5 files changed, 65 insertions, 18 deletions

diff --git a/crates/project_model/src/cargo_workspace.rs b/crates/project_model/src/cargo_workspace.rs
index f47898b9b..1d8d34a0b 100644
--- a/crates/project_model/src/cargo_workspace.rs
+++ b/crates/project_model/src/cargo_workspace.rs
@@ -44,6 +44,15 @@ impl ops::Index<Target> for CargoWorkspace {
     }
 }
 
+/// Describes how to set the rustc source directory.
+#[derive(Clone, Debug, PartialEq, Eq)]
+pub enum RustcSource {
+    /// Explicit path for the rustc source directory.
+    Path(AbsPathBuf),
+    /// Try to automatically detect where the rustc source directory is.
+    Discover,
+}
+
 #[derive(Default, Clone, Debug, PartialEq, Eq)]
 pub struct CargoConfig {
     /// Do not activate the `default` feature.
@@ -64,7 +73,7 @@ pub struct CargoConfig {
     pub no_sysroot: bool,
 
     /// rustc private crate source
-    pub rustc_source: Option<AbsPathBuf>,
+    pub rustc_source: Option<RustcSource>,
 }
 
 pub type Package = Idx<PackageData>;
diff --git a/crates/project_model/src/lib.rs b/crates/project_model/src/lib.rs
index d712095a6..a5b35ed95 100644
--- a/crates/project_model/src/lib.rs
+++ b/crates/project_model/src/lib.rs
@@ -21,8 +21,8 @@ use rustc_hash::FxHashSet;
 pub use crate::{
     build_data::{BuildDataCollector, BuildDataResult},
     cargo_workspace::{
-        CargoConfig, CargoWorkspace, Package, PackageData, PackageDependency, Target, TargetData,
-        TargetKind,
+        CargoConfig, CargoWorkspace, Package, PackageData, PackageDependency, RustcSource, Target,
+        TargetData, TargetKind,
     },
     project_json::{ProjectJson, ProjectJsonData},
     sysroot::Sysroot,
diff --git a/crates/project_model/src/sysroot.rs b/crates/project_model/src/sysroot.rs
index ff44dae4a..3b0ff506d 100644
--- a/crates/project_model/src/sysroot.rs
+++ b/crates/project_model/src/sysroot.rs
@@ -51,11 +51,18 @@ impl Sysroot {
     pub fn discover(cargo_toml: &AbsPath) -> Result<Sysroot> {
         log::debug!("Discovering sysroot for {}", cargo_toml.display());
         let current_dir = cargo_toml.parent().unwrap();
-        let sysroot_src_dir = discover_sysroot_src_dir(current_dir)?;
+        let sysroot_dir = discover_sysroot_dir(current_dir)?;
+        let sysroot_src_dir = discover_sysroot_src_dir(&sysroot_dir, current_dir)?;
         let res = Sysroot::load(&sysroot_src_dir)?;
         Ok(res)
     }
 
+    pub fn discover_rustc(cargo_toml: &AbsPath) -> Option<AbsPathBuf> {
+        log::debug!("Discovering rustc source for {}", cargo_toml.display());
+        let current_dir = cargo_toml.parent().unwrap();
+        discover_sysroot_dir(current_dir).ok().and_then(|sysroot_dir| get_rustc_src(&sysroot_dir))
+    }
+
     pub fn load(sysroot_src_dir: &AbsPath) -> Result<Sysroot> {
         let mut sysroot = Sysroot { crates: Arena::default() };
 
@@ -110,7 +117,18 @@ impl Sysroot {
     }
 }
 
-fn discover_sysroot_src_dir(current_dir: &AbsPath) -> Result<AbsPathBuf> {
+fn discover_sysroot_dir(current_dir: &AbsPath) -> Result<AbsPathBuf> {
+    let mut rustc = Command::new(toolchain::rustc());
+    rustc.current_dir(current_dir).args(&["--print", "sysroot"]);
+    log::debug!("Discovering sysroot by {:?}", rustc);
+    let stdout = utf8_stdout(rustc)?;
+    Ok(AbsPathBuf::assert(PathBuf::from(stdout)))
+}
+
+fn discover_sysroot_src_dir(
+    sysroot_path: &AbsPathBuf,
+    current_dir: &AbsPath,
+) -> Result<AbsPathBuf> {
     if let Ok(path) = env::var("RUST_SRC_PATH") {
         let path = AbsPathBuf::try_from(path.as_str())
             .map_err(|path| format_err!("RUST_SRC_PATH must be absolute: {}", path.display()))?;
@@ -122,14 +140,6 @@ fn discover_sysroot_src_dir(current_dir: &AbsPath) -> Result<AbsPathBuf> {
         log::debug!("RUST_SRC_PATH is set, but is invalid (no core: {:?}), ignoring", core);
     }
 
