implement transparent typedef feature

docs.md

@@ -282,7 +282,76 @@ fn bar() -> Foo { .. } // Will be emitted as `struct foo bar();`
 
 ### Struct Annotations
 
-* field-names=\[field1, field2, ...\] -- sets the names of all the fields in the output struct. These names will be output verbatim, and are not eligible for renaming.
+* field-names=\[field1, field2, ...\] -- sets the names of all the fields in the output
+  struct. These names will be output verbatim, and are not eligible for renaming.
+
+* transparent-typedef -- when emitting the typedef for a transparent struct, mark it as
+  transparent. All references to the struct will be replaced with the type of its underlying NZST
+  field, effectively making the struct invisible on the FFI side. For example, consider the
+  following Rust code:
+
+  ```rust
+  #[repr(transparent)]
+  pub struct Handle<T> {
+      ptr: NonNull<T>,
+  }
+
+  pub struct Foo { }
+
+  #[no_mangle]
+  pub extern "C" fn foo_operation(foo: Option<Handle<Foo>>) { }
+  ```
+
+  By default, the exported C++ code would fail to compile, because the function takes `Option<...>`
+  (which is an opaque type) by value:
+
+  ```cpp
+  template<typename T>
+  struct Option<T>;
+
+  template<typename T>
+  using Handle = T;
+
+  struct Foo;
+
+  void foo_operation(Option<Handle<Foo>> foo);
+  ```
+
+  If we annotate `Handle` with `transparent-typedef` (leaving the rest of the code unchanged):
+  ```rust
+  /// cbindgen:transparent-typedef
+  #[repr(transparent)]
+  pub struct Handle<T> {
+      ptr: NonNull<T>,
+  }
+  ```
+
+  Then cbindgen is able to simplify the exported C++ code to just:
+  ```cpp
+  struct Foo;
+
+  void foo_operation(Foo* foo);
+  ```
+
+  NOTE: This annotation does _NOT_ affect user-defined type aliases for transparent structs. If we
+  we adjust the previous example to use a type alias:
+
+  ```rust
+  type NullableFooHandle = Option<Handle<Foo>>;
+
+  #[no_mangle]
+  pub extern "C" fn foo_operation(foo: NullableFooHandle) { }
+  ```
+
+  Then the exported code will use it as expected:
+
+  ```cpp
+  struct Foo;
+
+  using NullableFooHandle = Foo*;
+
+  void foo_operation(NullableFooHandle foo);
+  ```
 
 The rest are just local overrides for the same options found in the cbindgen.toml:
 
@@ -299,27 +368,25 @@ The rest are just local overrides for the same options found in the cbindgen.tom
   / etc(if any). The idea is for this to be used to annotate the operator with
   attributes, for example:
 
-```rust
-/// cbindgen:eq-attributes=MY_ATTRIBUTES
-#[repr(C)]
-pub struct Foo { .. }
-```
-
-Will generate something like:
+  ```rust
+  /// cbindgen:eq-attributes=MY_ATTRIBUTES
+  #[repr(C)]
+  pub struct Foo { .. }
+  ```
 
-```
-  MY_ATTRIBUTES bool operator==(const Foo& other) const {
-    ...
-  }
-```
+  Will generate something like:
 
-Combined with something like:
+  ```
+    MY_ATTRIBUTES bool operator==(const Foo& other) const {
+      ...
+    }
+  ```
 
-```
-#define MY_ATTRIBUTES [[nodiscard]]
-```
+  Combined with something like:
 
-for example.
+  ```
+  #define MY_ATTRIBUTES [[nodiscard]]
+  ```
 
 ### Enum Annotations
 

src/bindgen/builder.rs

@@ -368,7 +368,7 @@ impl Builder {
         let mut result = Parse::new();
 
         if self.std_types {
-            result.add_std_types();
+            result.add_std_types(self.config.language);
         }
 
