Convert function_cast_as_integer lint suggestion to plain *const () cast

Author: GuillaumeGomez

compiler/rustc_lint/messages.ftl

@@ -266,7 +266,7 @@ lint_forgetting_references = calls to `std::mem::forget` with a reference instea
     .label = argument has type `{$arg_ty}`
 
 lint_function_casts_as_integer = direct cast of function item into an integer
-    .cast_as_fn = first cast to a function pointer `{$cast_from_ty}`
+    .cast_as_fn = first cast to a pointer `as *const ()`
 
 lint_hidden_glob_reexport = private item shadows public glob re-export
     .note_glob_reexport = the name `{$name}` in the {$namespace} namespace is supposed to be publicly re-exported here

compiler/rustc_lint/src/function_cast_as_integer.rs

@@ -39,7 +39,7 @@ impl<'tcx> LateLintPass<'tcx> for FunctionCastsAsInteger {
         let hir::ExprKind::Cast(cast_from_expr, cast_to_expr) = expr.kind else { return };
         let cast_to_ty = cx.typeck_results().expr_ty(expr);
         // Casting to a function (pointer?), so all good.
-        if matches!(cast_to_ty.kind(), ty::FnDef(..) | ty::FnPtr(..)) {
+        if matches!(cast_to_ty.kind(), ty::FnDef(..) | ty::FnPtr(..) | ty::RawPtr(..)) {
             return;
         }
         let cast_from_ty = cx.typeck_results().expr_ty(cast_from_expr);
@@ -51,8 +51,6 @@ impl<'tcx> LateLintPass<'tcx> for FunctionCastsAsInteger {
                 FunctionCastsAsIntegerDiag {
                     sugg: FunctionCastsAsIntegerSugg {
                         suggestion: cast_from_expr.span.shrink_to_hi(),
-                        // We get the function pointer to have a nice display.
-                        cast_from_ty: cx.typeck_results().expr_ty_adjusted(cast_from_expr),
                         cast_to_ty,
                     },
                 },

compiler/rustc_lint/src/lints.rs

@@ -3029,14 +3029,13 @@ pub(crate) struct FunctionCastsAsIntegerDiag<'tcx> {
 #[derive(Subdiagnostic)]
 #[suggestion(
     lint_cast_as_fn,
-    code = " as {cast_from_ty}",
+    code = " as *const ()",
     applicability = "machine-applicable",
     style = "verbose"
 )]
 pub(crate) struct FunctionCastsAsIntegerSugg<'tcx> {
     #[primary_span]
     pub suggestion: Span,
-    pub cast_from_ty: Ty<'tcx>,
     pub cast_to_ty: Ty<'tcx>,
 }
 

tests/ui/lint/function_casts_as_integer.fixed

@@ -6,8 +6,9 @@
 fn foo() {}
 
 fn main() {
-    let x = foo as fn() as usize; //~ ERROR: function_casts_as_integer
-    let x = String::len as for<'a> fn(&'a String) -> usize as usize; //~ ERROR: function_casts_as_integer
+    let x = foo as *const () as usize; //~ ERROR: function_casts_as_integer
+    let x = String::len as *const () as usize; //~ ERROR: function_casts_as_integer
     // Ok.
     let x = foo as fn() as usize;
+    let x = foo as *const () as usize;
 }

tests/ui/lint/function_casts_as_integer.rs

@@ -10,4 +10,5 @@ fn main() {
     let x = String::len as usize; //~ ERROR: function_casts_as_integer
     // Ok.
     let x = foo as fn() as usize;
+    let x = foo as *const () as usize;
 }

tests/ui/lint/function_casts_as_integer.stderr

@@ -9,21 +9,21 @@ note: the lint level is defined here
    |
 LL | #![deny(function_casts_as_integer)]
    |         ^^^^^^^^^^^^^^^^^^^^^^^^^
-help: first cast to a function pointer `fn()`
+help: first cast to a pointer `as *const ()`
    |
-LL |     let x = foo as fn() as usize;
-   |                 +++++++
+LL |     let x = foo as *const () as usize;
+   |                 ++++++++++++
 
 error: direct cast of function item into an integer
   --> $DIR/function_casts_as_integer.rs:10:25
    |
 LL |     let x = String::len as usize;
    |                         ^^^^^^^^
    |
-help: first cast to a function pointer `for<'a> fn(&'a String) -> usize`
+help: first cast to a pointer `as *const ()`
    |
-LL |     let x = String::len as for<'a> fn(&'a String) -> usize as usize;
-   |                         ++++++++++++++++++++++++++++++++++
+LL |     let x = String::len as *const () as usize;
+   |                         ++++++++++++
 
 error: aborting due to 2 previous errors