-    let sysroot_path = {
-        let mut rustc = Command::new(toolchain::rustc());
-        rustc.current_dir(current_dir).args(&["--print", "sysroot"]);
-        log::debug!("Discovering sysroot by {:?}", rustc);
-        let stdout = utf8_stdout(rustc)?;
-        AbsPathBuf::assert(PathBuf::from(stdout))
-    };
-
     get_rust_src(&sysroot_path)
         .or_else(|| {
             let mut rustup = Command::new(toolchain::rustup());
@@ -149,6 +159,16 @@ try installing the Rust source the same way you installed rustc",
         })
 }
 
+fn get_rustc_src(sysroot_path: &AbsPath) -> Option<AbsPathBuf> {
+    let rustc_src = sysroot_path.join("lib/rustlib/rustc-src/rust/compiler/rustc/Cargo.toml");
+    log::debug!("Checking for rustc source code: {}", rustc_src.display());
+    if rustc_src.exists() {
+        Some(rustc_src)
+    } else {
+        None
+    }
+}
+
 fn get_rust_src(sysroot_path: &AbsPath) -> Option<AbsPathBuf> {
     // Try the new path first since the old one still exists.
     let rust_src = sysroot_path.join("lib/rustlib/src/rust");
diff --git a/crates/project_model/src/workspace.rs b/crates/project_model/src/workspace.rs
index c30861976..0220efdb4 100644
--- a/crates/project_model/src/workspace.rs
+++ b/crates/project_model/src/workspace.rs
@@ -114,6 +114,7 @@ impl ProjectWorkspace {
                             cargo_version
                         )
                     })?;
+
                 let sysroot = if config.no_sysroot {
                     Sysroot::default()
                 } else {
@@ -125,7 +126,17 @@ impl ProjectWorkspace {
                     })?
                 };
 
-                let rustc = if let Some(rustc_dir) = &config.rustc_source {
+                let rustc_dir = if let Some(rustc_source) = &config.rustc_source {
+                    use cargo_workspace::RustcSource;
+                    match rustc_source {
+                        RustcSource::Path(path) => Some(path.clone()),
+                        RustcSource::Discover => Sysroot::discover_rustc(&cargo_toml),
+                    }
+                } else {
+                    None
+                };
+
+                let rustc = if let Some(rustc_dir) = rustc_dir {
                     Some(
                         CargoWorkspace::from_cargo_metadata(&rustc_dir, config, progress)
                             .with_context(|| {
diff --git a/crates/rust-analyzer/src/config.rs b/crates/rust-analyzer/src/config.rs
index cc0b22bff..f9098968a 100644
--- a/crates/rust-analyzer/src/config.rs
+++ b/crates/rust-analyzer/src/config.rs
@@ -18,7 +18,7 @@ use ide_db::helpers::{
 };
 use itertools::Itertools;
 use lsp_types::{ClientCapabilities, MarkupKind};
-use project_model::{CargoConfig, ProjectJson, ProjectJsonData, ProjectManifest};
+use project_model::{CargoConfig, ProjectJson, ProjectJsonData, ProjectManifest, RustcSource};
 use rustc_hash::FxHashSet;
 use serde::{de::DeserializeOwned, Deserialize};
 use vfs::AbsPathBuf;
@@ -177,8 +177,9 @@ config_data! {
         /// tests or binaries.\nFor example, it may be `--release`.
         runnables_cargoExtraArgs: Vec<String>   = "[]",
 
-        /// Path to the rust compiler sources, for usage in rustc_private projects.
-        rustcSource : Option<PathBuf> = "null",
+        /// Path to the rust compiler sources, for usage in rustc_private projects, or "discover"
+        /// to try to automatically find it.
+        rustcSource : Option<String> = "null",
 
         /// Additional arguments to `rustfmt`.
         rustfmt_extraArgs: Vec<String>               = "[]",
@@ -473,7 +474,13 @@ impl Config {
         self.data.cargo_loadOutDirsFromCheck
     }
     pub fn cargo(&self) -> CargoConfig {
-        let rustc_source = self.data.rustcSource.as_ref().map(|it| self.root_path.join(&it));
+        let rustc_source = self.data.rustcSource.as_ref().map(|rustc_src| {
+            if rustc_src == "discover" {
+                RustcSource::Discover
+            } else {
+                RustcSource::Path(self.root_path.join(rustc_src))
+            }
+        });
 
         CargoConfig {
             no_default_features: self.data.cargo_noDefaultFeatures,