         for x in &self.srcs {

src/bindgen/ir/constant.rs

@@ -13,8 +13,8 @@ use crate::bindgen::config::{Config, Language};
 use crate::bindgen::declarationtyperesolver::DeclarationTypeResolver;
 use crate::bindgen::dependencies::Dependencies;
 use crate::bindgen::ir::{
-    AnnotationSet, Cfg, ConditionWrite, Documentation, GenericParams, Item, ItemContainer, Path,
-    Struct, ToCondition, Type,
+    AnnotationSet, Cfg, ConditionWrite, Documentation, GenericArgument, GenericParams, Item,
+    ItemContainer, Path, Struct, ToCondition, TransparentTypeEraser, Type,
 };
 use crate::bindgen::language_backend::LanguageBackend;
 use crate::bindgen::library::Library;
@@ -597,6 +597,18 @@ impl Item for Constant {
     fn generic_params(&self) -> Option<&GenericParams> {
         None
     }
+
+    fn erase_transparent_types_inplace(
+        &mut self,
+        library: &Library,
+        eraser: &mut TransparentTypeEraser,
+        _generics: &[GenericArgument],
+    ) {
+        // NOTE: We also need to simplify the literal initializer value to match the underlying
+        // type, but that is true for all transparent structs (not just transparent-typedef
+        // structs), and is handled by the `write` method below.
+        eraser.erase_transparent_types_inplace(library, &mut self.ty, &[]);
+    }
 }
 
 impl Constant {

src/bindgen/ir/enumeration.rs

@@ -12,7 +12,7 @@ use crate::bindgen::dependencies::Dependencies;
 use crate::bindgen::ir::{
     AnnotationSet, AnnotationValue, Cfg, ConditionWrite, DeprecatedNoteKind, Documentation, Field,
     GenericArgument, GenericParams, GenericPath, Item, ItemContainer, Literal, Path, Repr,
-    ReprStyle, Struct, ToCondition, Type,
+    ReprStyle, Struct, ToCondition, TransparentTypeEraser, Type,
 };
 use crate::bindgen::language_backend::LanguageBackend;
 use crate::bindgen::library::Library;
@@ -247,12 +247,6 @@ impl EnumVariant {
         }
     }
 
-    fn simplify_standard_types(&mut self, config: &Config) {
-        if let VariantBody::Body { ref mut body, .. } = self.body {
-            body.simplify_standard_types(config);
-        }
-    }
-
     fn add_dependencies(&self, library: &Library, out: &mut Dependencies) {
         if let VariantBody::Body { ref body, .. } = self.body {
             body.add_dependencies(library, out);
@@ -500,6 +494,30 @@ impl Item for Enum {
         Some(&self.generic_params)
     }
 
+    fn erase_transparent_types_inplace(
+        &mut self,
+        library: &Library,
+        eraser: &mut TransparentTypeEraser,
+        generics: &[GenericArgument],
+    ) {
+        let mut skip_inline_tag_field = Self::inline_tag_field(&self.repr);
+        let generics = self.generic_params.defaulted_generics(generics);
+        let mappings = self.generic_params.call(self.name(), &generics);
+        for variant in self.variants.iter_mut() {
+            if let VariantBody::Body { ref mut body, .. } = variant.body {
+                for field in body.fields.iter_mut() {
+                    // Ignore the inline Tag field, if any (it's always first)
+                    if skip_inline_tag_field {
+                        debug!("Skipping inline Tag field {:?}", field);
+                        skip_inline_tag_field = false;
+                    } else {
+                        eraser.erase_transparent_types_inplace(library, &mut field.ty, &mappings);
+                    }
+                }
+            }
+        }
+    }
+
     fn rename_for_config(&mut self, config: &Config) {
         config.export.rename(&mut self.export_name);
 
@@ -1493,10 +1511,4 @@ impl Enum {
             }
         }
     }
-
-    pub fn simplify_standard_types(&mut self, config: &Config) {
-        for variant in &mut self.variants {
-            variant.simplify_standard_types(config);
-        }
-    }
 }

src/bindgen/ir/function.rs

@@ -9,7 +9,9 @@ use syn::ext::IdentExt;
 use crate::bindgen::config::{Config, Language};
 use crate::bindgen::declarationtyperesolver::DeclarationTypeResolver;
 use crate::bindgen::dependencies::Dependencies;
-use crate::bindgen::ir::{AnnotationSet, Cfg, Documentation, GenericPath, Path, Type};
+use crate::bindgen::ir::{
+    AnnotationSet, Cfg, Documentation, GenericPath, Path, TransparentTypeEraser, Type,
+};
 use crate::bindgen::library::Library;
 use crate::bindgen::monomorph::Monomorphs;
 use crate::bindgen::rename::{IdentifierType, RenameRule};
@@ -108,13 +110,6 @@ impl Function {
         &self.path
     }
 
-    pub fn simplify_standard_types(&mut self, config: &Config) {
-        self.ret.simplify_standard_types(config);
-        for arg in &mut self.args {
-            arg.ty.simplify_standard_types(config);
-        }
-    }
-
     pub fn add_dependencies(&self, library: &Library, out: &mut Dependencies) {
         self.ret.add_dependencies(library, out);
         for arg in &self.args {
@@ -143,6 +138,18 @@ impl Function {
         }
     }
 
