New lint: unbuffered_bytes

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`",
+            );
+        }
+    }
+}