adds lint logic and test for bytes_count_to_len

formats code with

fixes single match clippy error to replace with if let

swaps ident.name.as_str to ident.name == sym for count fn

CHANGELOG.md

@@ -3299,6 +3299,7 @@ Released 2018-09-13
 [`boxed_local`]: https://rust-lang.github.io/rust-clippy/master/index.html#boxed_local
 [`branches_sharing_code`]: https://rust-lang.github.io/rust-clippy/master/index.html#branches_sharing_code
 [`builtin_type_shadow`]: https://rust-lang.github.io/rust-clippy/master/index.html#builtin_type_shadow
+[`bytes_count_to_len`]: https://rust-lang.github.io/rust-clippy/master/index.html#bytes_count_to_len
 [`bytes_nth`]: https://rust-lang.github.io/rust-clippy/master/index.html#bytes_nth
 [`cargo_common_metadata`]: https://rust-lang.github.io/rust-clippy/master/index.html#cargo_common_metadata
 [`case_sensitive_file_extension_comparisons`]: https://rust-lang.github.io/rust-clippy/master/index.html#case_sensitive_file_extension_comparisons

clippy_lints/src/bytes_count_to_len.rs

@@ -0,0 +1,54 @@
+use clippy_utils::diagnostics::span_lint_and_note;
+use if_chain::if_chain;
+use rustc_hir as hir;
+use rustc_lint::{LateContext, LateLintPass};
+use rustc_session::{declare_lint_pass, declare_tool_lint};
+
+declare_clippy_lint! {
+    /// ### What it does
+    /// It checks for `str::bytes().count()` and suggests replacing it with
+    /// `str::len()`.
+    ///
+    /// ### Why is this bad?
+    /// `str::bytes().count()` is longer and may not be as performant as using
+    /// `str::len()`.
+    ///
+    /// ### Example
+    /// ```rust
+    /// "hello".bytes().count();
+    /// ```
+    /// Use instead:
+    /// ```rust
+    /// "hello".len();
+    /// ```
+    #[clippy::version = "1.60.0"]
+    pub BYTES_COUNT_TO_LEN,
+    complexity,
+    "Using bytest().count() when len() performs the same functionality"
+}
+
+declare_lint_pass!(BytesCountToLen => [BYTES_COUNT_TO_LEN]);
+
+impl<'tcx> LateLintPass<'tcx> for BytesCountToLen {
+    fn check_expr(&mut self, cx: &LateContext<'tcx>, expr: &'tcx hir::Expr<'_>) {
+        if_chain! {
+            //check for method call called "count"
+            if let hir::ExprKind::MethodCall(count_path, count_args, _) = &expr.kind;
+            if count_path.ident.name == rustc_span::sym::count;
+            if let [bytes_expr] = &**count_args;
+            //check for method call called "bytes" that was linked to "count"
+            if let hir::ExprKind::MethodCall(bytes_path, _, _) = &bytes_expr.kind;
+            if bytes_path.ident.name.as_str() == "bytes";
+            then {
+                span_lint_and_note(
+                    cx,
+                    BYTES_COUNT_TO_LEN,
+                    expr.span,
+                    "using long and hard to read `.bytes().count()`",
+                    None,
+                    "`.len()` achieves same functionality"
+                );
+            }
+        };
+    }
+}

clippy_lints/src/lib.register_all.rs

@@ -23,6 +23,7 @@ store.register_group(true, "clippy::all", Some("clippy_all"), vec![
     LintId::of(bool_assert_comparison::BOOL_ASSERT_COMPARISON),
     LintId::of(booleans::LOGIC_BUG),
     LintId::of(booleans::NONMINIMAL_BOOL),
+    LintId::of(bytes_count_to_len::BYTES_COUNT_TO_LEN),
     LintId::of(casts::CAST_ABS_TO_UNSIGNED),
     LintId::of(casts::CAST_ENUM_CONSTRUCTOR),
     LintId::of(casts::CAST_ENUM_TRUNCATION),

clippy_lints/src/lib.register_complexity.rs

@@ -5,6 +5,7 @@
 store.register_group(true, "clippy::complexity", Some("clippy_complexity"), vec![
     LintId::of(attrs::DEPRECATED_CFG_ATTR),
     LintId::of(booleans::NONMINIMAL_BOOL),
+    LintId::of(bytes_count_to_len::BYTES_COUNT_TO_LEN),
     LintId::of(casts::CHAR_LIT_AS_U8),
     LintId::of(casts::UNNECESSARY_CAST),
     LintId::of(derivable_impls::DERIVABLE_IMPLS),

clippy_lints/src/lib.register_lints.rs

@@ -64,6 +64,7 @@ store.register_lints(&[
     booleans::NONMINIMAL_BOOL,
     borrow_as_ptr::BORROW_AS_PTR,
     bytecount::NAIVE_BYTECOUNT,
+    bytes_count_to_len::BYTES_COUNT_TO_LEN,
     cargo::CARGO_COMMON_METADATA,
     cargo::MULTIPLE_CRATE_VERSIONS,
     cargo::NEGATIVE_FEATURE_NAMES,

clippy_lints/src/lib.rs

@@ -181,6 +181,7 @@ mod bool_assert_comparison;
 mod booleans;
 mod borrow_as_ptr;
 mod bytecount;
+mod bytes_count_to_len;
 mod cargo;
 mod case_sensitive_file_extension_comparisons;
 mod casts;
@@ -875,6 +876,7 @@ pub fn register_plugins(store: &mut rustc_lint::LintStore, sess: &Session, conf:
     store.register_late_pass(|| Box::new(unnecessary_owned_empty_strings::UnnecessaryOwnedEmptyStrings));
     store.register_early_pass(|| Box::new(pub_use::PubUse));
     store.register_late_pass(|| Box::new(format_push_string::FormatPushString));
+    store.register_late_pass(|| Box::new(bytes_count_to_len::BytesCountToLen));
     // add lints here, do not remove this comment, it's used in `new_lint`
 }
 

tests/ui/bytes_count_to_len.rs

@@ -0,0 +1,15 @@
+#![warn(clippy::bytes_count_to_len)]
+
+fn main() {
+    let s1 = String::from("world");
+
+    //test warning against a string literal
+    "hello".bytes().count();
+
+    //test warning against a string variable
+    s1.bytes().count();
+
+    //make sure using count() normally doesn't trigger warning
+    let vector = [0, 1, 2];
+    let size = vector.iter().count();
+}

tests/ui/bytes_count_to_len.stderr

@@ -0,0 +1,19 @@
+error: using long and hard to read `.bytes().count()`
+  --> $DIR/bytes_count_to_len.rs:7:5
+   |
+LL |     "hello".bytes().count();
+   |     ^^^^^^^^^^^^^^^^^^^^^^^
+   |
+   = note: `-D clippy::bytes-count-to-len` implied by `-D warnings`
+   = note: `.len()` achieves same functionality
+
+error: using long and hard to read `.bytes().count()`
+  --> $DIR/bytes_count_to_len.rs:10:5
+   |
+LL |     s1.bytes().count();
+   |     ^^^^^^^^^^^^^^^^^^
+   |
+   = note: `.len()` achieves same functionality
+
+error: aborting due to 2 previous errors
+