1 files changed, 41 insertions, 0 deletions
diff --git a/src/tools/clippy/clippy_lints/src/casts/char_lit_as_u8.rs b/src/tools/clippy/clippy_lints/src/casts/char_lit_as_u8.rs
new file mode 100644
index 000000000..7cc406018
--- /dev/null
+++ b/src/tools/clippy/clippy_lints/src/casts/char_lit_as_u8.rs
@@ -0,0 +1,41 @@
+use clippy_utils::diagnostics::span_lint_and_then;
+use clippy_utils::source::snippet_with_applicability;
+use if_chain::if_chain;
+use rustc_ast::LitKind;
+use rustc_errors::Applicability;
+use rustc_hir::{Expr, ExprKind};
+use rustc_lint::LateContext;
+use rustc_middle::ty::{self, UintTy};
+
+use super::CHAR_LIT_AS_U8;
+
+pub(super) fn check(cx: &LateContext<'_>, expr: &Expr<'_>) {
+    if_chain! {
+        if let ExprKind::Cast(e, _) = &expr.kind;
+        if let ExprKind::Lit(l) = &e.kind;
+        if let LitKind::Char(c) = l.node;
+        if ty::Uint(UintTy::U8) == *cx.typeck_results().expr_ty(expr).kind();
+        then {
+            let mut applicability = Applicability::MachineApplicable;
+            let snippet = snippet_with_applicability(cx, e.span, "'x'", &mut applicability);
+
+            span_lint_and_then(
+                cx,
+                CHAR_LIT_AS_U8,
+                expr.span,
+                "casting a character literal to `u8` truncates",
+                |diag| {
+                    diag.note("`char` is four bytes wide, but `u8` is a single byte");
+
+                    if c.is_ascii() {
+                        diag.span_suggestion(
+                            expr.span,
+                            "use a byte literal instead",
+                            format!("b{}", snippet),
+                            applicability,
+                        );
+                    }
+            });
+        }
+    }
+}