Merge #3549

3549: Implement env! macro r=matklad a=edwin0cheng

This PR implements `env!` macro by adding following things:

1. Added `additional_outdirs` settings in vscode. (naming to be bikeshed)
2. Added `ExternSourceId` which is a wrapping for SourceRootId but only used in extern sources. It is because `OUT_DIR` is not belonged to any crate and we have to access it behind an `AstDatabase`.
3. This PR does not implement the `OUT_DIR` parsing from `cargo check`. I don't have general design about this,  @kiljacken could we reuse some cargo watch code for that ?

~~Block on [#3536]~~

PS: After this PR , we (kind of) completed the `include!(concat!(env!('OUT_DIR'),  "foo.rs")` macro call combo. [Exodia Obliterate!](https://www.youtube.com/watch?v=RfqNH3FoGi0)

Co-authored-by: Edwin Cheng <[email protected]>

20 files changed, 320 insertions, 72 deletions

diff --git a/crates/ra_db/src/fixture.rs b/crates/ra_db/src/fixture.rs
index 7f43c2971..3dc86ca2d 100644
--- a/crates/ra_db/src/fixture.rs
+++ b/crates/ra_db/src/fixture.rs
@@ -61,7 +61,14 @@ fn with_single_file(db: &mut dyn SourceDatabaseExt, ra_fixture: &str) -> FileId
         };
 
         let mut crate_graph = CrateGraph::default();
-        crate_graph.add_crate_root(file_id, meta.edition, meta.krate, meta.cfg, meta.env);
+        crate_graph.add_crate_root(
+            file_id,
+            meta.edition,
+            meta.krate,
+            meta.cfg,
+            meta.env,
+            Default::default(),
+        );
         crate_graph
     } else {
         let mut crate_graph = CrateGraph::default();
@@ -71,6 +78,7 @@ fn with_single_file(db: &mut dyn SourceDatabaseExt, ra_fixture: &str) -> FileId
             None,
             CfgOptions::default(),
             Env::default(),
+            Default::default(),
         );
         crate_graph
     };
@@ -119,6 +127,7 @@ fn with_files(db: &mut dyn SourceDatabaseExt, fixture: &str) -> Option<FilePosit
                 Some(krate.clone()),
                 meta.cfg,
                 meta.env,
+                Default::default(),
             );
             let prev = crates.insert(krate.clone(), crate_id);
             assert!(prev.is_none());
@@ -155,6 +164,7 @@ fn with_files(db: &mut dyn SourceDatabaseExt, fixture: &str) -> Option<FilePosit
             None,
             CfgOptions::default(),
             Env::default(),
+            Default::default(),
         );
     } else {
         for (from, to) in crate_deps {
diff --git a/crates/ra_db/src/input.rs b/crates/ra_db/src/input.rs
index 1a1c64202..06d40db96 100644
--- a/crates/ra_db/src/input.rs
+++ b/crates/ra_db/src/input.rs
@@ -113,6 +113,7 @@ pub struct CrateData {
     pub display_name: Option<String>,
     pub cfg_options: CfgOptions,
     pub env: Env,
+    pub extern_source: ExternSource,
     pub dependencies: Vec<Dependency>,
 }
 
@@ -122,11 +123,22 @@ pub enum Edition {
     Edition2015,
 }
 
+#[derive(Debug, Clone, Copy, PartialEq, Eq, PartialOrd, Ord, Hash)]
+pub struct ExternSourceId(pub u32);
+
 #[derive(Default, Debug, Clone, PartialEq, Eq)]
 pub struct Env {
     entries: FxHashMap<String, String>,
 }
 
+// FIXME: Redesign vfs for solve the following limitation ?
+// Note: Some env variables (e.g. OUT_DIR) are located outside of the
+// crate. We store a map to allow remap it to ExternSourceId
+#[derive(Default, Debug, Clone, PartialEq, Eq)]
+pub struct ExternSource {
+    extern_paths: FxHashMap<String, ExternSourceId>,
+}
+
 #[derive(Debug, Clone, PartialEq, Eq)]
 pub struct Dependency {
     pub crate_id: CrateId,
@@ -141,6 +153,7 @@ impl CrateGraph {
         display_name: Option<String>,
         cfg_options: CfgOptions,
         env: Env,
+        extern_source: ExternSource,
     ) -> CrateId {
         let data = CrateData {
             root_file_id: file_id,
@@ -148,6 +161,7 @@ impl CrateGraph {
             display_name,
             cfg_options,
             env,
+            extern_source,
             dependencies: Vec::new(),
         };
         let crate_id = CrateId(self.arena.len() as u32);
@@ -271,6 +285,27 @@ impl Env {
     }
 }
 
