bitfields-derive: strip :: prefix from type if present (#1411)

The code was testing whether the initial `::` was present, but was not
taking it out of the string/iterator. That would then lead to a panic
when creating the `Ident` on an empty string.

Author: kkysen

c2rust-bitfields-derive/src/lib.rs

@@ -1,6 +1,7 @@
 #![recursion_limit = "512"]
 
-use proc_macro::{Span, TokenStream};
+use proc_macro::TokenStream;
+use proc_macro2::Span;
 use quote::quote;
 use syn::parse::Error;
 use syn::punctuated::Punctuated;
@@ -20,7 +21,7 @@ struct BFFieldAttr {
     field_name: Ident,
     name: String,
     ty: String,
-    bits: (String, proc_macro2::Span),
+    bits: (String, Span),
 }
 
 fn parse_bitfield_attr(
@@ -111,29 +112,19 @@ fn filter_and_parse_fields(field: &Field) -> Vec<Result<BFFieldAttr, Error>> {
 }
 
 fn parse_bitfield_ty_path(field: &BFFieldAttr) -> Path {
-    let leading_colon = if field.ty.starts_with("::") {
-        Some(Token![::]([
-            Span::call_site().into(),
-            Span::call_site().into(),
-        ]))
-    } else {
-        None
-    };
-
     let mut segments = Punctuated::new();
     let mut segment_strings = field.ty.split("::").peekable();
+    let colon = Token![::]([Span::call_site(), Span::call_site()]);
+    let leading_colon = segment_strings.next_if_eq(&"").map(|_| colon);
 
     while let Some(segment_string) = segment_strings.next() {
         segments.push_value(PathSegment {
-            ident: Ident::new(segment_string, Span::call_site().into()),
+            ident: Ident::new(segment_string, Span::call_site()),
             arguments: PathArguments::None,
         });
 
         if segment_strings.peek().is_some() {
-            segments.push_punct(Token![::]([
-                Span::call_site().into(),
-                Span::call_site().into(),
-            ]));
+            segments.push_punct(colon);
         }
     }
 
@@ -143,6 +134,21 @@ fn parse_bitfield_ty_path(field: &BFFieldAttr) -> Path {
     }
 }
 
+#[cfg(test)]
+#[test]
+fn test_parse_bitfield_ty_path_non_empty_idents() {
+    let tys = ["::core::ffi::c_int", "core::ffi::c_int"];
+    for ty in tys {
+        let field = BFFieldAttr {
+            field_name: Ident::new("field", Span::call_site()),
+            name: Default::default(),
+            ty: ty.into(),
+            bits: (Default::default(), Span::call_site()),
+        };
+        let _path = parse_bitfield_ty_path(&field);
+    }
+}
+
 #[proc_macro_derive(BitfieldStruct, attributes(bitfield))]
 pub fn bitfield_struct(input: TokenStream) -> TokenStream {
     let struct_item = parse_macro_input!(input as ItemStruct);
@@ -181,15 +187,15 @@ fn bitfield_struct_impl(struct_item: ItemStruct) -> Result<TokenStream, Error> {
     let field_types_setter_arg = &field_types;
     let method_names: Vec<_> = bitfields
         .iter()
-        .map(|field| Ident::new(&field.name, Span::call_site().into()))
+        .map(|field| Ident::new(&field.name, Span::call_site()))
         .collect();
     let field_names: Vec<_> = bitfields.iter().map(|field| &field.field_name).collect();
     let field_names_setters = &field_names;
     let field_names_getters = &field_names;
     let method_name_setters: Vec<_> = method_names
         .iter()
         .map(|field_ident| {
-            let span = Span::call_site().into();
+            let span = Span::call_site();
             let setter_name = &format!("set_{}", field_ident);
 
             Ident::new(setter_name, span)