feat: Add "env_prefix" attribute for struct derive

This attribute allows to add an annotation like so `#[envconfig(env_prefix = "...")]`
on top of a struct. The `env_prefix` will be applied to every config
attribute in the struct.

envconfig_derive/src/lib.rs

@@ -5,7 +5,7 @@ use proc_macro::TokenStream;
 use quote::quote;
 use syn::punctuated::Punctuated;
 use syn::token::Comma;
-use syn::{Attribute, DeriveInput, Field, Fields, Ident, Meta, Expr};
+use syn::{Attribute, DeriveInput, Field, Fields, Ident, Meta, Lit, Expr, ExprLit};
 
 #[proc_macro_derive(Envconfig, attributes(envconfig))]
 pub fn derive(input: TokenStream) -> TokenStream {
@@ -23,9 +23,27 @@ fn impl_envconfig(input: &DeriveInput) -> proc_macro2::TokenStream {
     use syn::Data::*;
     let struct_name = &input.ident;
 
+    let input_attr = fetch_envconfig_attr_from_attrs(&input.attrs);
+    let input_ident_opt: Option<Ident> = Some(input.ident.clone()); 
+    let env_prefix = match input_attr {
+        Some(ref attr) => find_item_in_attr_meta(&input_ident_opt, attr, "env_prefix"),
+        None => None,
+    };
+
+    let env_prefix_str = match env_prefix {
+        Some(Expr::Lit(ref prefix)) => {
+            match &prefix.lit {
+                Lit::Str(lit_prefix) => lit_prefix.value(),
+                _ => panic!("Expected `env_prefix` value to be a literal string"),
+            }
+        },
+        None => "".to_string(),
+        _ => panic!("Expected `env_prefix` value to be a literal string"),
+    };
+
     let inner_impl = match input.data {
         Struct(ref ds) => match ds.fields {
-            Fields::Named(ref fields) => impl_envconfig_for_struct(struct_name, &fields.named),
+            Fields::Named(ref fields) => impl_envconfig_for_struct(struct_name, &fields.named, env_prefix_str),
             _ => panic!("envconfig supports only named fields"),
         },
         _ => panic!("envconfig only supports non-tuple structs"),
@@ -37,13 +55,14 @@ fn impl_envconfig(input: &DeriveInput) -> proc_macro2::TokenStream {
 fn impl_envconfig_for_struct(
     struct_name: &Ident,
     fields: &Punctuated<Field, Comma>,
+    env_prefix: String,
 ) -> proc_macro2::TokenStream {
     let field_assigns_env = fields
         .iter()
-        .map(|field| gen_field_assign(field, Source::Environment));
+        .map(|field| gen_field_assign(field, Source::Environment, &env_prefix));
     let field_assigns_hashmap = fields
         .iter()
-        .map(|field| gen_field_assign(field, Source::HashMap));
+        .map(|field| gen_field_assign(field, Source::HashMap, &env_prefix));
 
     quote! {
         impl Envconfig for #struct_name {
@@ -68,8 +87,9 @@ fn impl_envconfig_for_struct(
     }
 }
 
-fn gen_field_assign(field: &Field, source: Source) -> proc_macro2::TokenStream {
+fn gen_field_assign(field: &Field, source: Source, env_prefix: &str) -> proc_macro2::TokenStream {
     let attr = fetch_envconfig_attr_from_attrs(&field.attrs);
+
     if let Some(attr) = attr {
         // if #[envconfig(...)] is there
 
@@ -80,25 +100,41 @@ fn gen_field_assign(field: &Field, source: Source) -> proc_macro2::TokenStream {
         }
 
         let opt_default = find_item_in_attr_meta(&field.ident, &attr, "default");
-
         let from_opt = find_item_in_attr_meta(&field.ident, &attr, "from");
-        let env_var = match from_opt {
-            Some(v) => quote! { #v },
-            None => field_to_env_var(field),
+
+        let env_var_name: String = match from_opt {
+            Some(Expr::Lit(v)) => format!("{}{}", env_prefix, get_exprlit_str_value(&v, &field.ident)),
+            None => field_to_env_var(field, env_prefix),
+            _ => panic!(
+                "Expected '{}' field option 'from' type to be a String",
+                to_s(&field.ident)
+            ),
         };
+        let env_var = quote! { #env_var_name };
 
         gen(field, env_var, opt_default, source)
     } else {
         // if #[envconfig(...)] is not present
-        let env_var = field_to_env_var(field);
+        let env_var_name = field_to_env_var(field, env_prefix);
+        let env_var = quote! { #env_var_name };
         gen(field, env_var, None, source)
     }
 }
 
-fn field_to_env_var(field: &Field) -> proc_macro2::TokenStream {
-    let field_name = field.clone().ident.unwrap().to_string().to_uppercase();
-    quote! { #field_name }
+fn get_exprlit_str_value(exprlit_obj: &ExprLit, field_ident: &Option<Ident>) -> String {
+    match &exprlit_obj.lit {
+        Lit::Str(lit_prefix) => lit_prefix.value(),
+        _ => panic!(
+            "Expected '{}' field option 'from' type to be a String",
+            to_s(field_ident)
+        ),
+    }
+}
+
 
+fn field_to_env_var(field: &Field, env_prefix: &str) -> String {
+    let ident_name = field.clone().ident.unwrap().to_string().to_uppercase();
+    format!("{}{}", env_prefix, ident_name)
 }
 
 fn gen(

test_suite/tests/env_prefix.rs

@@ -0,0 +1,52 @@
+extern crate envconfig;
+
+use envconfig::{Envconfig};
+use std::env;
+
+#[derive(Envconfig)]
+#[envconfig(env_prefix = "TEST_")]
+pub struct ConfigWithoutFrom {
+    pub db_host: String,
+    pub db_port: u16,
+}
+
+fn setup() {
+    env::remove_var("TEST_DB_HOST");
+    env::remove_var("TEST_DB_PORT");
+}
+
+#[test]
+fn test_init_from_env_with_env_prefix_and_no_envconfig_on_attributes() {
+    setup();
+
+    env::set_var("TEST_DB_HOST", "localhost");
+    env::set_var("TEST_DB_PORT", "5432");
+
+
+    let config = ConfigWithoutFrom::init_from_env().unwrap();
+    assert_eq!(config.db_host, "localhost");
+    assert_eq!(config.db_port, 5432u16);
+}
+
+#[derive(Envconfig)]
+#[envconfig(env_prefix = "TEST_")]
+pub struct ConfigWithFrom {
+    #[envconfig(from = "DB_HOST")]
+    pub db_host: String,
+
+    #[envconfig(from = "DB_PORT")]
+    pub db_port: u16,
+}
+
+#[test]
+fn test_init_from_env_with_env_prefix_and_envconfig_on_attributes() {
+    setup();
+
+    env::set_var("TEST_DB_HOST", "localhost");
+    env::set_var("TEST_DB_PORT", "5433");
+
+
+    let config = ConfigWithFrom::init_from_env().unwrap();
+    assert_eq!(config.db_host, "localhost");
+    assert_eq!(config.db_port, 5433u16);
+}