+impl ExternSource {
+    pub fn extern_path(&self, path: &str) -> Option<(ExternSourceId, RelativePathBuf)> {
+        self.extern_paths.iter().find_map(|(root_path, id)| {
+            if path.starts_with(root_path) {
+                let mut rel_path = &path[root_path.len()..];
+                if rel_path.starts_with("/") {
+                    rel_path = &rel_path[1..];
+                }
+                let rel_path = RelativePathBuf::from_path(rel_path).ok()?;
+                Some((id.clone(), rel_path))
+            } else {
+                None
+            }
+        })
+    }
+
+    pub fn set_extern_path(&mut self, root_path: &str, root: ExternSourceId) {
+        self.extern_paths.insert(root_path.to_owned(), root);
+    }
+}
+
 #[derive(Debug)]
 pub struct ParseEditionError {
     invalid_input: String,
@@ -300,6 +335,7 @@ mod tests {
             None,
             CfgOptions::default(),
             Env::default(),
+            Default::default(),
         );
         let crate2 = graph.add_crate_root(
             FileId(2u32),
@@ -307,6 +343,7 @@ mod tests {
             None,
             CfgOptions::default(),
             Env::default(),
+            Default::default(),
         );
         let crate3 = graph.add_crate_root(
             FileId(3u32),
@@ -314,6 +351,7 @@ mod tests {
             None,
             CfgOptions::default(),
             Env::default(),
+            Default::default(),
         );
         assert!(graph.add_dep(crate1, CrateName::new("crate2").unwrap(), crate2).is_ok());
         assert!(graph.add_dep(crate2, CrateName::new("crate3").unwrap(), crate3).is_ok());
@@ -329,6 +367,7 @@ mod tests {
             None,
             CfgOptions::default(),
             Env::default(),
+            Default::default(),
         );
         let crate2 = graph.add_crate_root(
             FileId(2u32),
@@ -336,6 +375,7 @@ mod tests {
             None,
             CfgOptions::default(),
             Env::default(),
+            Default::default(),
         );
         let crate3 = graph.add_crate_root(
             FileId(3u32),
@@ -343,6 +383,7 @@ mod tests {
             None,
             CfgOptions::default(),
             Env::default(),
+            Default::default(),
         );
         assert!(graph.add_dep(crate1, CrateName::new("crate2").unwrap(), crate2).is_ok());
         assert!(graph.add_dep(crate2, CrateName::new("crate3").unwrap(), crate3).is_ok());
@@ -357,6 +398,7 @@ mod tests {
             None,
             CfgOptions::default(),
             Env::default(),
+            Default::default(),
         );
         let crate2 = graph.add_crate_root(
             FileId(2u32),
@@ -364,6 +406,7 @@ mod tests {
             None,
             CfgOptions::default(),
             Env::default(),
+            Default::default(),
         );
         assert!(graph
             .add_dep(crate1, CrateName::normalize_dashes("crate-name-with-dashes"), crate2)
diff --git a/crates/ra_db/src/lib.rs b/crates/ra_db/src/lib.rs
index fb002d717..d500d5e85 100644
--- a/crates/ra_db/src/lib.rs
+++ b/crates/ra_db/src/lib.rs
@@ -11,7 +11,8 @@ use ra_syntax::{ast, Parse, SourceFile, TextRange, TextUnit};
 pub use crate::{
     cancellation::Canceled,
     input::{
-        CrateGraph, CrateId, CrateName, Dependency, Edition, Env, FileId, SourceRoot, SourceRootId,
+        CrateGraph, CrateId, CrateName, Dependency, Edition, Env, ExternSource, ExternSourceId,
+        FileId, SourceRoot, SourceRootId,
     },
 };
 pub use relative_path::{RelativePath, RelativePathBuf};
@@ -87,6 +88,12 @@ pub trait FileLoader {
     fn resolve_relative_path(&self, anchor: FileId, relative_path: &RelativePath)
         -> Option<FileId>;
     fn relevant_crates(&self, file_id: FileId) -> Arc<Vec<CrateId>>;
+
+    fn resolve_extern_path(
+        &self,
+        extern_id: ExternSourceId,
+        relative_path: &RelativePath,
+    ) -> Option<FileId>;
 }
 
