as_pointer_underscore: reduce applicability for unnameable inferred type

don't emit machine applicable suggestion when the inferred type is
unnameable, for example fn item pointer

changelog: [`as_pointer_underscore`]: reduce applicability for
unnameable inferred type

Signed-off-by: Zihan <[email protected]>

Author: zihan0822

clippy_lints/src/casts/as_pointer_underscore.rs

@@ -1,19 +1,32 @@
+use clippy_utils::ty::ty_cannot_be_named;
 use rustc_errors::Applicability;
 use rustc_lint::LateContext;
-use rustc_middle::ty::Ty;
+use rustc_middle::ty::{self, Ty};
 
 pub fn check<'tcx>(cx: &LateContext<'tcx>, ty_into: Ty<'_>, cast_to_hir: &'tcx rustc_hir::Ty<'tcx>) {
     if let rustc_hir::TyKind::Ptr(rustc_hir::MutTy { ty, .. }) = cast_to_hir.kind
         && matches!(ty.kind, rustc_hir::TyKind::Infer(()))
+        && let ty::RawPtr(pointee, _) = ty_into.kind()
     {
-        clippy_utils::diagnostics::span_lint_and_sugg(
-            cx,
-            super::AS_POINTER_UNDERSCORE,
-            cast_to_hir.span,
-            "using inferred pointer cast",
-            "use explicit type",
-            ty_into.to_string(),
-            Applicability::MachineApplicable,
-        );
+        if ty_cannot_be_named(*pointee) {
+            clippy_utils::diagnostics::span_lint_and_help(
+                cx,
+                super::AS_POINTER_UNDERSCORE,
+                cast_to_hir.span,
+                format!("using inferred pointer cast to unnameable type: `{ty_into}`"),
+                None,
+                "use explicit type",
+            );
+        } else {
+            clippy_utils::diagnostics::span_lint_and_sugg(
+                cx,
+                super::AS_POINTER_UNDERSCORE,
+                cast_to_hir.span,
+                "using inferred pointer cast",
+                "use explicit type",
+                ty_into.to_string(),
+                Applicability::MachineApplicable,
+            );
+        };
     }
 }

tests/ui/as_pointer_underscore_unfixable.rs

@@ -0,0 +1,10 @@
+#![warn(clippy::as_pointer_underscore)]
+#![crate_type = "lib"]
+#![no_std]
+
+fn issue_15281() {
+    fn bar(_: usize) {}
+    // pointer to fn item, lint should not trigger
+    let _ = &bar as *const _;
+    //~^ as_pointer_underscore
+}

tests/ui/as_pointer_underscore_unfixable.stderr

@@ -0,0 +1,12 @@
+error: using inferred pointer cast to unnameable type: `*const fn(usize) {issue_15281::bar}`
+  --> tests/ui/as_pointer_underscore_unfixable.rs:8:21
+   |
+LL |     let _ = &bar as *const _;
+   |                     ^^^^^^^^
+   |
+   = help: use explicit type
+   = note: `-D clippy::as-pointer-underscore` implied by `-D warnings`
+   = help: to override `-D warnings` add `#[allow(clippy::as_pointer_underscore)]`
+
+error: aborting due to 1 previous error
+