Add support for Rust 2018 edition projects.

Summary:
After enabling 2018 edition on my cargo project, the use statements
generated in the interface module no longer resolve.

Prefixing module paths with `crate::` brings things back into alignment.

Unsure if this is how you'd like to approach the issue, but I'm happy to revise the patch based on your feedback.

Test Plan:
STEPS TO REPRODUCE
1. create a new cargo project based on the template
2. update your toolchain to >= 1.31
3. add `edition = "2018"` to the package section of your Cargo.toml
4. run `cargo build`

OBSERVED RESULT

Build fails since generated code tries to import crate-local modules as if they are top level crates.

EXPECTED RESULT

Build completes successfully.

Reviewers: vandenoever

Differential Revision: https://phabricator.kde.org/D17447

Cargo.toml

@@ -16,3 +16,4 @@ serde = "1.0"
 serde_derive = "1.0"
 serde_json = "1.0"
 serde-xml-rs = "0.2.1"
+toml = "0.4.10"

src/configuration.rs

@@ -1,5 +1,6 @@
 use configuration_private::*;
 use serde_json;
+use toml;
 use std::collections::{BTreeMap, BTreeSet};
 use std::error::Error;
 use std::fs;
@@ -57,11 +58,28 @@ mod json {
     }
 }
 
+pub enum RustEdition {
+    Rust2015,
+    Rust2018,
+    Unknown
+}
+
+impl ::std::convert::From<Option<&str>> for RustEdition {
+    fn from(str: Option<&str>) -> RustEdition {
+        match str {
+            None | Some("2015") => RustEdition::Rust2015,
+            Some("2018") => RustEdition::Rust2018,
+            _ => RustEdition::Unknown,
+        }
+    }
+}
+
 pub struct Config {
     pub config_file: PathBuf,
     pub cpp_file: PathBuf,
     pub objects: BTreeMap<String, Rc<Object>>,
     pub rust: Rust,
+    pub rust_edition: RustEdition,
     pub overwrite_implementation: bool,
 }
 
@@ -447,11 +465,21 @@ fn post_process(config_file: &Path, json: json::Config) -> Result<Config, Box<Er
     for object in &json.objects {
         post_process_object(object, &mut objects, &json.objects)?;
     }
+
+    let rust_edition: RustEdition = {
+        let mut buf = config_file.to_path_buf();
+        buf.pop();
+        buf.push("Cargo.toml");
+        let manifest: toml::Value = fs::read_to_string(&buf)?.parse()?;
+        manifest["package"].get("edition").and_then(|val| val.as_str()).into()
+    };
+
     Ok(Config {
         config_file: config_file.into(),
         cpp_file: json.cpp_file,
         objects,
         rust: json.rust,
+        rust_edition,
         overwrite_implementation: json.overwrite_implementation,
     })
 }

src/lib.rs

@@ -3,6 +3,7 @@ extern crate regex;
 extern crate serde_derive;
 extern crate serde_json;
 extern crate serde_xml_rs;
+extern crate toml;
 
 pub mod build;
 pub mod configuration;

src/rust.rs

@@ -1264,7 +1264,8 @@ use std::sync::Arc;
 use std::sync::atomic::{{AtomicPtr, Ordering}};
 use std::ptr::null;
 
-use {}::*;",
+use {}{}::*;",
+        get_module_prefix(conf),
         conf.rust.implementation_module
     )?;
 
@@ -1518,8 +1519,9 @@ pub fn write_implementation(conf: &Config) -> Result<()> {
         "#![allow(unused_imports)]
 #![allow(unused_variables)]
 #![allow(dead_code)]
-use {}::*;
+use {}{}::*;
 ",
+        get_module_prefix(conf),
         conf.rust.interface_module
     )?;
 
@@ -1528,3 +1530,16 @@ use {}::*;
     }
     write_if_different(file, &r)
 }
+
+/// Inspects the rust edition of the target crate to decide how the module
+/// imports should be written.
+///
+/// As of Rust 2018, modules inside the crate should be prefixed with `crate::`.
+/// Prior to the 2018 edition, crate-local modules could be imported without
+/// this prefix.
+fn get_module_prefix(conf: &Config) -> &'static str {
+    match conf.rust_edition {
+        RustEdition::Rust2018 => "crate::",
+        _ => ""
+    }
+}