 /// Database which stores all significant input facts: source code and project
@@ -164,4 +171,13 @@ impl<T: SourceDatabaseExt> FileLoader for FileLoaderDelegate<&'_ T> {
         let source_root = self.0.file_source_root(file_id);
         self.0.source_root_crates(source_root)
     }
+
+    fn resolve_extern_path(
+        &self,
+        extern_id: ExternSourceId,
+        relative_path: &RelativePath,
+    ) -> Option<FileId> {
+        let source_root = self.0.source_root(SourceRootId(extern_id.0));
+        source_root.file_by_relative_path(&relative_path)
+    }
 }
diff --git a/crates/ra_hir_def/src/test_db.rs b/crates/ra_hir_def/src/test_db.rs
index 1568820e9..0756916a8 100644
--- a/crates/ra_hir_def/src/test_db.rs
+++ b/crates/ra_hir_def/src/test_db.rs
@@ -6,7 +6,7 @@ use std::{
 };
 
 use crate::db::DefDatabase;
-use ra_db::{salsa, CrateId, FileId, FileLoader, FileLoaderDelegate, RelativePath};
+use ra_db::{salsa, CrateId, ExternSourceId, FileId, FileLoader, FileLoaderDelegate, RelativePath};
 
 #[salsa::database(
     ra_db::SourceDatabaseExtStorage,
@@ -52,6 +52,14 @@ impl FileLoader for TestDB {
     fn relevant_crates(&self, file_id: FileId) -> Arc<Vec<CrateId>> {
         FileLoaderDelegate(self).relevant_crates(file_id)
     }
+
+    fn resolve_extern_path(
+        &self,
+        extern_id: ExternSourceId,
+        relative_path: &RelativePath,
+    ) -> Option<FileId> {
+        FileLoaderDelegate(self).resolve_extern_path(extern_id, relative_path)
+    }
 }
 
 impl TestDB {
diff --git a/crates/ra_hir_expand/src/builtin_macro.rs b/crates/ra_hir_expand/src/builtin_macro.rs
index 3f60b1cca..a90007f26 100644
--- a/crates/ra_hir_expand/src/builtin_macro.rs
+++ b/crates/ra_hir_expand/src/builtin_macro.rs
@@ -90,15 +90,15 @@ register_builtin! {
     (line, Line) => line_expand,
     (stringify, Stringify) => stringify_expand,
     (format_args, FormatArgs) => format_args_expand,
-    (env, Env) => env_expand,
-    (option_env, OptionEnv) => option_env_expand,
     // format_args_nl only differs in that it adds a newline in the end,
     // so we use the same stub expansion for now
     (format_args_nl, FormatArgsNl) => format_args_expand,
 
     EAGER:
     (concat, Concat) => concat_expand,
-    (include, Include) => include_expand
+    (include, Include) => include_expand,
+    (env, Env) => env_expand,
+    (option_env, OptionEnv) => option_env_expand
 }
 
 fn line_expand(
@@ -137,31 +137,6 @@ fn stringify_expand(
     Ok(expanded)
 }
 
-fn env_expand(
-    _db: &dyn AstDatabase,
-    _id: LazyMacroId,
-    _tt: &tt::Subtree,
-) -> Result<tt::Subtree, mbe::ExpandError> {
-    // dummy implementation for type-checking purposes
-    // we cannot use an empty string here, because for
-    // `include!(concat!(env!("OUT_DIR"), "/foo.rs"))` will become
-    // `include!("foo.rs"), which maybe infinite loop
-    let expanded = quote! { "__RA_UNIMPLEMENTATED__" };
-
-    Ok(expanded)
-}
-
-fn option_env_expand(
-    _db: &dyn AstDatabase,
-    _id: LazyMacroId,
-    _tt: &tt::Subtree,
-) -> Result<tt::Subtree, mbe::ExpandError> {
-    // dummy implementation for type-checking purposes
-    let expanded = quote! { std::option::Option::None::<&str> };
-
-    Ok(expanded)
-}
-
 fn column_expand(
     _db: &dyn AstDatabase,
     _id: LazyMacroId,
@@ -278,30 +253,37 @@ fn concat_expand(
 
