Add default value macro

README.md

@@ -118,12 +118,33 @@ your crate:
 extern crate dotenv_codegen;
 ```
 
-Then, in your crate:
+Then, in your crate you may retrieve a value like so:
 
 ```rust
 fn main() {
   println!("{}", dotenv!("MEANING_OF_LIFE"));
 }
 ```
 
+You may also panic with a supplied error message if the variable does
+not exist:
+
+```rust
+fn main() {
+    dotenv!("A_MISSING_VARIABLE", "This is an error message!");
+}
+```
+
+Or you can supply a default value if the variable does not exist:
+
+```rust
+fn main() {
+    let meaning_of_life: &str = dotenv_or_default!(
+        "A_MISSING_VARIABLE",
+        "42"
+    );
+    println!("{}", meaning_of_life);
+}
+```
+
 [dotenv]: https://github.com/bkeepers/dotenv

dotenv_codegen/Cargo.toml

@@ -20,3 +20,6 @@ edition = "2018"
 [dependencies]
 dotenv_codegen_implementation = { version = "0.15", path = "../dotenv_codegen_implementation" }
 proc-macro-hack = "0.5"
+
+[dev-dependencies]
+trybuild = "1.0.32"

dotenv_codegen/src/lib.rs

@@ -1,4 +1,4 @@
 use proc_macro_hack::proc_macro_hack;
 
 #[proc_macro_hack]
-pub use dotenv_codegen_implementation::dotenv;
+pub use dotenv_codegen_implementation::{dotenv, dotenv_or_default};

dotenv_codegen/tests/basic_dotenv_macro.rs

@@ -17,3 +17,15 @@ fn two_argument_form_works() {
         "'quotes within quotes'"
     );
 }
+
+#[test]
+fn dotenv_or_default_works() {
+    let default_value: &str = dotenv_or_default!("CODEGEN_TEST_NONEXISTING_VARIABLE", "hello!");
+    assert_eq!(default_value, "hello!");
+}
+
+#[test]
+fn ui() {
+    let t = trybuild::TestCases::new();
+    t.compile_fail("tests/ui/*.rs");
+}

dotenv_codegen/tests/ui/dotenv_no_args.rs

@@ -0,0 +1,6 @@
+#[macro_use]
+extern crate dotenv_codegen;
+
+pub fn main() {
+    dotenv!();
+}

dotenv_codegen/tests/ui/dotenv_no_args.stderr

@@ -0,0 +1,16 @@
+error: proc-macro derive panicked
+ --> $DIR/dotenv_no_args.rs:5:5
+  |
+5 |     dotenv!();
+  |     ^^^^^^^^^^
+  |
+  = help: message: expected 1 or 2 arguments, found none
+  = note: this error originates in a macro (in Nightly builds, run with -Z macro-backtrace for more info)
+
+error: cannot find macro `proc_macro_call` in this scope
+ --> $DIR/dotenv_no_args.rs:5:5
+  |
+5 |     dotenv!();
+  |     ^^^^^^^^^^
+  |
+  = note: this error originates in a macro (in Nightly builds, run with -Z macro-backtrace for more info)

dotenv_codegen/tests/ui/dotenv_or_default_no_args.rs

@@ -0,0 +1,6 @@
+#[macro_use]
+extern crate dotenv_codegen;
+
+pub fn main() {
+    dotenv_or_default!();
+}

dotenv_codegen/tests/ui/dotenv_or_default_no_args.stderr

@@ -0,0 +1,16 @@
+error: proc-macro derive panicked
+ --> $DIR/dotenv_or_default_no_args.rs:5:5
+  |
+5 |     dotenv_or_default!();
+  |     ^^^^^^^^^^^^^^^^^^^^^
+  |
+  = help: message: expected 1 or 2 arguments, found none
+  = note: this error originates in a macro (in Nightly builds, run with -Z macro-backtrace for more info)
+
+error: cannot find macro `proc_macro_call` in this scope
+ --> $DIR/dotenv_or_default_no_args.rs:5:5
+  |
+5 |     dotenv_or_default!();
+  |     ^^^^^^^^^^^^^^^^^^^^^
+  |
+  = note: this error originates in a macro (in Nightly builds, run with -Z macro-backtrace for more info)

dotenv_codegen/tests/ui/dotenv_or_default_three_args.rs

@@ -0,0 +1,6 @@
+#[macro_use]
+extern crate dotenv_codegen;
+
+pub fn main() {
+    dotenv_or_default!("a", "b", "c");
+}

dotenv_codegen/tests/ui/dotenv_or_default_three_args.stderr

@@ -0,0 +1,16 @@
+error: proc-macro derive panicked
+ --> $DIR/dotenv_or_default_three_args.rs:5:5
+  |
+5 |     dotenv_or_default!("a", "b", "c");
+  |     ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
+  |
+  = help: message: expected 1 or 2 arguments, found 3 or more
+  = note: this error originates in a macro (in Nightly builds, run with -Z macro-backtrace for more info)
+
+error: cannot find macro `proc_macro_call` in this scope
