New lint needless_as_bytes

Author: samueltardieu

CHANGELOG.md

@@ -5725,6 +5725,7 @@ Released 2018-09-13
 [`mutex_integer`]: https://rust-lang.github.io/rust-clippy/master/index.html#mutex_integer
 [`naive_bytecount`]: https://rust-lang.github.io/rust-clippy/master/index.html#naive_bytecount
 [`needless_arbitrary_self_type`]: https://rust-lang.github.io/rust-clippy/master/index.html#needless_arbitrary_self_type
+[`needless_as_bytes`]: https://rust-lang.github.io/rust-clippy/master/index.html#needless_as_bytes
 [`needless_bitwise_bool`]: https://rust-lang.github.io/rust-clippy/master/index.html#needless_bitwise_bool
 [`needless_bool`]: https://rust-lang.github.io/rust-clippy/master/index.html#needless_bool
 [`needless_bool_assign`]: https://rust-lang.github.io/rust-clippy/master/index.html#needless_bool_assign

clippy_lints/src/.byte_char_slices.rs.swp

@@ -526,6 +526,7 @@ pub static LINTS: &[&crate::LintInfo] = &[
     crate::mutex_atomic::MUTEX_ATOMIC_INFO,
     crate::mutex_atomic::MUTEX_INTEGER_INFO,
     crate::needless_arbitrary_self_type::NEEDLESS_ARBITRARY_SELF_TYPE_INFO,
+    crate::needless_as_bytes::NEEDLESS_AS_BYTES_INFO,
     crate::needless_bool::BOOL_COMPARISON_INFO,
     crate::needless_bool::NEEDLESS_BOOL_INFO,
     crate::needless_bool::NEEDLESS_BOOL_ASSIGN_INFO,

clippy_lints/src/declared_lints.rs

@@ -249,6 +249,7 @@ mod mut_reference;
 mod mutable_debug_assertion;
 mod mutex_atomic;
 mod needless_arbitrary_self_type;
+mod needless_as_bytes;
 mod needless_bool;
 mod needless_borrowed_ref;
 mod needless_borrows_for_generic_args;
@@ -944,5 +945,6 @@ pub fn register_lints(store: &mut rustc_lint::LintStore, conf: &'static Conf) {
     store.register_late_pass(|_| Box::new(manual_is_power_of_two::ManualIsPowerOfTwo));
     store.register_late_pass(|_| Box::new(non_zero_suggestions::NonZeroSuggestions));
     store.register_late_pass(move |_| Box::new(unused_trait_names::UnusedTraitNames::new(conf)));
+    store.register_late_pass(|_| Box::new(needless_as_bytes::NeedlessAsBytes));
     // add lints here, do not remove this comment, it's used in `new_lint`
 }

clippy_lints/src/lib.rs

@@ -0,0 +1,60 @@
+use clippy_utils::diagnostics::span_lint_and_sugg;
+use clippy_utils::sugg::Sugg;
+use clippy_utils::ty::is_type_lang_item;
+use rustc_errors::Applicability;
+use rustc_hir::{Expr, ExprKind, LangItem};
+use rustc_lint::{LateContext, LateLintPass};
+use rustc_session::declare_lint_pass;
+use rustc_span::sym;
+
+declare_clippy_lint! {
+    /// ### What it does
+    /// It detects useless calls to `str::as_bytes()` before calling `len()` or `is_empty()`.
+    ///
+    /// ### Why is this bad?
+    /// The `len()` and `is_empty()` methods are also directly available on strings, and they
+    /// return identical results. In particular, `len()` on a string returns the number of
+    /// bytes.
+    ///
+    /// ### Example
+    /// ```
+    /// let len = "some string".as_bytes().len();
+    /// let b = "some string".as_bytes().is_empty();
+    /// ```
+    /// Use instead:
+    /// ```
+    /// let len = "some string".len();
+    /// let b = "some string".is_empty();
+    /// ```
+    #[clippy::version = "1.83.0"]
+    pub NEEDLESS_AS_BYTES,
+    complexity,
+    "detect useless calls to `as_bytes()`"
+}
+
+declare_lint_pass!(NeedlessAsBytes => [NEEDLESS_AS_BYTES]);
+
+impl LateLintPass<'_> for NeedlessAsBytes {
+    fn check_expr(&mut self, cx: &LateContext<'_>, expr: &Expr<'_>) {
+        if let ExprKind::MethodCall(method2, receiver2, &[], _) = expr.kind
+            && cx.typeck_results().expr_ty_adjusted(receiver2).peel_refs().is_slice()
+            && (method2.ident.name == sym::len || method2.ident.name.as_str() == "is_empty")
+            && let ExprKind::MethodCall(method1, receiver1, &[], _) = receiver2.kind
+            && let ty1 = cx.typeck_results().expr_ty_adjusted(receiver1).peel_refs()
+            && (is_type_lang_item(cx, ty1, LangItem::String) || ty1.is_str())
+            && method1.ident.name.as_str() == "as_bytes"
+        {
+            let mut app = Applicability::MachineApplicable;
+            let sugg = Sugg::hir_with_context(cx, receiver1, expr.span.ctxt(), "..", &mut app);
+            span_lint_and_sugg(
+                cx,
+                NEEDLESS_AS_BYTES,
+                expr.span,
+                "needless call to `as_bytes()`",
+                format!("`{}()` can be called directly on strings", method2.ident.name),
+                format!("{sugg}.{}()", method2.ident.name),
+                app,
+            );
+        }
+    }
+}