 fn relative_file(db: &dyn AstDatabase, call_id: MacroCallId, path: &str) -> Option<FileId> {
     let call_site = call_id.as_file().original_file(db);
-    let path = RelativePath::new(&path);
 
-    let res = db.resolve_relative_path(call_site, &path)?;
-    // Prevent include itself
-    if res == call_site {
-        return None;
+    // Handle trivial case
+    if let Some(res) = db.resolve_relative_path(call_site, &RelativePath::new(&path)) {
+        // Prevent include itself
+        return if res == call_site { None } else { Some(res) };
     }
-    Some(res)
+
+    // Extern paths ?
+    let krate = db.relevant_crates(call_site).get(0)?.clone();
+    let (extern_source_id, relative_file) =
+        db.crate_graph()[krate].extern_source.extern_path(path)?;
+
+    db.resolve_extern_path(extern_source_id, &relative_file)
 }
 
-fn include_expand(
-    db: &dyn AstDatabase,
-    arg_id: EagerMacroId,
-    tt: &tt::Subtree,
-) -> Result<(tt::Subtree, FragmentKind), mbe::ExpandError> {
-    let path = tt
-        .token_trees
+fn parse_string(tt: &tt::Subtree) -> Result<String, mbe::ExpandError> {
+    tt.token_trees
         .get(0)
         .and_then(|tt| match tt {
             tt::TokenTree::Leaf(tt::Leaf::Literal(it)) => unquote_str(&it),
             _ => None,
         })
-        .ok_or_else(|| mbe::ExpandError::ConversionError)?;
+        .ok_or_else(|| mbe::ExpandError::ConversionError)
+}
 
+fn include_expand(
+    db: &dyn AstDatabase,
+    arg_id: EagerMacroId,
+    tt: &tt::Subtree,
+) -> Result<(tt::Subtree, FragmentKind), mbe::ExpandError> {
+    let path = parse_string(tt)?;
     let file_id =
         relative_file(db, arg_id.into(), &path).ok_or_else(|| mbe::ExpandError::ConversionError)?;
 
@@ -314,12 +296,58 @@ fn include_expand(
     Ok((res, FragmentKind::Items))
 }
 
+fn get_env_inner(db: &dyn AstDatabase, arg_id: EagerMacroId, key: &str) -> Option<String> {
+    let call_id: MacroCallId = arg_id.into();
+    let original_file = call_id.as_file().original_file(db);
+
+    let krate = db.relevant_crates(original_file).get(0)?.clone();
+    db.crate_graph()[krate].env.get(key)
+}
+
+fn env_expand(
+    db: &dyn AstDatabase,
+    arg_id: EagerMacroId,
+    tt: &tt::Subtree,
+) -> Result<(tt::Subtree, FragmentKind), mbe::ExpandError> {
+    let key = parse_string(tt)?;
+
+    // FIXME:
+    // If the environment variable is not defined int rustc, then a compilation error will be emitted.
+    // We might do the same if we fully support all other stuffs.
+    // But for now on, we should return some dummy string for better type infer purpose.
+    // However, we cannot use an empty string here, because for
+    // `include!(concat!(env!("OUT_DIR"), "/foo.rs"))` will become
+    // `include!("foo.rs"), which might go to infinite loop
+    let s = get_env_inner(db, arg_id, &key).unwrap_or("__RA_UNIMPLEMENTATED__".to_string());
+    let expanded = quote! { #s };
+
+    Ok((expanded, FragmentKind::Expr))
+}
+
+fn option_env_expand(
+    db: &dyn AstDatabase,
+    arg_id: EagerMacroId,
+    tt: &tt::Subtree,
+) -> Result<(tt::Subtree, FragmentKind), mbe::ExpandError> {
+    let key = parse_string(tt)?;
+    let expanded = match get_env_inner(db, arg_id, &key) {
+        None => quote! { std::option::Option::None::<&str> },
+        Some(s) => quote! { std::option::Some(#s) },
+    };
+
+    Ok((expanded, FragmentKind::Expr))
+}
+
 #[cfg(test)]
 mod tests {
     use super::*;
-    use crate::{name::AsName, test_db::TestDB, AstNode, MacroCallId, MacroCallKind, MacroCallLoc};
+    use crate::{
+        name::AsName, test_db::TestDB, AstNode, EagerCallLoc, MacroCallId, MacroCallKind,
+        MacroCallLoc,
+    };
     use ra_db::{fixture::WithFixture, SourceDatabase};
     use ra_syntax::ast::NameOwner;
+    use std::sync::Arc;
 