+ --> $DIR/dotenv_or_default_three_args.rs:5:5
+  |
+5 |     dotenv_or_default!("a", "b", "c");
+  |     ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
+  |
+  = note: this error originates in a macro (in Nightly builds, run with -Z macro-backtrace for more info)

dotenv_codegen/tests/ui/dotenv_three_args.rs

@@ -0,0 +1,6 @@
+#[macro_use]
+extern crate dotenv_codegen;
+
+pub fn main() {
+    dotenv!("a", "b", "c");
+}

dotenv_codegen/tests/ui/dotenv_three_args.stderr

@@ -0,0 +1,16 @@
+error: proc-macro derive panicked
+ --> $DIR/dotenv_three_args.rs:5:5
+  |
+5 |     dotenv!("a", "b", "c");
+  |     ^^^^^^^^^^^^^^^^^^^^^^^
+  |
+  = help: message: expected 1 or 2 arguments, found 3 or more
+  = note: this error originates in a macro (in Nightly builds, run with -Z macro-backtrace for more info)
+
+error: cannot find macro `proc_macro_call` in this scope
+ --> $DIR/dotenv_three_args.rs:5:5
+  |
+5 |     dotenv!("a", "b", "c");
+  |     ^^^^^^^^^^^^^^^^^^^^^^^
+  |
+  = note: this error originates in a macro (in Nightly builds, run with -Z macro-backtrace for more info)

dotenv_codegen_implementation/src/lib.rs

@@ -12,14 +12,56 @@ use syn::Token;
 #[proc_macro_hack]
 pub fn dotenv(input: TokenStream) -> TokenStream {
     if let Err(err) = dotenv::dotenv() {
-        panic!("Error loading .env file: {}", err);
+        let err_msg = format!("Error loading .env file: {}", err);
+        return quote! {
+            compile_error!(#err_msg);
+        }
+        .into();
     }
 
     // Either everything was fine, or we didn't find an .env file (which we ignore)
-    expand_env(input)
+    let (var_name, second_value) = expand_env(input);
+
+    let err_msg = match second_value {
+        Some(e) => e,
+        None => format!("environment variable `{}` not defined", var_name),
+    };
+
+    match env::var(var_name) {
+        Ok(val) => quote!(#val).into(),
+        Err(VarError::NotPresent) | Err(VarError::NotUnicode(_)) => panic!("{}", err_msg),
+    }
+}
+
+#[proc_macro_hack]
+pub fn dotenv_or_default(input: TokenStream) -> TokenStream {
+    if let Err(err) = dotenv::dotenv() {
+        let err_msg = format!("Error loading .env file: {}", err);
+        return quote! {
+            compile_error!(#err_msg);
+        }
+        .into();
+    }
+
+    // Either everything was fine, or we didn't find an .env file (which we ignore)
+    let (var_name, second_value) = expand_env(input);
+
+    match second_value {
+        Some(default) => match env::var(var_name) {
+            Ok(val) => quote!(#val).into(),
+            Err(VarError::NotPresent) | Err(VarError::NotUnicode(_)) => quote!(#default).into(),
+        },
+        None => {
+            let err_msg = format!("Missing default value for: {}", var_name);
+            (quote! {
+                compile_error!(#err_msg)
+            })
+            .into()
+        }
+    }
 }
 
-fn expand_env(input_raw: TokenStream) -> TokenStream {
+fn expand_env(input_raw: TokenStream) -> (String, Option<String>) {
     let args = <Punctuated<syn::LitStr, Token![,]>>::parse_terminated
         .parse(input_raw)
         .expect("expected macro to be called with a comma-separated list of string literals");
@@ -31,17 +73,14 @@ fn expand_env(input_raw: TokenStream) -> TokenStream {
         None => panic!("expected 1 or 2 arguments, found none"),
     };
 
-    let err_msg = match iter.next() {
-        Some(lit) => lit.value(),
-        None => format!("environment variable `{}` not defined", var_name),
+    let second_value = match iter.next() {
+        Some(lit) => Some(lit.value()),
+        None => None,
     };
 
     if iter.next().is_some() {
         panic!("expected 1 or 2 arguments, found 3 or more");
     }
 
-    match env::var(var_name) {
-        Ok(val) => quote!(#val).into(),
-        Err(VarError::NotPresent) | Err(VarError::NotUnicode(_)) => panic!("{}", err_msg),
-    }
+    (var_name, second_value)
 }