Auto merge of #12496 - Jacherr:issue-12492, r=blyxyas

Disable `cast_lossless` when casting to u128 from any (u)int type

Fixes #12492

Disables `cast_lossless` when casting to u128 from any int or uint type. The lint states that when casting to any int type, there can potentially be lossy behaviour if the source type ever exceeds the size of the destination type in the future, which is impossible with a destination of u128.

It's possible this is a bit of a niche edge case which is better addressed by just disabling the lint in code, but I personally couldn't think of any good reason to still lint in this specific case - maybe except if the source is a bool, for readability reasons :).

changelog: FP: `cast_lossless`: disable lint when casting to u128 from any (u)int type

clippy_lints/src/casts/cast_lossless.rs

@@ -6,7 +6,7 @@ use clippy_utils::ty::is_isize_or_usize;
 use rustc_errors::Applicability;
 use rustc_hir::{Expr, ExprKind, QPath, TyKind};
 use rustc_lint::LateContext;
-use rustc_middle::ty::{self, FloatTy, Ty};
+use rustc_middle::ty::{self, FloatTy, Ty, UintTy};
 
 use super::{utils, CAST_LOSSLESS};
 
@@ -77,7 +77,10 @@ pub(super) fn check(
 
 fn should_lint(cx: &LateContext<'_>, expr: &Expr<'_>, cast_from: Ty<'_>, cast_to: Ty<'_>, msrv: &Msrv) -> bool {
     // Do not suggest using From in consts/statics until it is valid to do so (see #2267).
-    if in_constant(cx, expr.hir_id) {
+    //
+    // If destination is u128, do not lint because source type cannot be larger
+    // If source is bool, still lint due to the lint message differing (refers to style)
+    if in_constant(cx, expr.hir_id) || (!cast_from.is_bool() && matches!(cast_to.kind(), ty::Uint(UintTy::U128))) {
         return false;
     }
 

tests/ui/cast_lossless_integer.fixed

@@ -2,6 +2,7 @@
 #![warn(clippy::cast_lossless)]
 
 type I64 = i64;
+type U128 = u128;
 
 fn main() {
     // Test clippy::cast_lossless with casts to integer types
@@ -28,6 +29,11 @@ fn main() {
     let _ = u16::from(1u8 + 1u8);
 
     let _ = I64::from(1i8);
+
+    // Do not lint if destination type is u128
+    // see https://github.com/rust-lang/rust-clippy/issues/12492
+    let _ = 1u8 as u128;
+    let _ = 1u8 as U128;
 }
 
 // The lint would suggest using `f64::from(input)` here but the `XX::from` function is not const,

tests/ui/cast_lossless_integer.rs

@@ -2,6 +2,7 @@
 #![warn(clippy::cast_lossless)]
 
 type I64 = i64;
+type U128 = u128;
 
 fn main() {
     // Test clippy::cast_lossless with casts to integer types
@@ -28,6 +29,11 @@ fn main() {
     let _ = (1u8 + 1u8) as u16;
 
     let _ = 1i8 as I64;
+
+    // Do not lint if destination type is u128
+    // see https://github.com/rust-lang/rust-clippy/issues/12492
+    let _ = 1u8 as u128;
+    let _ = 1u8 as U128;
 }
 
 // The lint would suggest using `f64::from(input)` here but the `XX::from` function is not const,

tests/ui/cast_lossless_integer.stderr

@@ -1,5 +1,5 @@
 error: casting `i8` to `i16` may become silently lossy if you later change the type
-  --> tests/ui/cast_lossless_integer.rs:8:13
+  --> tests/ui/cast_lossless_integer.rs:9:13
    |
 LL |     let _ = 1i8 as i16;
    |             ^^^^^^^^^^ help: try: `i16::from(1i8)`
@@ -8,127 +8,127 @@ LL |     let _ = 1i8 as i16;
    = help: to override `-D warnings` add `#[allow(clippy::cast_lossless)]`
 
 error: casting `i8` to `i32` may become silently lossy if you later change the type
-  --> tests/ui/cast_lossless_integer.rs:9:13
+  --> tests/ui/cast_lossless_integer.rs:10:13
    |
 LL |     let _ = 1i8 as i32;
    |             ^^^^^^^^^^ help: try: `i32::from(1i8)`
 
 error: casting `i8` to `i64` may become silently lossy if you later change the type
-  --> tests/ui/cast_lossless_integer.rs:10:13
+  --> tests/ui/cast_lossless_integer.rs:11:13
    |
 LL |     let _ = 1i8 as i64;
    |             ^^^^^^^^^^ help: try: `i64::from(1i8)`
 
 error: casting `u8` to `i16` may become silently lossy if you later change the type
-  --> tests/ui/cast_lossless_integer.rs:11:13
+  --> tests/ui/cast_lossless_integer.rs:12:13
    |
 LL |     let _ = 1u8 as i16;
    |             ^^^^^^^^^^ help: try: `i16::from(1u8)`
 
 error: casting `u8` to `i32` may become silently lossy if you later change the type
-  --> tests/ui/cast_lossless_integer.rs:12:13
+  --> tests/ui/cast_lossless_integer.rs:13:13
    |
 LL |     let _ = 1u8 as i32;
    |             ^^^^^^^^^^ help: try: `i32::from(1u8)`
 
 error: casting `u8` to `i64` may become silently lossy if you later change the type
-  --> tests/ui/cast_lossless_integer.rs:13:13
+  --> tests/ui/cast_lossless_integer.rs:14:13
    |
 LL |     let _ = 1u8 as i64;
    |             ^^^^^^^^^^ help: try: `i64::from(1u8)`
 