+    // NOTE: No `generics` arg because Functions do not support generics and do not `impl Item`.
+    pub fn erase_transparent_types_inplace(
+        &mut self,
+        library: &Library,
+        eraser: &mut TransparentTypeEraser,
+    ) {
+        eraser.erase_transparent_types_inplace(library, &mut self.ret, &[]);
+        for arg in &mut self.args {
+            eraser.erase_transparent_types_inplace(library, &mut arg.ty, &[]);
+        }
+    }
+
     pub fn rename_for_config(&mut self, config: &Config) {
         // Rename the types used in arguments
         let generic_params = Default::default();

src/bindgen/ir/generic_path.rs

@@ -1,3 +1,5 @@
+use std::borrow::Cow;
+use std::collections::HashMap;
 use std::io::Write;
 use std::ops::Deref;
 
@@ -8,16 +10,17 @@ use crate::bindgen::config::{Config, Language};
 use crate::bindgen::declarationtyperesolver::{DeclarationType, DeclarationTypeResolver};
 use crate::bindgen::ir::{ConstExpr, Path, Type};
 use crate::bindgen::language_backend::LanguageBackend;
+use crate::bindgen::library::Library;
 use crate::bindgen::utilities::IterHelpers;
 use crate::bindgen::writer::SourceWriter;
 
-#[derive(Debug, Clone)]
+#[derive(Debug, Clone, PartialEq, Eq, Hash, PartialOrd, Ord)]
 pub enum GenericParamType {
     Type,
     Const(Type),
 }
 
-#[derive(Debug, Clone)]
+#[derive(Debug, Clone, PartialEq, Eq, Hash, PartialOrd, Ord)]
 pub struct GenericParam {
     name: Path,
     ty: GenericParamType,
@@ -97,6 +100,24 @@ impl GenericParams {
         Ok(GenericParams(params))
     }
 
+    /// If `generics` is empty, create a set of "default" generic arguments, which preserves the
+    /// existing parameter name. Useful to allow `call` to work when no generics are provided.
+    pub fn defaulted_generics<'a>(
+        &self,
+        generics: &'a [GenericArgument],
+    ) -> Cow<'a, [GenericArgument]> {
+        if !self.is_empty() && generics.is_empty() {
+            Cow::Owned(
+                self.iter()
+                    .map(|param| Type::Path(GenericPath::new(param.name.clone(), vec![])))
+                    .map(GenericArgument::Type)
+                    .collect(),
+            )
+        } else {
+            Cow::Borrowed(generics)
+        }
+    }
+
     /// Associate each parameter with an argument.
     pub fn call<'out>(
         &'out self,
@@ -228,6 +249,48 @@ impl GenericArgument {
     }
 }
 
+/// Helper for erasing transparent types, which memoizes already-seen types to avoid repeated work.
+#[derive(Default)]
+pub struct TransparentTypeEraser {
+    // Remember paths we've already visited, so we don't repeat unnecessary work.
+    // TODO: how to handle recursive types such as `struct Foo { next: Box<Foo> }`?
+    known_types: HashMap<Type, Option<Type>>,
+}
+
+impl TransparentTypeEraser {
+    pub fn erase_transparent_types_inplace(
+        &mut self,
+        library: &Library,
+        target: &mut Type,
+        mappings: &[(&Path, &GenericArgument)],
+    ) {
+        if let Some(erased_type) = self.erase_transparent_types(library, target, mappings) {
+            *target = erased_type;
+        }
+    }
+
+    #[must_use]
+    pub fn erase_transparent_types(
+        &mut self,
+        library: &Library,
+        target: &Type,
+        mappings: &[(&Path, &GenericArgument)],
+    ) -> Option<Type> {
+        let known_type = self.known_types.get(target);
+        let unknown_type = known_type.is_none();
+        let erased_type = if let Some(ty) = known_type {
+            ty.clone()
+        } else {
+            target.erase_transparent_types(library, mappings, self)
+        };
+        if unknown_type {
+            debug!("Caching erasure of {:?} as {:?}", target, erased_type);
+            self.known_types.insert(target.clone(), erased_type.clone());
+        }
+        erased_type
+    }
+}
+
 #[derive(Debug, Clone, PartialEq, Eq, Hash, PartialOrd, Ord)]
 pub struct GenericPath {
     path: Path,