New lint: unbuffered_bytes

clippy_lints/src/declared_lints.rs

@@ -482,6 +482,7 @@ pub static LINTS: &[&crate::LintInfo] = &[
     crate::methods::SUSPICIOUS_SPLITN_INFO,
     crate::methods::SUSPICIOUS_TO_OWNED_INFO,
     crate::methods::TYPE_ID_ON_BOX_INFO,
+    crate::methods::UNBUFFERED_BYTES_INFO,
     crate::methods::UNINIT_ASSUMED_INIT_INFO,
     crate::methods::UNIT_HASH_INFO,
     crate::methods::UNNECESSARY_FALLIBLE_CONVERSIONS_INFO,

clippy_lints/src/methods/mod.rs

@@ -113,6 +113,7 @@ mod suspicious_map;
 mod suspicious_splitn;
 mod suspicious_to_owned;
 mod type_id_on_box;
+mod unbuffered_bytes;
 mod uninit_assumed_init;
 mod unit_hash;
 mod unnecessary_fallible_conversions;
@@ -4406,6 +4407,33 @@ declare_clippy_lint! {
     "using `Option::and_then` or `Result::and_then` to chain a computation that returns an `Option` or a `Result`"
 }
 
+declare_clippy_lint! {
+    /// ### What it does
+    /// Checks for calls to `Read::bytes` on types which don't implement `BufRead`.
+    ///
+    /// ### Why is this bad?
+    /// The default implementation calls `read` for each byte, which can be very inefficient for data that’s not in memory, such as `File`.
+    ///
+    /// ### Example
+    /// ```no_run
+    /// use std::io::Read;
+    /// use std::fs::File;
+    /// let file = File::open("./bytes.txt").unwrap();
+    /// file.bytes();
+    /// ```
+    /// Use instead:
+    /// ```no_run
+    /// use std::io::{BufReader, Read};
+    /// use std::fs::File;
+    /// let file = BufReader::new(std::fs::File::open("./bytes.txt").unwrap());
+    /// file.bytes();
+    /// ```
+    #[clippy::version = "1.86.0"]
+    pub UNBUFFERED_BYTES,
+    perf,
+    "calling .bytes() is very inefficient when data is not in memory"
+}
+
 #[expect(clippy::struct_excessive_bools)]
 pub struct Methods {
     avoid_breaking_exported_api: bool,
@@ -4580,6 +4608,7 @@ impl_lint_pass!(Methods => [
     MANUAL_REPEAT_N,
     SLICED_STRING_AS_BYTES,
     RETURN_AND_THEN,
+    UNBUFFERED_BYTES,
 ]);
 
 /// Extracts a method call name, args, and `Span` of the method name.
@@ -4856,6 +4885,7 @@ impl Methods {
                 ("as_ptr", []) => manual_c_str_literals::check_as_ptr(cx, expr, recv, &self.msrv),
                 ("as_ref", []) => useless_asref::check(cx, expr, "as_ref", recv),
                 ("assume_init", []) => uninit_assumed_init::check(cx, expr, recv),
+                ("bytes", []) => unbuffered_bytes::check(cx, expr, recv),
                 ("cloned", []) => {
                     cloned_instead_of_copied::check(cx, expr, recv, span, &self.msrv);
                     option_as_ref_cloned::check(cx, recv, span);

clippy_lints/src/methods/unbuffered_bytes.rs

@@ -0,0 +1,31 @@
+use super::UNBUFFERED_BYTES;
+use clippy_utils::diagnostics::span_lint_and_help;
+use clippy_utils::ty::implements_trait;
+use clippy_utils::{get_trait_def_id, is_trait_method, paths};
+use rustc_hir as hir;
+use rustc_lint::LateContext;
+use rustc_span::sym;
+
+pub(super) fn check(cx: &LateContext<'_>, expr: &hir::Expr<'_>, recv: &hir::Expr<'_>) {
+    let ty = cx.typeck_results().expr_ty_adjusted(recv);
+
+    // If the .bytes() call is a call from the Read trait
+    if is_trait_method(cx, expr, sym::IoRead) {
+        // Retrieve the DefId of the BufRead trait
+        // FIXME: add a diagnostic item for `BufRead`
+        let Some(buf_read) = get_trait_def_id(cx.tcx, &paths::BUF_READ) else {
+            return;
+        };
+        // And the implementor of the trait is not buffered
+        if !implements_trait(cx, ty, buf_read, &[]) {
+            span_lint_and_help(
+                cx,
+                UNBUFFERED_BYTES,
+                expr.span,
+                "calling .bytes() is very inefficient when data is not in memory",
+                None,
+                "consider using `BufReader`",
+            );
+        }
+    }
+}