.rs:297:24
    |
 LL |         Box::new(build(y.to_string()))
    |                        ^^^^^^^^^^^^^ help: use: `y`
 
 error: unnecessary use of `to_string`
-  --> tests/ui/unnecessary_to_owned.rs:387:12
+  --> tests/ui/unnecessary_to_owned.rs:406:12
    |
 LL |         id("abc".to_string())
    |            ^^^^^^^^^^^^^^^^^ help: use: `"abc"`
 
 error: unnecessary use of `to_vec`
-  --> tests/ui/unnecessary_to_owned.rs:530:37
+  --> tests/ui/unnecessary_to_owned.rs:549:37
    |
 LL |         IntoFuture::into_future(foo([].to_vec(), &0));
    |                                     ^^^^^^^^^^^ help: use: `[]`
 
 error: unnecessary use of `to_vec`
-  --> tests/ui/unnecessary_to_owned.rs:540:18
+  --> tests/ui/unnecessary_to_owned.rs:559:18
    |
 LL |         s.remove(&a.to_vec());
    |                  ^^^^^^^^^^^ help: replace it with: `a`
 
 error: unnecessary use of `to_owned`
-  --> tests/ui/unnecessary_to_owned.rs:544:14
+  --> tests/ui/unnecessary_to_owned.rs:563:14
    |
 LL |     s.remove(&"b".to_owned());
    |              ^^^^^^^^^^^^^^^ help: replace it with: `"b"`
 
 error: unnecessary use of `to_string`
-  --> tests/ui/unnecessary_to_owned.rs:545:14
+  --> tests/ui/unnecessary_to_owned.rs:564:14
    |
 LL |     s.remove(&"b".to_string());
    |              ^^^^^^^^^^^^^^^^ help: replace it with: `"b"`
 
 error: unnecessary use of `to_vec`
-  --> tests/ui/unnecessary_to_owned.rs:550:14
+  --> tests/ui/unnecessary_to_owned.rs:569:14
    |
 LL |     s.remove(&["b"].to_vec());
    |              ^^^^^^^^^^^^^^^ help: replace it with: `["b"].as_slice()`
 
 error: unnecessary use of `to_vec`
-  --> tests/ui/unnecessary_to_owned.rs:551:14
+  --> tests/ui/unnecessary_to_owned.rs:570:14
    |
 LL |     s.remove(&(&["b"]).to_vec());
    |              ^^^^^^^^^^^^^^^^^^ help: replace it with: `(&["b"]).as_slice()`
 
-error: aborting due to 85 previous errors
+error: aborting due to 88 previous errors