1 files changed, 128 insertions, 0 deletions
diff --git a/third_party/rust/inherent/src/expand.rs b/third_party/rust/inherent/src/expand.rs
new file mode 100644
index 0000000000..1e3ce2142f
--- /dev/null
+++ b/third_party/rust/inherent/src/expand.rs
@@ -0,0 +1,128 @@
+use crate::parse::TraitImpl;
+use proc_macro2::{Span, TokenStream, TokenTree};
+use quote::{quote, quote_spanned};
+use syn::{FnArg, Ident, ImplItem, ImplItemMethod, Item, Pat, Path, Stmt, Visibility};
+
+pub fn inherent(mut input: TraitImpl) -> TokenStream {
+    let impl_token = &input.impl_token;
+    let generics = &input.generics;
+    let where_clause = &input.generics.where_clause;
+    let trait_ = &input.trait_;
+    let ty = &input.self_ty;
+
+    let fwd_methods: Vec<_> = input
+        .items
+        .iter()
+        .filter_map(|item| match item {
+            ImplItem::Method(method) => Some(fwd_method(trait_, method)),
+            _ => None,
+        })
+        .collect();
+
+    input.items = input
+        .items
+        .into_iter()
+        .filter_map(|item| match item {
+            ImplItem::Method(mut method) => {
+                if inherit_default_implementation(&method) {
+                    None
+                } else {
+                    method.vis = Visibility::Inherited;
+                    Some(ImplItem::Method(method))
+                }
+            }
+            item => Some(item),
+        })
+        .collect();
+
+    let body = quote_spanned!(input.brace_token.span=> { #(#fwd_methods)* });
+
+    quote! {
+        #impl_token #generics #ty #where_clause #body
+
+        #input
+    }
+}
+
+fn fwd_method(trait_: &Path, method: &ImplItemMethod) -> TokenStream {
+    let attrs = &method.attrs;
+    let vis = &method.vis;
+    let constness = &method.sig.constness;
+    let asyncness = &method.sig.asyncness;
+    let unsafety = &method.sig.unsafety;
+    let abi = &method.sig.abi;
+    let fn_token = method.sig.fn_token;
+    let ident = &method.sig.ident;
+    let generics = &method.sig.generics;
+    let output = &method.sig.output;
+    let where_clause = &method.sig.generics.where_clause;
+
+    let (arg_pat, arg_val): (Vec<_>, Vec<_>) = method
+        .sig
+        .inputs
+        .pairs()
+        .enumerate()
+        .map(|(i, pair)| {
+            let (input, comma_token) = pair.into_tuple();
+            match input {
+                FnArg::Receiver(receiver) => {
+                    let self_token = receiver.self_token;
+                    if receiver.reference.is_some() {
+                        (quote!(#receiver #comma_token), quote!(#self_token))
+                    } else {
+                        (quote!(#self_token #comma_token), quote!(#self_token))
+                    }
+                }
+                FnArg::Typed(arg) => {
+                    let var = match arg.pat.as_ref() {
+                        Pat::Ident(pat) => pat.ident.clone(),
+                        _ => Ident::new(&format!("__arg{}", i), Span::call_site()),
+                    };
+                    let colon_token = arg.colon_token;
+                    let ty = &arg.ty;
+                    (quote!(#var #colon_token #ty #comma_token), quote!(#var))
+                }
+            }
+        })
+        .unzip();
+
+    let types = generics.type_params().map(|param| &param.ident);
+    let body = quote!(<Self as #trait_>::#ident::<#(#types,)*>(#(#arg_val,)*));
+    let block = quote_spanned!(method.block.brace_token.span=> { #body });
+    let args = quote_spanned!(method.sig.paren_token.span=> (#(#arg_pat)*));
+
+    let has_doc = attrs.iter().any(|attr| attr.path.is_ident("doc"));
+    let default_doc = if has_doc {
+        None
+    } else {
+        let mut link = String::new();
+        for segment in &trait_.segments {
+            link += &segment.ident.to_string();
+            link += "::";
+        }
+        let msg = format!("See [`{}{}`]", link, ident);
+        Some(quote!(#[doc = #msg]))
+    };
+
+    quote! {
+        #(#attrs)*
+        #default_doc
+        #vis #constness #asyncness #unsafety #abi #fn_token #ident #generics #args #output #where_clause #block
+    }
+}
+
+fn inherit_default_implementation(method: &ImplItemMethod) -> bool {
+    method.block.stmts.len() == 1
+        && match &method.block.stmts[0] {
+            Stmt::Item(Item::Verbatim(verbatim)) => {
+                let mut iter = verbatim.clone().into_iter();
+                match iter.next() {
+                    Some(TokenTree::Punct(punct)) => {
+                        punct.as_char() == ';' && iter.next().is_none()
+                    }
+                    _ => false,
+                }
+            }
+            _ => false,
+        }
+}