Fix name collision between C enum and typedef (#2326)

Fixes #2008.

Example:

```c
enum Enum { Variant };
typedef int16_t Enum;
```

This is valid and idiomatic C (though not valid C++). `cbindgen` uses this idiom as the default C translation of Rust enums, the equivalent of what would be `enum Enum : int16_t { Variant };` in C++.

`bindgen header.h` before:

```rust
pub const Enum_Variant: Enum = 0;
pub type Enum = ::std::os::raw::c_uint;
pub type Enum = i16;
```

```console
error[E0428]: the name `Enum` is defined multiple times
 --> generated.rs:3:1
  |
2 | pub type Enum = ::std::os::raw::c_uint;
  | --------------------------------------- previous definition of the type `Enum` here
3 | pub type Enum = i16;
  | ^^^^^^^^^^^^^^^^^^^^ `Enum` redefined here
  |
  = note: `Enum` must be defined only once in the type namespace of this module
```

After:

```rust

pub const Enum_Variant: Enum = 0;
pub type Enum = i16;
```

5 files changed, 126 insertions, 5 deletions

diff --git a/bindgen-tests/tests/expectations/tests/enum-typedef.rs b/bindgen-tests/tests/expectations/tests/enum-typedef.rs
new file mode 100644
index 00000000..dc78eb11
--- /dev/null
+++ b/bindgen-tests/tests/expectations/tests/enum-typedef.rs
@@ -0,0 +1,11 @@
+#![allow(
+    dead_code,
+    non_snake_case,
+    non_camel_case_types,
+    non_upper_case_globals
+)]
+
+pub const Enum_Variant: Enum = 0;
+pub type Enum = i16;
+pub type TypedefFirst = i16;
+pub const TypedefFirst_Variant2: TypedefFirst = 0;
diff --git a/bindgen-tests/tests/headers/enum-typedef.h b/bindgen-tests/tests/headers/enum-typedef.h
new file mode 100644
index 00000000..f345f4de
--- /dev/null
+++ b/bindgen-tests/tests/headers/enum-typedef.h
@@ -0,0 +1,18 @@
+typedef short int16_t;
+
+// `cbindgen` emits this C idiom as the translation of:
+//
+//     #[repr(i16)]
+//     pub enum Enum {
+//         Variant,
+//     }
+enum Enum {
+  Variant,
+};
+typedef int16_t Enum;
+
+// C is also fine with the typedef coming before the enum.
+typedef int16_t TypedefFirst;
+enum TypedefFirst {
+  Variant2,
+};
diff --git a/bindgen/codegen/mod.rs b/bindgen/codegen/mod.rs
index b4050637..154d7fd1 100644
--- a/bindgen/codegen/mod.rs
+++ b/bindgen/codegen/mod.rs
@@ -2709,6 +2709,7 @@ impl<'a> EnumBuilder<'a> {
         mut attrs: Vec<proc_macro2::TokenStream>,
         repr: proc_macro2::TokenStream,
         enum_variation: EnumVariation,
+        has_typedef: bool,
     ) -> Self {
         let ident = Ident::new(name, Span::call_site());
 
@@ -2741,10 +2742,12 @@ impl<'a> EnumBuilder<'a> {
             EnumVariation::Consts => {
                 let mut variants = Vec::new();
 
-                variants.push(quote! {
-                    #( #attrs )*
-                    pub type #ident = #repr;
-                });
+                if !has_typedef {
+                    variants.push(quote! {
+                        #( #attrs )*
+                        pub type #ident = #repr;
+                    });
+                }
 
                 EnumBuilder::Consts { variants }
             }
@@ -3157,8 +3160,10 @@ impl CodeGenerator for Enum {
         }
 
         let repr = repr.to_rust_ty_or_opaque(ctx, item);
+        let has_typedef = ctx.is_enum_typedef_combo(item.id());
 
-        let mut builder = EnumBuilder::new(&name, attrs, repr, variation);
+        let mut builder =
+            EnumBuilder::new(&name, attrs, repr, variation, has_typedef);
 
         // A map where we keep a value -> variant relation.
         let mut seen_values = HashMap::<_, Ident>::default();
diff --git a/bindgen/ir/context.rs b/bindgen/ir/context.rs
index 3cc30f1c..c5df37d7 100644
--- a/bindgen/ir/context.rs
+++ b/bindgen/ir/context.rs
@@ -398,6 +398,22 @@ pub struct BindgenContext {
     /// bitfield allocation units computed. Drained in `compute_bitfield_units`.
     need_bitfield_allocation: Vec<ItemId>,
 
