Allow fallback on value package.rust-src if XARGO_RUST_SRC doesn't exist

src/lib.rs

@@ -134,12 +134,18 @@ fn run(cargo_mode: XargoMode) -> Result<Option<ExitStatus>> {
         // We can't build sysroot with stable or beta due to unstable features
         let sysroot = rustc::sysroot(verbose)?;
         let src = match meta.channel {
-            Channel::Dev => rustc::Src::from_env().ok_or(
+            Channel::Dev => if let Some(env) = rustc::Src::from_env() {
+                Some(env)
+            } else {
+                xargo::toml_src(&root)?
+            }.ok_or(
                 "The XARGO_RUST_SRC env variable must be set and point to the \
                  Rust source directory when working with the 'dev' channel",
             )?,
             Channel::Nightly => if let Some(src) = rustc::Src::from_env() {
                 src
+            } else if let Some(src) = xargo::toml_src(&root)? {
+                src
             } else {
                 sysroot.src()?
             },

src/rustc.rs

@@ -108,6 +108,12 @@ impl Sysroot {
     }
 }
 
+impl From<PathBuf> for Src {
+    fn from(path: PathBuf) -> Self {
+        Self { path }
+    }
+}
+
 #[derive(Debug)]
 pub enum Target {
     Builtin { triple: String },

src/xargo.rs

@@ -13,6 +13,7 @@ use errors::*;
 use extensions::CommandExt;
 use flock::{FileLock, Filesystem};
 use {cargo, util};
+use rustc::Src;
 
 pub fn run(
     args: &Args,
@@ -118,6 +119,11 @@ impl Toml {
     pub fn patch(&self) -> Option<&Value> {
         self.table.lookup("patch")
     }
+
+    /// Returns the `rust-src` part of Xargo.toml
+    pub fn package(&self) -> Option<&Value> {
+        self.table.lookup("package")
+    }
 }
 
 /// Returns the closest directory containing a 'Xargo.toml' and the parsed
@@ -130,3 +136,21 @@ pub fn toml(root: &Root) -> Result<(Option<&Path>, Option<Toml>)> {
         Ok((None, None))
     }
 }
+
+/// Returns the closest directory containing a 'Xargo.toml' and the parsed
+/// content of this 'Xargo.toml'
+pub fn toml_src(root: &Root) -> Result<Option<Src>> {
+    Ok(if let Some(toml) = toml(root)?.1 {
+        if let Some(Value::Table(table)) = toml.package() {
+            if let Some(src) = table.get("rust-src").map(Value::as_str).flatten() {
+                env::current_dir().map(|cur_dir| Src::from(cur_dir.join(src))).ok()
+            } else {
+                None
+            }
+        } else {
+            None
+        }
+    } else {
+        None
+    })
+}