 error: casting `u8` to `u16` may become silently lossy if you later change the type
-  --> tests/ui/cast_lossless_integer.rs:14:13
+  --> tests/ui/cast_lossless_integer.rs:15:13
    |
 LL |     let _ = 1u8 as u16;
    |             ^^^^^^^^^^ help: try: `u16::from(1u8)`
 
 error: casting `u8` to `u32` may become silently lossy if you later change the type
-  --> tests/ui/cast_lossless_integer.rs:15:13
+  --> tests/ui/cast_lossless_integer.rs:16:13
    |
 LL |     let _ = 1u8 as u32;
    |             ^^^^^^^^^^ help: try: `u32::from(1u8)`
 
 error: casting `u8` to `u64` may become silently lossy if you later change the type
-  --> tests/ui/cast_lossless_integer.rs:16:13
+  --> tests/ui/cast_lossless_integer.rs:17:13
    |
 LL |     let _ = 1u8 as u64;
    |             ^^^^^^^^^^ help: try: `u64::from(1u8)`
 
 error: casting `i16` to `i32` may become silently lossy if you later change the type
-  --> tests/ui/cast_lossless_integer.rs:17:13
+  --> tests/ui/cast_lossless_integer.rs:18:13
    |
 LL |     let _ = 1i16 as i32;
    |             ^^^^^^^^^^^ help: try: `i32::from(1i16)`
 
 error: casting `i16` to `i64` may become silently lossy if you later change the type
-  --> tests/ui/cast_lossless_integer.rs:18:13
+  --> tests/ui/cast_lossless_integer.rs:19:13
    |
 LL |     let _ = 1i16 as i64;
    |             ^^^^^^^^^^^ help: try: `i64::from(1i16)`
 
 error: casting `u16` to `i32` may become silently lossy if you later change the type
-  --> tests/ui/cast_lossless_integer.rs:19:13
+  --> tests/ui/cast_lossless_integer.rs:20:13
    |
 LL |     let _ = 1u16 as i32;
    |             ^^^^^^^^^^^ help: try: `i32::from(1u16)`
 
 error: casting `u16` to `i64` may become silently lossy if you later change the type
-  --> tests/ui/cast_lossless_integer.rs:20:13
+  --> tests/ui/cast_lossless_integer.rs:21:13
    |
 LL |     let _ = 1u16 as i64;
    |             ^^^^^^^^^^^ help: try: `i64::from(1u16)`
 
 error: casting `u16` to `u32` may become silently lossy if you later change the type
-  --> tests/ui/cast_lossless_integer.rs:21:13
+  --> tests/ui/cast_lossless_integer.rs:22:13
    |
 LL |     let _ = 1u16 as u32;
    |             ^^^^^^^^^^^ help: try: `u32::from(1u16)`
 
 error: casting `u16` to `u64` may become silently lossy if you later change the type
-  --> tests/ui/cast_lossless_integer.rs:22:13
+  --> tests/ui/cast_lossless_integer.rs:23:13
    |
 LL |     let _ = 1u16 as u64;
    |             ^^^^^^^^^^^ help: try: `u64::from(1u16)`
 
 error: casting `i32` to `i64` may become silently lossy if you later change the type
-  --> tests/ui/cast_lossless_integer.rs:23:13
+  --> tests/ui/cast_lossless_integer.rs:24:13
    |
 LL |     let _ = 1i32 as i64;
    |             ^^^^^^^^^^^ help: try: `i64::from(1i32)`
 
 error: casting `u32` to `i64` may become silently lossy if you later change the type
-  --> tests/ui/cast_lossless_integer.rs:24:13
+  --> tests/ui/cast_lossless_integer.rs:25:13
    |
 LL |     let _ = 1u32 as i64;
    |             ^^^^^^^^^^^ help: try: `i64::from(1u32)`
 
 error: casting `u32` to `u64` may become silently lossy if you later change the type
-  --> tests/ui/cast_lossless_integer.rs:25:13
+  --> tests/ui/cast_lossless_integer.rs:26:13
    |
 LL |     let _ = 1u32 as u64;
    |             ^^^^^^^^^^^ help: try: `u64::from(1u32)`
 
 error: casting `u8` to `u16` may become silently lossy if you later change the type
-  --> tests/ui/cast_lossless_integer.rs:28:13
+  --> tests/ui/cast_lossless_integer.rs:29:13
    |
 LL |     let _ = (1u8 + 1u8) as u16;
    |             ^^^^^^^^^^^^^^^^^^ help: try: `u16::from(1u8 + 1u8)`
 
 error: casting `i8` to `I64` may become silently lossy if you later change the type
-  --> tests/ui/cast_lossless_integer.rs:30:13
+  --> tests/ui/cast_lossless_integer.rs:31:13
    |
 LL |     let _ = 1i8 as I64;
    |             ^^^^^^^^^^ help: try: `I64::from(1i8)`
 
 error: casting `i8` to `i32` may become silently lossy if you later change the type
-  --> tests/ui/cast_lossless_integer.rs:64:13
+  --> tests/ui/cast_lossless_integer.rs:70:13
    |
 LL |     let _ = sign_cast!(x, u8, i8) as i32;
    |             ^^^^^^^^^^^^^^^^^^^^^^^^^^^^ help: try: `i32::from(sign_cast!(x, u8, i8))`
 
 error: casting `i8` to `i32` may become silently lossy if you later change the type
-  --> tests/ui/cast_lossless_integer.rs:65:13
+  --> tests/ui/cast_lossless_integer.rs:71:13
    |
 LL |     let _ = (sign_cast!(x, u8, i8) + 1) as i32;
    |             ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^ help: try: `i32::from(sign_cast!(x, u8, i8) + 1)`