     fn expand_builtin_macro(ra_fixture: &str) -> String {
         let (db, file_id) = TestDB::with_single_file(&ra_fixture);
@@ -330,27 +358,61 @@ mod tests {
         let ast_id_map = db.ast_id_map(file_id.into());
 
         let expander = find_by_name(&macro_calls[0].name().unwrap().as_name()).unwrap();
-        let expander = expander.left().unwrap();
 
-        // the first one should be a macro_rules
-        let def = MacroDefId {
-            krate: Some(CrateId(0)),
-            ast_id: Some(AstId::new(file_id.into(), ast_id_map.ast_id(&macro_calls[0]))),
-            kind: MacroDefKind::BuiltIn(expander),
-        };
+        let file_id = match expander {
+            Either::Left(expander) => {
+                // the first one should be a macro_rules
+                let def = MacroDefId {
+                    krate: Some(CrateId(0)),
+                    ast_id: Some(AstId::new(file_id.into(), ast_id_map.ast_id(&macro_calls[0]))),
+                    kind: MacroDefKind::BuiltIn(expander),
+                };
 
-        let loc = MacroCallLoc {
-            def,
-            kind: MacroCallKind::FnLike(AstId::new(
-                file_id.into(),
-                ast_id_map.ast_id(&macro_calls[1]),
-            )),
-        };
+                let loc = MacroCallLoc {
+                    def,
+                    kind: MacroCallKind::FnLike(AstId::new(
+                        file_id.into(),
+                        ast_id_map.ast_id(&macro_calls[1]),
+                    )),
+                };
+
+                let id: MacroCallId = db.intern_macro(loc).into();
+                id.as_file()
+            }
+            Either::Right(expander) => {
+                // the first one should be a macro_rules
+                let def = MacroDefId {
+                    krate: Some(CrateId(0)),
+                    ast_id: Some(AstId::new(file_id.into(), ast_id_map.ast_id(&macro_calls[0]))),
+                    kind: MacroDefKind::BuiltInEager(expander),
+                };
 
-        let id: MacroCallId = db.intern_macro(loc).into();
-        let parsed = db.parse_or_expand(id.as_file()).unwrap();
+                let args = macro_calls[1].token_tree().unwrap();
+                let parsed_args = mbe::ast_to_token_tree(&args).unwrap().0;
+
+                let arg_id = db.intern_eager_expansion({
+                    EagerCallLoc {
+                        def,
+                        fragment: FragmentKind::Expr,
+                        subtree: Arc::new(parsed_args.clone()),
+                        file_id: file_id.into(),
+                    }
+                });
+
+                let (subtree, fragment) = expander.expand(&db, arg_id, &parsed_args).unwrap();
+                let eager = EagerCallLoc {
+                    def,
+                    fragment,
+                    subtree: Arc::new(subtree),
+                    file_id: file_id.into(),
+                };
+
+                let id: MacroCallId = db.intern_eager_expansion(eager.into()).into();
+                id.as_file()
+            }
+        };
 
-        parsed.text().to_string()
+        db.parse_or_expand(file_id).unwrap().to_string()
     }
 
