From d8a313dd422c78fb018cfe4249b526fe3e9dc851 Mon Sep 17 00:00:00 2001
From: Martin Fischer <martin@push-f.com>
Date: Fri, 19 Nov 2021 10:35:56 +0100
Subject: allow same-named method generics if they have same bounds

---
 README.md      |  3 +++
 src/lib.rs     | 25 +++++++++++++++++++++++--
 tests/tests.rs |  8 +++++---
 3 files changed, 31 insertions(+), 5 deletions(-)

diff --git a/README.md b/README.md
index 332de4f..967738e 100644
--- a/README.md
+++ b/README.md
@@ -117,3 +117,6 @@ trait TraitWithCallback {
 
 Note that since in order to be object-safe methods must not have generics,
 dynamize simply moves the generic from the method to the trait definition.
+
+If two method type parameters have the same name, dynamize enforces that they
+also have the same bounds and only adds the parameter once to the trait.
diff --git a/src/lib.rs b/src/lib.rs
index 6e88136..5385181 100644
--- a/src/lib.rs
+++ b/src/lib.rs
@@ -1,5 +1,7 @@
 #![doc = include_str!("../README.md")]
 
+use std::collections::HashMap;
+
 use proc_macro::TokenStream;
 use proc_macro2::Group;
 use quote::format_ident;
@@ -8,6 +10,7 @@ use quote::quote_spanned;
 use quote::ToTokens;
 use syn::parse_macro_input;
 use syn::punctuated::Punctuated;
+use syn::spanned::Spanned;
 use syn::token::Brace;
 use syn::token::Gt;
 use syn::token::Lt;
@@ -171,6 +174,14 @@ pub fn dynamize(_attr: TokenStream, input: TokenStream) -> TokenStream {
         items: Vec::new(),
     };
 
+    let mut generic_map = HashMap::new();
+
+    for generic in &dyn_trait.generics.params {
+        if let GenericParam::Type(type_param) = generic {
+            generic_map.insert(type_param.ident.clone(), type_param.bounds.clone());
+        }
+    }
+
     for (signature, parsed_method) in objectifiable_methods {
         let mut new_method = TraitItemMethod {
             attrs: Vec::new(),
@@ -235,8 +246,18 @@ pub fn dynamize(_attr: TokenStream, input: TokenStream) -> TokenStream {
             // FUTURE: use Vec::drain_filter once it's stable
             let mut i = 0;
             while i < params.len() {
-                if matches!(params[i], GenericParam::Type(_)) {
-                    dyn_trait.generics.params.push(params.remove(i));
+                if let GenericParam::Type(type_param) = &params[i] {
+                    if let Some(bounds) = generic_map.get(&type_param.ident) {
+                        if *bounds == type_param.bounds {
+                            params.remove(i);
+                            continue;
+                        } else {
+                            return abort!(type_param.span(), "dynamize failure: there exists a same-named method generic with different bounds");
+                        }
+                    } else {
+                        generic_map.insert(type_param.ident.clone(), type_param.bounds.clone());
+                        dyn_trait.generics.params.push(params.remove(i));
+                    }
                 } else {
                     i += 1;
                 }
diff --git a/tests/tests.rs b/tests/tests.rs
index 6bccc04..4d09847 100644
--- a/tests/tests.rs
+++ b/tests/tests.rs
@@ -100,12 +100,14 @@ fn test1<T: Bar1<X>, X, A, B, C>(some: T) {
 trait Buz<X> {
     type C: Into<String>;
 
-    fn foobar<A>(&self, x: X) -> Result<A, Self::C>;
+    fn foobar1<A>(&self, x: X) -> Result<A, Self::C>;
+    fn foobar2<A>(&self) -> Result<A, Self::C>;
 }
 
-fn test2<T: Buz<X>, X, A>(some: T, x: X) -> Result<A, String> {
+fn test2<T: Buz<X>, X, A>(some: T, x: X) {
     let dyn_trait: &dyn DynBuz<X, A> = &some;
-    dyn_trait.foobar(x)
+    let _: Result<A, String> = dyn_trait.foobar1(x);
+    let _: Result<A, String> = dyn_trait.foobar2();
 }
 
 #[dynamize::dynamize]
-- 
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