3 files changed, 16 insertions, 2 deletions

diff --git a/crates/ra_hir_def/src/nameres/collector.rs b/crates/ra_hir_def/src/nameres/collector.rs
index 7e6083961..36a61dc3d 100644
--- a/crates/ra_hir_def/src/nameres/collector.rs
+++ b/crates/ra_hir_def/src/nameres/collector.rs
@@ -27,6 +27,7 @@ pub(super) fn collect_defs(db: &impl DefDatabase2, mut def_map: CrateDefMap) ->
     let crate_graph = db.crate_graph();
 
     // populate external prelude
+    let mut prelude_is_core = false;
     for dep in crate_graph.dependencies(def_map.krate) {
         let dep_def_map = db.crate_def_map(dep.crate_id);
         log::debug!("crate dep {:?} -> {:?}", dep.name, dep.crate_id);
@@ -36,10 +37,13 @@ pub(super) fn collect_defs(db: &impl DefDatabase2, mut def_map: CrateDefMap) ->
         );
 
         // look for the prelude
-        if def_map.prelude.is_none() {
+        // If the prelude is the "core" prelude, try to replace it with a higher
+        // level prelude (e.g. "std") if available.
+        if def_map.prelude.is_none() || prelude_is_core {
             let map = db.crate_def_map(dep.crate_id);
             if map.prelude.is_some() {
                 def_map.prelude = map.prelude;
+                prelude_is_core = dep.name == "core";
             }
         }
     }
diff --git a/crates/ra_project_model/src/lib.rs b/crates/ra_project_model/src/lib.rs
index 8b8663a78..b1268f29b 100644
--- a/crates/ra_project_model/src/lib.rs
+++ b/crates/ra_project_model/src/lib.rs
@@ -199,6 +199,7 @@ impl ProjectWorkspace {
                     }
                 }
 
+                let libcore = sysroot.core().and_then(|it| sysroot_crates.get(&it).copied());
                 let libstd = sysroot.std().and_then(|it| sysroot_crates.get(&it).copied());
 
                 let mut pkg_to_lib_crate = FxHashMap::default();
@@ -226,7 +227,7 @@ impl ProjectWorkspace {
                         }
                     }
 
-                    // Set deps to the std and to the lib target of the current package
+                    // Set deps to the core, std and to the lib target of the current package
                     for &from in pkg_crates.get(&pkg).into_iter().flatten() {
                         if let Some(to) = lib_tgt {
                             if to != from {
@@ -240,6 +241,11 @@ impl ProjectWorkspace {
                                 }
                             }
                         }
+                        if let Some(core) = libcore {
+                            if let Err(_) = crate_graph.add_dep(from, "core".into(), core) {
+                                log::error!("cyclic dependency on core for {}", pkg.name(&cargo))
+                            }
+                        }
                         if let Some(std) = libstd {
                             if let Err(_) = crate_graph.add_dep(from, "std".into(), std) {
                                 log::error!("cyclic dependency on std for {}", pkg.name(&cargo))
diff --git a/crates/ra_project_model/src/sysroot.rs b/crates/ra_project_model/src/sysroot.rs
index 35d6df5cb..3d827809e 100644
--- a/crates/ra_project_model/src/sysroot.rs
+++ b/crates/ra_project_model/src/sysroot.rs
@@ -27,6 +27,10 @@ struct SysrootCrateData {
 }
 
 impl Sysroot {
+    pub fn core(&self) -> Option<SysrootCrate> {
+        self.by_name("core")
+    }
+
     pub fn std(&self) -> Option<SysrootCrate> {
         self.by_name("std")
     }