     #[test]
diff --git a/crates/ra_hir_expand/src/test_db.rs b/crates/ra_hir_expand/src/test_db.rs
index 918736e2a..c1fb762de 100644
--- a/crates/ra_hir_expand/src/test_db.rs
+++ b/crates/ra_hir_expand/src/test_db.rs
@@ -5,7 +5,7 @@ use std::{
     sync::{Arc, Mutex},
 };
 
-use ra_db::{salsa, CrateId, FileId, FileLoader, FileLoaderDelegate, RelativePath};
+use ra_db::{salsa, CrateId, ExternSourceId, FileId, FileLoader, FileLoaderDelegate, RelativePath};
 
 #[salsa::database(
     ra_db::SourceDatabaseExtStorage,
@@ -51,4 +51,11 @@ impl FileLoader for TestDB {
     fn relevant_crates(&self, file_id: FileId) -> Arc<Vec<CrateId>> {
         FileLoaderDelegate(self).relevant_crates(file_id)
     }
+    fn resolve_extern_path(
+        &self,
+        anchor: ExternSourceId,
+        relative_path: &RelativePath,
+    ) -> Option<FileId> {
+        FileLoaderDelegate(self).resolve_extern_path(anchor, relative_path)
+    }
 }
diff --git a/crates/ra_hir_ty/src/test_db.rs b/crates/ra_hir_ty/src/test_db.rs
index c794f7b84..0be2fea4b 100644
--- a/crates/ra_hir_ty/src/test_db.rs
+++ b/crates/ra_hir_ty/src/test_db.rs
@@ -67,6 +67,13 @@ impl FileLoader for TestDB {
     fn relevant_crates(&self, file_id: FileId) -> Arc<Vec<CrateId>> {
         FileLoaderDelegate(self).relevant_crates(file_id)
     }
+    fn resolve_extern_path(
+        &self,
+        extern_id: ra_db::ExternSourceId,
+        relative_path: &RelativePath,
+    ) -> Option<FileId> {
+        FileLoaderDelegate(self).resolve_extern_path(extern_id, relative_path)
+    }
 }
 
 impl TestDB {
diff --git a/crates/ra_hir_ty/src/tests/macros.rs b/crates/ra_hir_ty/src/tests/macros.rs
index ffa78b046..32457bbf7 100644
--- a/crates/ra_hir_ty/src/tests/macros.rs
+++ b/crates/ra_hir_ty/src/tests/macros.rs
@@ -550,6 +550,26 @@ fn main() {
 }
 
 #[test]
+fn infer_builtin_macros_env() {
+    assert_snapshot!(
+        infer(r#"
+//- /main.rs env:foo=bar
+#[rustc_builtin_macro]
+macro_rules! env {() => {}}
+
+fn main() {
+    let x = env!("foo");
+}
+"#),
+        @r###"
+    ![0; 5) '"bar"': &str
+    [88; 116) '{     ...o"); }': ()
+    [98; 99) 'x': &str
+    "###
+    );
+}
+
+#[test]
 fn infer_derive_clone_simple() {
     let (db, pos) = TestDB::with_position(
         r#"
diff --git a/crates/ra_ide/src/lib.rs b/crates/ra_ide/src/lib.rs
index 9f45003d3..015fae195 100644
--- a/crates/ra_ide/src/lib.rs
+++ b/crates/ra_ide/src/lib.rs
@@ -212,6 +212,7 @@ impl Analysis {
             None,
             cfg_options,
             Env::default(),
+            Default::default(),
         );
         change.add_file(source_root, file_id, "main.rs".into(), Arc::new(text));
         change.set_crate_graph(crate_graph);
diff --git a/crates/ra_ide/src/mock_analysis.rs b/crates/ra_ide/src/mock_analysis.rs
index 90f84b052..25816cf6f 100644
--- a/crates/ra_ide/src/mock_analysis.rs
+++ b/crates/ra_ide/src/mock_analysis.rs
@@ -102,6 +102,7 @@ impl MockAnalysis {
                     None,
                     cfg_options,
                     Env::default(),
+                    Default::default(),
                 ));
             } else if path.ends_with("/lib.rs") {
                 let crate_name = path.parent().unwrap().file_name().unwrap();
@@ -111,6 +112,7 @@ impl MockAnalysis {
                     Some(crate_name.to_owned()),
                     cfg_options,
                     Env::default(),
+                    Default::default(),
                 );
                 if let Some(root_crate) = root_crate {
                     crate_graph
diff --git a/crates/ra_ide/src/parent_module.rs b/crates/ra_ide/src/parent_module.rs
index b73cefd97..76d130b9b 100644
--- a/crates/ra_ide/src/parent_module.rs
+++ b/crates/ra_ide/src/parent_module.rs
@@ -136,6 +136,7 @@ mod tests {
             None,
             CfgOptions::default(),
             Env::default(),
+            Default::default(),
         );
         let mut change = AnalysisChange::new();
         change.set_crate_graph(crate_graph);
diff --git a/crates/ra_ide_db/src/lib.rs b/crates/ra_ide_db/src/lib.rs
index 6bcccc848..fc1b19def 100644
--- a/crates/ra_ide_db/src/lib.rs
+++ b/crates/ra_ide_db/src/lib.rs
@@ -55,6 +55,13 @@ impl FileLoader for RootDatabase {
     fn relevant_crates(&self, file_id: FileId) -> Arc<Vec<CrateId>> {
         FileLoaderDelegate(self).relevant_crates(file_id)
     }
+    fn resolve_extern_path(
+        &self,
+        extern_id: ra_db::ExternSourceId,
+        relative_path: &RelativePath,
+    ) -> Option<FileId> {
+        FileLoaderDelegate(self).resolve_extern_path(extern_id, relative_path)
+    }
 }
 