+    /// The set of enums that are defined by a pair of `enum` and `typedef`,
+    /// which is legal in C (but not C++).
+    ///
+    /// ```c++
+    /// // in either order
+    /// enum Enum { Variants... };
+    /// typedef int16_t Enum;
+    /// ```
+    ///
+    /// The stored `ItemId` is that of the `TypeKind::Enum`, not of the
+    /// `TypeKind::Alias`.
+    ///
+    /// This is populated when we enter codegen by `compute_enum_typedef_combos`
+    /// and is always `None` before that and `Some` after.
+    enum_typedef_combos: Option<HashSet<ItemId>>,
+
     /// The set of (`ItemId`s of) types that can't derive debug.
     ///
     /// This is populated when we enter codegen by `compute_cannot_derive_debug`
@@ -565,6 +581,7 @@ If you encounter an error missing from this list, please file an issue or a PR!"
             codegen_items: None,
             used_template_parameters: None,
             need_bitfield_allocation: Default::default(),
+            enum_typedef_combos: None,
             cannot_derive_debug: None,
             cannot_derive_default: None,
             cannot_derive_copy: None,
@@ -1157,6 +1174,7 @@ If you encounter an error missing from this list, please file an issue or a PR!"
         self.compute_sizedness();
         self.compute_has_destructor();
         self.find_used_template_parameters();
+        self.compute_enum_typedef_combos();
         self.compute_cannot_derive_debug();
         self.compute_cannot_derive_default();
         self.compute_cannot_derive_copy();
@@ -2476,6 +2494,70 @@ If you encounter an error missing from this list, please file an issue or a PR!"
         self.generated_bindgen_complex.get()
     }
 
+    /// Compute which `enum`s have an associated `typedef` definition.
+    fn compute_enum_typedef_combos(&mut self) {
+        let _t = self.timer("compute_enum_typedef_combos");
+        assert!(self.enum_typedef_combos.is_none());
+
+        let mut enum_typedef_combos = HashSet::default();
+        for item in &self.items {
+            if let Some(ItemKind::Module(module)) =
+                item.as_ref().map(Item::kind)
+            {
+                // Find typedefs in this module, and build set of their names.
+                let mut names_of_typedefs = HashSet::default();
+                for child_id in module.children() {
+                    if let Some(ItemKind::Type(ty)) =
+                        self.items[child_id.0].as_ref().map(Item::kind)
+                    {
+                        if let (Some(name), TypeKind::Alias(type_id)) =
+                            (ty.name(), ty.kind())
+                        {
+                            // We disregard aliases that refer to the enum
+                            // itself, such as in `typedef enum { ... } Enum;`.
+                            if type_id
+                                .into_resolver()
+                                .through_type_refs()
+                                .through_type_aliases()
+                                .resolve(self)
+                                .expect_type()
+                                .is_int()
+                            {
+                                names_of_typedefs.insert(name);
+                            }
+                        }
+                    }
+                }
+
+                // Find enums in this module, and record the id of each one that
+                // has a typedef.
+                for child_id in module.children() {
+                    if let Some(ItemKind::Type(ty)) =
+                        self.items[child_id.0].as_ref().map(Item::kind)
+                    {
+                        if let (Some(name), true) = (ty.name(), ty.is_enum()) {
+                            if names_of_typedefs.contains(name) {
+                                enum_typedef_combos.insert(*child_id);
+                            }
+                        }
+                    }
+                }
+            }
+        }
+
+        self.enum_typedef_combos = Some(enum_typedef_combos);
+    }
+
+    /// Look up whether `id` refers to an `enum` whose underlying type is
+    /// defined by a `typedef`.
+    pub fn is_enum_typedef_combo(&self, id: ItemId) -> bool {
+        assert!(
+            self.in_codegen_phase(),
+            "We only compute enum_typedef_combos when we enter codegen",
+        );
+        self.enum_typedef_combos.as_ref().unwrap().contains(&id)
+    }
+
     /// Compute whether we can derive debug.
     fn compute_cannot_derive_debug(&mut self) {
         let _t = self.timer("compute_cannot_derive_debug");
diff --git a/bindgen/ir/ty.rs b/bindgen/ir/ty.rs
index ed3331ad..fef340de 100644
--- a/bindgen/ir/ty.rs
+++ b/bindgen/ir/ty.rs
@@ -95,6 +95,11 @@ impl Type {
         matches!(self.kind, TypeKind::BlockPointer(..))
     }
 
+    /// Is this an integer type, including `bool` or `char`?
+    pub fn is_int(&self) -> bool {
+        matches!(self.kind, TypeKind::Int(_))
+    }
+
     /// Is this a compound type?
     pub fn is_comp(&self) -> bool {
         matches!(self.kind, TypeKind::Comp(..))