Improve error messages on wrong query/schema file path

Author: tomhoule

graphql_query_derive/src/lib.rs

@@ -45,6 +45,26 @@ pub fn graphql_query_derive(input: proc_macro::TokenStream) -> proc_macro::Token
     gen.into()
 }
 
+fn read_file(
+    path: impl AsRef<::std::path::Path> + ::std::fmt::Debug,
+) -> Result<String, failure::Error> {
+    use std::io::prelude::*;
+
+    let mut out = String::new();
+    let mut file = ::std::fs::File::open(&path).map_err(|io_err| {
+        let err: failure::Error = io_err.into();
+        err.context(format!(
+            r#"
+            Could not find file with path: {:?}
+            Hint: file paths in the GraphQLQuery attribute are relative to the project root (location of the Cargo.toml). Example: query_path = "src/my_query.graphql".
+            "#,
+            path
+        ))
+    })?;
+    file.read_to_string(&mut out)?;
+    Ok(out)
+}
+
 fn impl_gql_query(input: &syn::DeriveInput) -> Result<TokenStream, failure::Error> {
     use std::io::prelude::*;
 
@@ -56,14 +76,12 @@ fn impl_gql_query(input: &syn::DeriveInput) -> Result<TokenStream, failure::Erro
 
     // We need to qualify the query with the path to the crate it is part of
     let query_path = format!("{}/{}", cargo_manifest_dir, query_path);
-    let mut query_string = String::new();
-    ::std::fs::File::open(query_path)?.read_to_string(&mut query_string)?;
+    let query_string = read_file(&query_path)?;
     let query = graphql_parser::parse_query(&query_string)?;
 
     // We need to qualify the schema with the path to the crate it is part of
     let schema_path = ::std::path::Path::new(&cargo_manifest_dir).join(schema_path);
-    let mut schema_string = String::new();
-    ::std::fs::File::open(&schema_path)?.read_to_string(&mut schema_string)?;
+    let schema_string = read_file(&schema_path)?;
 
     let extension = schema_path
         .extension()