 impl salsa::Database for RootDatabase {
diff --git a/crates/ra_project_model/src/lib.rs b/crates/ra_project_model/src/lib.rs
index 37845ca56..a6274709d 100644
--- a/crates/ra_project_model/src/lib.rs
+++ b/crates/ra_project_model/src/lib.rs
@@ -14,7 +14,7 @@ use std::{
 
 use anyhow::{bail, Context, Result};
 use ra_cfg::CfgOptions;
-use ra_db::{CrateGraph, CrateName, Edition, Env, FileId};
+use ra_db::{CrateGraph, CrateName, Edition, Env, ExternSource, ExternSourceId, FileId};
 use rustc_hash::FxHashMap;
 use serde_json::from_reader;
 
@@ -162,6 +162,7 @@ impl ProjectWorkspace {
     pub fn to_crate_graph(
         &self,
         default_cfg_options: &CfgOptions,
+        outdirs: &FxHashMap<String, (ExternSourceId, String)>,
         load: &mut dyn FnMut(&Path) -> Option<FileId>,
     ) -> CrateGraph {
         let mut crate_graph = CrateGraph::default();
@@ -185,6 +186,8 @@ impl ProjectWorkspace {
                             }
                             opts
                         };
+
+                        // FIXME: No crate name in json definition such that we cannot add OUT_DIR to env
                         crates.insert(
                             crate_id,
                             crate_graph.add_crate_root(
@@ -194,6 +197,7 @@ impl ProjectWorkspace {
                                 None,
                                 cfg_options,
                                 Env::default(),
+                                Default::default(),
                             ),
                         );
                     }
@@ -231,12 +235,20 @@ impl ProjectWorkspace {
                             opts
                         };
 