clippy_lints/src/needless_as_bytes.rs

@@ -0,0 +1,25 @@
+#![warn(clippy::needless_as_bytes)]
+#![allow(clippy::const_is_empty)]
+
+struct S;
+
+impl S {
+    fn as_bytes(&self) -> &[u8] {
+        &[]
+    }
+}
+
+fn main() {
+    if "some string".is_empty() {
+        println!("len = {}", "some string".len());
+    }
+
+    let s = String::from("yet another string");
+    if s.is_empty() {
+        println!("len = {}", s.len());
+    }
+
+    // Do not lint
+    let _ = S.as_bytes().is_empty();
+    let _ = S.as_bytes().len();
+}

tests/ui/needless_as_bytes.fixed

@@ -0,0 +1,25 @@
+#![warn(clippy::needless_as_bytes)]
+#![allow(clippy::const_is_empty)]
+
+struct S;
+
+impl S {
+    fn as_bytes(&self) -> &[u8] {
+        &[]
+    }
+}
+
+fn main() {
+    if "some string".as_bytes().is_empty() {
+        println!("len = {}", "some string".as_bytes().len());
+    }
+
+    let s = String::from("yet another string");
+    if s.as_bytes().is_empty() {
+        println!("len = {}", s.as_bytes().len());
+    }
+
+    // Do not lint
+    let _ = S.as_bytes().is_empty();
+    let _ = S.as_bytes().len();
+}

tests/ui/needless_as_bytes.rs

@@ -0,0 +1,29 @@
+error: needless call to `as_bytes()`
+  --> tests/ui/needless_as_bytes.rs:13:8
+   |
+LL |     if "some string".as_bytes().is_empty() {
+   |        ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^ help: `is_empty()` can be called directly on strings: `"some string".is_empty()`
+   |
+   = note: `-D clippy::needless-as-bytes` implied by `-D warnings`
+   = help: to override `-D warnings` add `#[allow(clippy::needless_as_bytes)]`
+
+error: needless call to `as_bytes()`
+  --> tests/ui/needless_as_bytes.rs:14:30
+   |
+LL |         println!("len = {}", "some string".as_bytes().len());
+   |                              ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^ help: `len()` can be called directly on strings: `"some string".len()`
+
+error: needless call to `as_bytes()`
+  --> tests/ui/needless_as_bytes.rs:18:8
+   |
+LL |     if s.as_bytes().is_empty() {
+   |        ^^^^^^^^^^^^^^^^^^^^^^^ help: `is_empty()` can be called directly on strings: `s.is_empty()`
+
+error: needless call to `as_bytes()`
+  --> tests/ui/needless_as_bytes.rs:19:30
+   |
+LL |         println!("len = {}", s.as_bytes().len());
+   |                              ^^^^^^^^^^^^^^^^^^ help: `len()` can be called directly on strings: `s.len()`
+
+error: aborting due to 4 previous errors
+