fix: cast-lossless should not suggest when casting type is from macro input

Author: profetia

clippy_lints/src/casts/cast_lossless.rs

@@ -46,6 +46,12 @@ pub(super) fn check(
                         applicability,
                     );
                 },
+                _ if expr.span.from_expansion() && !cast_to_hir.span.eq_ctxt(expr.span)  => {
+                    diag.span_help(
+                        expr.span,
+                        format!("use `<{ty}>::from` instead")
+                    );
+                },
                 // Don't suggest `A<_>::B::From(x)` or `macro!()::from(x)`
                 kind if matches!(kind, TyKind::Path(QPath::Resolved(_, path)) if path.segments.iter().any(|s| s.args.is_some()))
                     || !cast_to_hir.span.eq_ctxt(expr.span) =>

tests/ui/cast_lossless_integer_unfixable.rs

@@ -0,0 +1,19 @@
+#![warn(clippy::cast_lossless)]
+
+fn issue15348() {
+    macro_rules! zero {
+        ($int:ty) => {{
+            let data: [u8; 3] = [0, 0, 0];
+            data[0] as $int
+            //~^ cast_lossless
+            //~| cast_lossless
+            //~| cast_lossless
+            //~| cast_lossless
+        }};
+    }
+
+    let _ = zero!(u16);
+    let _ = zero!(u32);
+    let _ = zero!(u64);
+    let _ = zero!(u128);
+}

tests/ui/cast_lossless_integer_unfixable.stderr

@@ -0,0 +1,84 @@
+error: casts from `u8` to `u16` can be expressed infallibly using `From`
+  --> tests/ui/cast_lossless_integer_unfixable.rs:7:13
+   |
+LL |             data[0] as $int
+   |             ^^^^^^^^^^^^^^^
+...
+LL |     let _ = zero!(u16);
+   |             ---------- in this macro invocation
+   |
+   = help: an `as` cast can become silently lossy if the types change in the future
+help: use `<u16>::from` instead
+  --> tests/ui/cast_lossless_integer_unfixable.rs:7:13
+   |
+LL |             data[0] as $int
+   |             ^^^^^^^^^^^^^^^
+...
+LL |     let _ = zero!(u16);
+   |             ---------- in this macro invocation
+   = note: `-D clippy::cast-lossless` implied by `-D warnings`
+   = help: to override `-D warnings` add `#[allow(clippy::cast_lossless)]`
+   = note: this error originates in the macro `zero` (in Nightly builds, run with -Z macro-backtrace for more info)
+
+error: casts from `u8` to `u32` can be expressed infallibly using `From`
+  --> tests/ui/cast_lossless_integer_unfixable.rs:7:13
+   |
+LL |             data[0] as $int
+   |             ^^^^^^^^^^^^^^^
+...
+LL |     let _ = zero!(u32);
+   |             ---------- in this macro invocation
+   |
+   = help: an `as` cast can become silently lossy if the types change in the future
+help: use `<u32>::from` instead
+  --> tests/ui/cast_lossless_integer_unfixable.rs:7:13
+   |
+LL |             data[0] as $int
+   |             ^^^^^^^^^^^^^^^
+...
+LL |     let _ = zero!(u32);
+   |             ---------- in this macro invocation
+   = note: this error originates in the macro `zero` (in Nightly builds, run with -Z macro-backtrace for more info)
+
+error: casts from `u8` to `u64` can be expressed infallibly using `From`
+  --> tests/ui/cast_lossless_integer_unfixable.rs:7:13
+   |
+LL |             data[0] as $int
+   |             ^^^^^^^^^^^^^^^
+...
+LL |     let _ = zero!(u64);
+   |             ---------- in this macro invocation
+   |
+   = help: an `as` cast can become silently lossy if the types change in the future
+help: use `<u64>::from` instead
+  --> tests/ui/cast_lossless_integer_unfixable.rs:7:13
+   |
+LL |             data[0] as $int
+   |             ^^^^^^^^^^^^^^^
+...
+LL |     let _ = zero!(u64);
+   |             ---------- in this macro invocation
+   = note: this error originates in the macro `zero` (in Nightly builds, run with -Z macro-backtrace for more info)
+
+error: casts from `u8` to `u128` can be expressed infallibly using `From`
+  --> tests/ui/cast_lossless_integer_unfixable.rs:7:13
+   |
+LL |             data[0] as $int
+   |             ^^^^^^^^^^^^^^^
+...
+LL |     let _ = zero!(u128);
+   |             ----------- in this macro invocation
+   |
+   = help: an `as` cast can become silently lossy if the types change in the future
+help: use `<u128>::from` instead
+  --> tests/ui/cast_lossless_integer_unfixable.rs:7:13
+   |
+LL |             data[0] as $int
+   |             ^^^^^^^^^^^^^^^
+...
+LL |     let _ = zero!(u128);
+   |             ----------- in this macro invocation
+   = note: this error originates in the macro `zero` (in Nightly builds, run with -Z macro-backtrace for more info)
+
+error: aborting due to 4 previous errors
+