Resolve CARGO_MANIFEST_DIR at runtime (#28)

Author: heaths

Cargo.lock

@@ -1,6 +1,6 @@
 [package]
 name = "include-file"
-version = "0.3.0"
+version = "0.4.0"
 description = "Include sections of files into Rust source code"
 readme = "README.md"
 authors = ["Heath Stewart (https://github.com/heaths)"]

Cargo.toml

@@ -12,7 +12,7 @@ mod textile;
 
 use proc_macro2::{Span, TokenStream};
 use std::{
-    fmt, fs,
+    env, fmt, fs,
     io::{self, BufRead},
     path::PathBuf,
 };
@@ -237,8 +237,8 @@ where
 }
 
 fn open(path: &str) -> io::Result<fs::File> {
-    let manifest_dir: PathBuf = option_env!("CARGO_MANIFEST_DIR")
-        .ok_or_else(|| io::Error::other("no manifest directory"))?
+    let manifest_dir: PathBuf = env::var("CARGO_MANIFEST_DIR")
+        .map_err(|_| io::Error::other("no manifest directory"))?
         .into();
     let path = manifest_dir.join(path);
     fs::File::open(path)