fix: infallible structs in derive macro (#58)

* fix: infallible structs in derive macro

* lint: clippy

Author: prestwich

from-env-derive/Cargo.toml

@@ -2,7 +2,7 @@
 name = "init4-from-env-derive"
 
 description = "A derive macro for `init4_bin_base::FromEnv`"
-version = "0.1.1"
+version = "0.1.2"
 edition = "2021"
 rust-version = "1.81"
 authors = ["init4", "James Prestwich"]
@@ -20,4 +20,4 @@ syn = { version = "2.0.100", features = ["full", "parsing"] }
 proc-macro = true
 
 [dev-dependencies]
-init4-bin-base = "0.3"
+init4-bin-base = "0.9"

from-env-derive/src/field.rs

@@ -114,7 +114,7 @@ impl Field {
             return field_name.clone();
         }
 
-        let n = format!("field_{}", idx);
+        let n = format!("field_{idx}");
         syn::parse_str::<Ident>(&n)
             .map_err(|_| syn::Error::new(self.span, "Failed to create field name"))
             .unwrap()
@@ -199,7 +199,7 @@ impl Field {
         })
     }
 
-    pub(crate) fn expand_item_from_env(&self, err_ident: &Ident, idx: usize) -> TokenStream {
+    pub(crate) fn expand_item_from_env(&self, err_ident: &syn::Path, idx: usize) -> TokenStream {
         // Produces code fo the following form:
         // ```rust
         // // EITHER

from-env-derive/src/lib.rs

@@ -102,15 +102,23 @@ impl Input {
         }
     }
 
-    fn error_ident(&self) -> syn::Ident {
+    fn error_ident(&self) -> syn::Path {
+        if self.is_infallible() {
+            return syn::parse_str("::std::convert::Infallible").unwrap();
+        }
+
         let error_name = format!("{}EnvError", self.ident);
-        syn::parse_str::<syn::Ident>(&error_name)
+        syn::parse_str::<syn::Path>(&error_name)
             .map_err(|_| {
                 syn::Error::new(self.ident.span(), "Failed to parse error ident").to_compile_error()
             })
             .unwrap()
     }
 
+    fn is_infallible(&self) -> bool {
+        self.error_variants().is_empty()
+    }
+
     fn error_variants(&self) -> Vec<TokenStream> {
         self.fields
             .iter()
@@ -152,6 +160,10 @@ impl Input {
         let error_variant_displays = self.error_variant_displays();
         let error_variant_sources = self.expand_variant_sources();
 
+        if error_variants.is_empty() {
+            return Default::default();
+        }
+
         quote! {
             #[doc = "Generated error type for [`FromEnv`] for"]
             #[doc = #struct_name_str]
@@ -195,6 +207,7 @@ impl Input {
     fn expand_impl(&self) -> TokenStream {
         let env_item_info = self.env_item_info();
         let struct_name = &self.ident;
+
         let error_ident = self.error_ident();
 
         let item_from_envs = self.item_from_envs();
@@ -226,16 +239,25 @@ impl Input {
 
     fn expand_mod(&self) -> TokenStream {
         // let expanded_impl = expand_impl(input);
-        let expanded_error = self.expand_error();
         let expanded_impl = self.expand_impl();
         let crate_name = &self.crate_name;
-        let error_ident = self.error_ident();
 
         let mod_ident =
             syn::parse_str::<syn::Ident>(&format!("__from_env_impls_{}", self.ident)).unwrap();
 
+        let expanded_error = self.expand_error();
+
+        let use_err = if !expanded_error.is_empty() {
+            let error_ident = self.error_ident();
+            quote! {
+                pub use #mod_ident::#error_ident;
+            }
+        } else {
+            quote! {}
+        };
+
         quote! {
-            pub use #mod_ident::#error_ident;
+            #use_err
             mod #mod_ident {
                 use super::*;
                 use #crate_name::utils::from_env::{FromEnv, FromEnvErr, FromEnvVar, EnvItemInfo};

from-env-derive/tests/macro.rs

@@ -1,5 +1,16 @@
 use init4_from_env_derive::FromEnv;
 
+// Compile check for a struct with no fallible fields (and therefore no
+// associated env error).
+#[derive(Debug, FromEnv)]
+pub struct InfallibleConfig {
+    #[from_env(var = "INTERNAL", desc = "An infallible string", infallible)]
+    pub internal: String,
+
+    #[from_env(var = "INTERNAL_COW", desc = "An infallible Cow<str>", infallible)]
+    pub internal_cow: std::borrow::Cow<'static, str>,
+}
+
 #[derive(FromEnv, Debug)]
 pub struct FromEnvTest {
     /// This is a guy named tony
@@ -81,10 +92,10 @@ mod test {
     }
 
     fn assert_contains(vec: &Vec<&'static EnvItemInfo>, item: &EnvItemInfo) {
-        let item = vec.iter().find(|i| i.var == item.var).unwrap();
-        assert_eq!(item.var, item.var);
-        assert_eq!(item.description, item.description);
-        assert_eq!(item.optional, item.optional);
+        let i = vec.iter().find(|i| i.var == item.var).unwrap();
+        assert_eq!(i.var, item.var);
+        assert_eq!(i.description, item.description);
+        assert_eq!(i.optional, item.optional);
     }
 
     #[test]