Fix u16 enum repr.

A previous change assumed we could always treat u16 as char16_t, since
it appeared only to be used to represent char16_t. It turns out it's
also used for the representation of 16-bit-wide enums, so we need to
keep track of the distinction between these two types as they travel
through autocxx-bindgen.

Relates to google/autocxx#1214

Author: adetaylor

bindgen/codegen/mod.rs

@@ -3941,9 +3941,7 @@ impl TryToRustTy for Type {
                     IntKind::I8 => Ok(quote! { i8 }.into()),
                     IntKind::U8 => Ok(quote! { u8 }.into()),
                     IntKind::I16 => Ok(quote! { i16 }.into()),
-                    IntKind::U16 if ctx.options().use_distinct_char16_t => {
-                        Ok(quote! { c_char16_t }.into())
-                    }
+                    IntKind::Char16 => Ok(quote! { c_char16_t }.into()),
                     IntKind::U16 => Ok(quote! { u16 }.into()),
                     IntKind::I32 => Ok(quote! { i32 }.into()),
                     IntKind::U32 => Ok(quote! { u32 }.into()),

bindgen/ir/context.rs

@@ -1947,6 +1947,7 @@ If you encounter an error missing from this list, please file an issue or a PR!"
             CXType_Short => TypeKind::Int(IntKind::Short),
             CXType_UShort => TypeKind::Int(IntKind::UShort),
             CXType_WChar => TypeKind::Int(IntKind::WChar),
+            CXType_Char16 if self.options().use_distinct_char16_t => TypeKind::Int(IntKind::Char16),
             CXType_Char16 => TypeKind::Int(IntKind::U16),
             CXType_Char32 => TypeKind::Int(IntKind::U32),
             CXType_Long => TypeKind::Int(IntKind::Long),

bindgen/ir/int.rs

@@ -54,9 +54,12 @@ pub enum IntKind {
     /// A 16-bit signed integer.
     I16,
 
-    /// Either a `char16_t` or a `wchar_t`.
+    /// A 16-bit integer, used only for enum size representation.
     U16,
 
+    /// Either a `char16_t` or a `wchar_t`.
+    Char16,
+
     /// A 32-bit signed integer.
     I32,
 
@@ -93,7 +96,7 @@ impl IntKind {
             // TODO(emilio): wchar_t can in theory be signed, but we have no way
             // to know whether it is or not right now (unlike char, there's no
             // WChar_S / WChar_U).
-            Bool | UChar | UShort | UInt | ULong | ULongLong | U8 | U16 |
+            Bool | UChar | UShort | UInt | ULong | ULongLong | U8 | U16 | Char16 |
             WChar | U32 | U64 | U128 => false,
 
             SChar | Short | Int | Long | LongLong | I8 | I16 | I32 | I64 |
@@ -112,7 +115,7 @@ impl IntKind {
         use self::IntKind::*;
         Some(match *self {
             Bool | UChar | SChar | U8 | I8 | Char { .. } => 1,
-            U16 | I16 => 2,
+            U16 | I16 | Char16 => 2,
             U32 | I32 => 4,
             U64 | I64 => 8,
             I128 | U128 => 16,