ptr_cast_constness: avoid suggesting unresolvable method call (rust-lang#15540)

changelog: [`ptr_cast_constness`]: avoid suggesting unresolvable method
call

Spell check says "unresolvable" isn't a word. Which repository do I open
a PR against to fix that?

Fixes rust-lang#11317

Author: Alexendoo

clippy_lints/src/casts/ptr_cast_constness.rs

@@ -4,7 +4,7 @@ use clippy_utils::source::snippet_with_applicability;
 use clippy_utils::sugg::Sugg;
 use clippy_utils::{std_or_core, sym};
 use rustc_errors::Applicability;
-use rustc_hir::{Expr, ExprKind, QPath};
+use rustc_hir::{self as hir, Expr, ExprKind, QPath};
 use rustc_lint::LateContext;
 use rustc_middle::ty::{self, Ty, TypeVisitableExt};
 
@@ -51,7 +51,16 @@ pub(super) fn check<'tcx>(
 
         if msrv.meets(cx, msrvs::POINTER_CAST_CONSTNESS) {
             let mut app = Applicability::MachineApplicable;
-            let sugg = Sugg::hir_with_context(cx, cast_from_expr, expr.span.ctxt(), "_", &mut app);
+            let sugg = if let ExprKind::Cast(nested_from, nested_hir_ty) = cast_from_expr.kind
+                && let hir::TyKind::Ptr(ptr_ty) = nested_hir_ty.kind
+                && let hir::TyKind::Infer(()) = ptr_ty.ty.kind
+            {
+                // `(foo as *const _).cast_mut()` fails method name resolution
+                // avoid this by `as`-ing the full type
+                Sugg::hir_with_context(cx, nested_from, expr.span.ctxt(), "_", &mut app).as_ty(cast_from)
+            } else {
+                Sugg::hir_with_context(cx, cast_from_expr, expr.span.ctxt(), "_", &mut app)
+            };
             let constness = to_mutbl.ptr_str();
 
             span_lint_and_sugg(

tests/ui/ptr_cast_constness.fixed

@@ -106,3 +106,9 @@ fn issue14621() {
     let _ = std::ptr::addr_of_mut!(local).cast_const();
     //~^ ptr_cast_constness
 }
+
+fn issue11317() {
+    let r = &0_u32;
+    let _ptr: *mut u32 = (r as *const u32).cast_mut();
+    //~^ ptr_cast_constness
+}

tests/ui/ptr_cast_constness.rs

@@ -106,3 +106,9 @@ fn issue14621() {
     let _ = std::ptr::addr_of_mut!(local) as *const _;
     //~^ ptr_cast_constness
 }
+
+fn issue11317() {
+    let r = &0_u32;
+    let _ptr: *mut u32 = r as *const _ as *mut _;
+    //~^ ptr_cast_constness
+}

tests/ui/ptr_cast_constness.stderr

@@ -89,5 +89,11 @@ error: `as` casting between raw pointers while changing only its constness
 LL |     let _ = std::ptr::addr_of_mut!(local) as *const _;
    |             ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^ help: try `pointer::cast_const`, a safer alternative: `std::ptr::addr_of_mut!(local).cast_const()`
 
-error: aborting due to 14 previous errors
+error: `as` casting between raw pointers while changing only its constness
+  --> tests/ui/ptr_cast_constness.rs:112:26
+   |
+LL |     let _ptr: *mut u32 = r as *const _ as *mut _;
+   |                          ^^^^^^^^^^^^^^^^^^^^^^^ help: try `pointer::cast_mut`, a safer alternative: `(r as *const u32).cast_mut()`
+
+error: aborting due to 15 previous errors