Fix c_void false positive caused by libc refactoring

The path of `libc::c_void` has changes in rust-lang/libc@5c1a6b8
The DefId path is now always platform specific like
`libc::windows::c_void`. This fixes our c_void detection to only check
the first and last elements.

Author: phansch

Diff for: clippy_lints/src/types.rs

@@ -28,8 +28,9 @@ use crate::syntax::source_map::Span;
 use crate::utils::paths;
 use crate::utils::{
     clip, comparisons, differing_macro_contexts, higher, in_constant, in_macro, int_bits, last_path_segment,
-    match_def_path, match_path, match_type, multispan_sugg, opt_def_id, same_tys, sext, snippet, snippet_opt,
+    match_def_path, match_path, multispan_sugg, opt_def_id, same_tys, sext, snippet, snippet_opt,
     snippet_with_applicability, span_help_and_lint, span_lint, span_lint_and_sugg, span_lint_and_then, unsext,
+    AbsolutePathBuffer
 };
 use if_chain::if_chain;
 use std::borrow::Cow;
@@ -1023,6 +1024,21 @@ impl LintPass for CastPass {
     }
 }
 
+// Check if the given type is either `core::ffi::c_void` or
+// one of the platform specific `libc::<platform>::c_void` of libc.
+fn is_c_void(tcx: TyCtxt<'_, '_, '_>, ty: Ty<'_>) -> bool {
+    if let ty::Adt(adt, _) = ty.sty {
+        let mut apb = AbsolutePathBuffer { names: vec![] };
+        tcx.push_item_path(&mut apb, adt.did, false);
+
+        if apb.names.is_empty() { return false }
+        if apb.names[0] == "libc" || apb.names[0] == "core" && *apb.names.last().unwrap() == "c_void" {
+            return true
+        }
+    }
+    false
+}
+
 impl<'a, 'tcx> LateLintPass<'a, 'tcx> for CastPass {
     fn check_expr(&mut self, cx: &LateContext<'a, 'tcx>, expr: &'tcx Expr) {
         if let ExprKind::Cast(ref ex, _) = expr.node {
@@ -1114,10 +1130,7 @@ impl<'a, 'tcx> LateLintPass<'a, 'tcx> for CastPass {
                 if let Some(to_align) = cx.layout_of(to_ptr_ty.ty).ok().map(|a| a.align.abi);
                 if from_align < to_align;
                 // with c_void, we inherently need to trust the user
-                if ! (
-                    match_type(cx, from_ptr_ty.ty, &paths::C_VOID)
-                    || match_type(cx, from_ptr_ty.ty, &paths::C_VOID_LIBC)
-                );
+                if !is_c_void(cx.tcx, from_ptr_ty.ty);
                 then {
                     span_lint(
                         cx,

Diff for: clippy_lints/src/utils/mod.rs

@@ -29,6 +29,7 @@ use crate::syntax::attr;
 use crate::syntax::errors::DiagnosticBuilder;
 use crate::syntax::source_map::{Span, DUMMY_SP};
 use crate::syntax::symbol::{keywords, Symbol};
+use crate::syntax::symbol;
 use if_chain::if_chain;
 use matches::matches;
 use std::borrow::Cow;
@@ -74,6 +75,25 @@ pub fn in_macro(span: Span) -> bool {
     span.ctxt().outer().expn_info().is_some()
 }
 
+/// Used to store the absolute path to a type.
+///
+/// See `match_def_path` for usage.
+#[derive(Debug)]
+pub struct AbsolutePathBuffer {
+    pub names: Vec<symbol::LocalInternedString>,
+}
+
+impl ty::item_path::ItemPathBuffer for AbsolutePathBuffer {
+    fn root_mode(&self) -> &ty::item_path::RootMode {
+        const ABSOLUTE: &ty::item_path::RootMode = &ty::item_path::RootMode::Absolute;
+        ABSOLUTE
+    }
+
+    fn push(&mut self, text: &str) {
+        self.names.push(symbol::Symbol::intern(text).as_str());
+    }
+}
+
 /// Check if a `DefId`'s path matches the given absolute type path usage.
 ///
 /// # Examples
@@ -83,24 +103,6 @@ pub fn in_macro(span: Span) -> bool {
 ///
 /// See also the `paths` module.
 pub fn match_def_path(tcx: TyCtxt<'_, '_, '_>, def_id: DefId, path: &[&str]) -> bool {
-    use crate::syntax::symbol;
-
-    #[derive(Debug)]
-    struct AbsolutePathBuffer {
-        names: Vec<symbol::LocalInternedString>,
-    }
-
-    impl ty::item_path::ItemPathBuffer for AbsolutePathBuffer {
-        fn root_mode(&self) -> &ty::item_path::RootMode {
-            const ABSOLUTE: &ty::item_path::RootMode = &ty::item_path::RootMode::Absolute;
-            ABSOLUTE
-        }
-
-        fn push(&mut self, text: &str) {
-            self.names.push(symbol::Symbol::intern(text).as_str());
-        }
-    }
-
     let mut apb = AbsolutePathBuffer { names: vec![] };
 
     tcx.push_item_path(&mut apb, def_id, false);

Diff for: clippy_lints/src/utils/paths.rs

@@ -27,8 +27,6 @@ pub const CMP_MAX: [&str; 3] = ["core", "cmp", "max"];
 pub const CMP_MIN: [&str; 3] = ["core", "cmp", "min"];
 pub const COW: [&str; 3] = ["alloc", "borrow", "Cow"];
 pub const CSTRING_NEW: [&str; 5] = ["std", "ffi", "c_str", "CString", "new"];
-pub const C_VOID: [&str; 3] = ["core", "ffi", "c_void"];
-pub const C_VOID_LIBC: [&str; 2] = ["libc", "c_void"];
 pub const DEFAULT_TRAIT: [&str; 3] = ["core", "default", "Default"];
 pub const DEFAULT_TRAIT_METHOD: [&str; 4] = ["core", "default", "Default", "default"];
 pub const DEREF_TRAIT_METHOD: [&str; 5] = ["core", "ops", "deref", "Deref", "deref"];