zeroize_derive: Inject where clauses; skip unused

The where clauses I was previously injecting *were* load bearing, but
they needed to be filtered for skipped fields.

Fixes RustCrypto#878

zeroize/derive/Cargo.toml

@@ -17,7 +17,7 @@ proc-macro = true
 [dependencies]
 proc-macro2 = "1"
 quote = "1"
-syn = {version = "2", features = ["full", "extra-traits"]}
+syn = {version = "2", features = ["full", "extra-traits", "visit"]}
 
 [package.metadata.docs.rs]
 rustdoc-args = ["--document-private-items"]

zeroize/derive/src/lib.rs

@@ -8,8 +8,10 @@ use proc_macro2::{Ident, TokenStream};
 use quote::{format_ident, quote};
 use syn::{
     parse::{Parse, ParseStream},
+    parse_quote,
     punctuated::Punctuated,
     token::Comma,
+    visit::Visit,
     Attribute, Data, DeriveInput, Expr, ExprLit, Field, Fields, Lit, Meta, Result, Variant,
     WherePredicate,
 };
@@ -36,12 +38,19 @@ pub fn derive_zeroize(input: proc_macro::TokenStream) -> proc_macro::TokenStream
 fn derive_zeroize_impl(input: DeriveInput) -> TokenStream {
     let attributes = ZeroizeAttrs::parse(&input);
 
+    let mut generics = input.generics.clone();
+
     let extra_bounds = match attributes.bound {
         Some(bounds) => bounds.0,
-        None => Default::default(),
+        None => attributes
+            .auto_params
+            .iter()
+            .map(|type_param| -> WherePredicate {
+                parse_quote! {#type_param: Zeroize}
+            })
+            .collect(),
     };
 
-    let mut generics = input.generics.clone();
     generics.make_where_clause().predicates.extend(extra_bounds);
 
     let ty_name = &input.ident;
@@ -117,6 +126,8 @@ struct ZeroizeAttrs {
     drop: bool,
     /// Custom bounds as defined by the user
     bound: Option<Bounds>,
+    /// Type parameters in use by fields
+    auto_params: Vec<Ident>,
 }
 
 /// Parsing helper for custom bounds
@@ -128,10 +139,37 @@ impl Parse for Bounds {
     }
 }
 
+struct BoundAccumulator<'a> {
+    generics: &'a syn::Generics,
+    params: Vec<Ident>,
+}
+
+impl<'ast> Visit<'ast> for BoundAccumulator<'ast> {
+    fn visit_path(&mut self, path: &'ast syn::Path) {
+        if path.segments.len() != 1 {
+            return;
+        }
+
+        if let Some(segment) = path.segments.first() {
+            for param in &self.generics.params {
+                if let syn::GenericParam::Type(type_param) = param {
+                    if type_param.ident == segment.ident && !self.params.contains(&segment.ident) {
+                        self.params.push(type_param.ident.clone());
+                    }
+                }
+            }
+        }
+    }
+}
+
 impl ZeroizeAttrs {
     /// Parse attributes from the incoming AST
     fn parse(input: &DeriveInput) -> Self {
         let mut result = Self::default();
+        let mut bound_accumulator = BoundAccumulator {
+            generics: &input.generics,
+            params: Vec::new(),
+        };
 
         for attr in &input.attrs {
             result.parse_attr(attr, None, None);
@@ -147,6 +185,9 @@ impl ZeroizeAttrs {
                         for attr in &field.attrs {
                             result.parse_attr(attr, Some(variant), Some(field));
                         }
+                        if !attr_skip(&field.attrs) {
+                            bound_accumulator.visit_type(&field.ty);
+                        }
                     }
                 }
             }
@@ -155,11 +196,16 @@ impl ZeroizeAttrs {
                     for attr in &field.attrs {
                         result.parse_attr(attr, None, Some(field));
                     }
+                    if !attr_skip(&field.attrs) {
+                        bound_accumulator.visit_type(&field.ty);
+                    }
                 }
             }
             syn::Data::Union(union_) => panic!("Unsupported untagged union {:?}", union_),
         }
 
+        result.auto_params = bound_accumulator.params;
+
         result
     }
 

zeroize/tests/zeroize_derive.rs

@@ -351,3 +351,12 @@ fn derive_zeroize_with_marker() {
 
     trait Marker {}
 }
+
+#[test]
+// Issue #878
+fn derive_zeroize_used_param() {
+    #[derive(Zeroize)]
+    struct Z<T> {
+        used: T
+    }
+}