cast_lossless: lint when converting char as well

Author: lapla-cogito

clippy_lints/src/casts/cast_lossless.rs

@@ -96,7 +96,16 @@ fn should_lint(cx: &LateContext<'_>, cast_from: Ty<'_>, cast_to: Ty<'_>, msrv: M
             };
             !is_isize_or_usize(cast_from) && from_nbits < to_nbits
         },
-        (false, true) if matches!(cast_from.kind(), ty::Bool) && msrv.meets(cx, msrvs::FROM_BOOL) => true,
+        (false, true)
+            if (*cast_from.kind() == ty::Char
+                && matches!(
+                    cast_to.kind(),
+                    ty::Uint(ty::UintTy::U32 | ty::UintTy::U64 | ty::UintTy::U128)
+                ))
+                || (*cast_from.kind() == ty::Bool && msrv.meets(cx, msrvs::FROM_BOOL)) =>
+        {
+            true
+        },
         (_, _) => {
             matches!(cast_from.kind(), ty::Float(FloatTy::F32)) && matches!(cast_to.kind(), ty::Float(FloatTy::F64))
         },

tests/ui/cast_lossless_char.fixed

@@ -0,0 +1,20 @@
+#![warn(clippy::cast_lossless)]
+#![allow(clippy::char_lit_as_u8)]
+
+type I32 = i32;
+type U32 = u32;
+
+fn main() {
+    let _ = u32::from('a');
+    //~^ cast_lossless
+    let _ = 'a' as i32;
+    let _ = u64::from('a');
+    //~^ cast_lossless
+    let _ = 'a' as i64;
+    let _ = U32::from('a');
+    //~^ cast_lossless
+    let _ = 'a' as I32;
+
+    let _ = 'a' as u8;
+    let _ = 'a' as i8;
+}

tests/ui/cast_lossless_char.rs

@@ -0,0 +1,20 @@
+#![warn(clippy::cast_lossless)]
+#![allow(clippy::char_lit_as_u8)]
+
+type I32 = i32;
+type U32 = u32;
+
+fn main() {
+    let _ = 'a' as u32;
+    //~^ cast_lossless
+    let _ = 'a' as i32;
+    let _ = 'a' as u64;
+    //~^ cast_lossless
+    let _ = 'a' as i64;
+    let _ = 'a' as U32;
+    //~^ cast_lossless
+    let _ = 'a' as I32;
+
+    let _ = 'a' as u8;
+    let _ = 'a' as i8;
+}

tests/ui/cast_lossless_char.stderr

@@ -0,0 +1,43 @@
+error: casts from `char` to `u32` can be expressed infallibly using `From`
+  --> tests/ui/cast_lossless_char.rs:8:13
+   |
+LL |     let _ = 'a' as u32;
+   |             ^^^^^^^^^^
+   |
+   = help: an `as` cast can become silently lossy if the types change in the future
+   = note: `-D clippy::cast-lossless` implied by `-D warnings`
+   = help: to override `-D warnings` add `#[allow(clippy::cast_lossless)]`
+help: use `u32::from` instead
+   |
+LL -     let _ = 'a' as u32;
+LL +     let _ = u32::from('a');
+   |
+
+error: casts from `char` to `u64` can be expressed infallibly using `From`
+  --> tests/ui/cast_lossless_char.rs:11:13
+   |
+LL |     let _ = 'a' as u64;
+   |             ^^^^^^^^^^
+   |
+   = help: an `as` cast can become silently lossy if the types change in the future
+help: use `u64::from` instead
+   |
+LL -     let _ = 'a' as u64;
+LL +     let _ = u64::from('a');
+   |
+
+error: casts from `char` to `u32` can be expressed infallibly using `From`
+  --> tests/ui/cast_lossless_char.rs:14:13
+   |
+LL |     let _ = 'a' as U32;
+   |             ^^^^^^^^^^
+   |
+   = help: an `as` cast can become silently lossy if the types change in the future
+help: use `U32::from` instead
+   |
+LL -     let _ = 'a' as U32;
+LL +     let _ = U32::from('a');
+   |
+
+error: aborting due to 3 previous errors
+