Auto merge of #1311 - cynecx:canonicalize_pointer_ty, r=emilio

 Canonicalize a type (pointer type) first before checking for constness

This fixes cases like:

```cpp
int sodium_munlock(int* const addr); // addr itself is const but the pointer points to mutable data
```

Before fix:

```rust
extern "C" {
    pub fn sodium_munlock(
        addr: *const ::std::os::raw::c_int,
    ) -> ::std::os::raw::c_int;
}

```

After this patch:
```rust
extern "C" {
    pub fn sodium_munlock(
        addr: *mut ::std::os::raw::c_int,
    ) -> ::std::os::raw::c_int;
}
```

Author: bors-servo

src/ir/ty.rs

@@ -1194,7 +1194,23 @@ impl Type {
         };
 
         let name = if name.is_empty() { None } else { Some(name) };
-        let is_const = ty.is_const();
+
+        // Just using ty.is_const() is wrong here, because when we declare an
+        // argument like 'int* const arg0', arg0 is considered
+        // const but the pointer itself points to mutable data.
+        //
+        // Without canonicalizing the type to the pointer type, we'll get the
+        // following mapping:
+        //
+        // arg0: *const c_int
+        //
+        // So by canonicalizing the type first, we can check constness by
+        // calling is_const() on the pointer type.
+        let is_const = if let Some(pty) = ty.pointee_type() {
+            pty.is_const()
+        } else {
+            ty.is_const()
+        };
 
         let ty = Type::new(name, layout, kind, is_const);
         // TODO: maybe declaration.canonical()?

tests/expectations/tests/const_tparam.rs

@@ -1,14 +1,12 @@
 /* automatically generated by rust-bindgen */
 
-
 #![allow(dead_code, non_snake_case, non_camel_case_types, non_upper_case_globals)]
 
-
 #[repr(C)]
 #[derive(Debug, Copy, Clone)]
 pub struct C<T> {
     pub foo: *const T,
-    pub bar: *mut T,
+    pub bar: *const T,
     pub _phantom_0: ::std::marker::PhantomData<::std::cell::UnsafeCell<T>>,
 }
 impl<T> Default for C<T> {

tests/expectations/tests/derive-hash-struct-with-pointer.rs

@@ -1,15 +1,12 @@
 /* automatically generated by rust-bindgen */
 
-
 #![allow(dead_code, non_snake_case, non_camel_case_types, non_upper_case_globals)]
 
-
-
 /// Pointers can derive Hash/PartialOrd/Ord/PartialEq/Eq
 #[repr(C)]
 #[derive(Debug, Copy, Clone, Hash, PartialOrd, Ord, PartialEq, Eq)]
 pub struct ConstPtrMutObj {
-    pub bar: *const ::std::os::raw::c_int,
+    pub bar: *mut ::std::os::raw::c_int,
 }
 #[test]
 fn bindgen_test_layout_ConstPtrMutObj() {

tests/expectations/tests/issue-511.rs

@@ -0,0 +1,20 @@
+/* automatically generated by rust-bindgen */
+
+#![allow(dead_code, non_snake_case, non_camel_case_types, non_upper_case_globals)]
+
+extern "C" {
+    #[link_name = "\u{1}a"]
+    pub static mut a: *mut ::std::os::raw::c_char;
+}
+extern "C" {
+    #[link_name = "\u{1}b"]
+    pub static mut b: *const ::std::os::raw::c_char;
+}
+extern "C" {
+    #[link_name = "\u{1}c"]
+    pub static mut c: *mut ::std::os::raw::c_char;
+}
+extern "C" {
+    #[link_name = "\u{1}d"]
+    pub static mut d: *const ::std::os::raw::c_char;
+}

tests/expectations/tests/layout_array.rs

@@ -27,7 +27,7 @@ pub type rte_mempool_free_t = ::std::option::Option<unsafe extern "C" fn(mp: *mu
 pub type rte_mempool_enqueue_t = ::std::option::Option<
     unsafe extern "C" fn(
         mp: *mut rte_mempool,
-        obj_table: *const *const ::std::os::raw::c_void,
+        obj_table: *const *mut ::std::os::raw::c_void,
         n: ::std::os::raw::c_uint,
     ) -> ::std::os::raw::c_int,
 >;

tests/headers/issue-511.h

@@ -0,0 +1,4 @@
+char * a;
+const char * b;
+char * const c;
+const char * const d;