+                        let mut env = Env::default();
+                        let mut extern_source = ExternSource::default();
+                        if let Some((id, path)) = outdirs.get(krate.name(&sysroot)) {
+                            env.set("OUT_DIR", path.clone());
+                            extern_source.set_extern_path(&path, *id);
+                        }
+
                         let crate_id = crate_graph.add_crate_root(
                             file_id,
                             Edition::Edition2018,
                             Some(krate.name(&sysroot).to_string()),
                             cfg_options,
-                            Env::default(),
+                            env,
+                            extern_source,
                         );
                         sysroot_crates.insert(krate, crate_id);
                     }
@@ -275,12 +287,19 @@ impl ProjectWorkspace {
                                 opts.insert_features(pkg.features(&cargo).iter().map(Into::into));
                                 opts
                             };
+                            let mut env = Env::default();
+                            let mut extern_source = ExternSource::default();
+                            if let Some((id, path)) = outdirs.get(pkg.name(&cargo)) {
+                                env.set("OUT_DIR", path.clone());
+                                extern_source.set_extern_path(&path, *id);
+                            }
                             let crate_id = crate_graph.add_crate_root(
                                 file_id,
                                 edition,
                                 Some(pkg.name(&cargo).to_string()),
                                 cfg_options,
-                                Env::default(),
+                                env,
+                                extern_source,
                             );
                             if tgt.kind(&cargo) == TargetKind::Lib {
                                 lib_tgt = Some(crate_id);
diff --git a/crates/rust-analyzer/src/cli/load_cargo.rs b/crates/rust-analyzer/src/cli/load_cargo.rs
index 5df29a383..2ce69c9b3 100644
--- a/crates/rust-analyzer/src/cli/load_cargo.rs
+++ b/crates/rust-analyzer/src/cli/load_cargo.rs
@@ -52,7 +52,10 @@ pub(crate) fn load_cargo(
         opts
     };
 
-    let crate_graph = ws.to_crate_graph(&default_cfg_options, &mut |path: &Path| {
+    // FIXME: outdirs?
+    let outdirs = FxHashMap::default();
+
+    let crate_graph = ws.to_crate_graph(&default_cfg_options, &outdirs, &mut |path: &Path| {
         let vfs_file = vfs.load(path);
         log::debug!("vfs file {:?} -> {:?}", path, vfs_file);
         vfs_file.map(vfs_file_to_id)
diff --git a/crates/rust-analyzer/src/config.rs b/crates/rust-analyzer/src/config.rs
index 3314269ec..a8bf29ddf 100644
--- a/crates/rust-analyzer/src/config.rs
+++ b/crates/rust-analyzer/src/config.rs
@@ -44,6 +44,9 @@ pub struct ServerConfig {
     /// Fine grained feature flags to disable specific features.
     pub feature_flags: FxHashMap<String, bool>,
 
+    /// Fine grained controls for additional `OUT_DIR` env variables
+    pub additional_out_dirs: FxHashMap<String, String>,
+
     pub rustfmt_args: Vec<String>,
 
     /// Cargo feature configurations.
@@ -64,6 +67,7 @@ impl Default for ServerConfig {
             cargo_watch_all_targets: true,
             with_sysroot: true,
             feature_flags: FxHashMap::default(),
+            additional_out_dirs: FxHashMap::default(),
             cargo_features: Default::default(),
             rustfmt_args: Vec::new(),
         }
diff --git a/crates/rust-analyzer/src/main_loop.rs b/crates/rust-analyzer/src/main_loop.rs
index f9de712a0..4f7aac754 100644
--- a/crates/rust-analyzer/src/main_loop.rs
+++ b/crates/rust-analyzer/src/main_loop.rs
@@ -196,6 +196,7 @@ pub fn main_loop(
             Watch(!config.use_client_watching),
             options,
             feature_flags,
+            config.additional_out_dirs,
         )
     };
 
diff --git a/crates/rust-analyzer/src/world.rs b/crates/rust-analyzer/src/world.rs
index 1ddc3c1a5..9ef368529 100644
--- a/crates/rust-analyzer/src/world.rs
+++ b/crates/rust-analyzer/src/world.rs
@@ -26,6 +26,8 @@ use crate::{
     vfs_glob::{Glob, RustPackageFilterBuilder},
     LspError, Result,
 };
+use ra_db::ExternSourceId;
+use rustc_hash::{FxHashMap, FxHashSet};
 
 #[derive(Debug, Clone)]
 pub struct Options {
@@ -78,6 +80,7 @@ impl WorldState {
         watch: Watch,
         options: Options,
         feature_flags: FeatureFlags,
+        additional_out_dirs: FxHashMap<String, String>,
     ) -> WorldState {
         let mut change = AnalysisChange::new();
 
@@ -99,6 +102,19 @@ impl WorldState {
                 RootEntry::new(pkg_root.path().clone(), filter.into_vfs_filter())
             }));
         }
+
+        let extern_dirs: FxHashSet<_> =
+            additional_out_dirs.iter().map(|(_, path)| (PathBuf::from(path))).collect();
+        let mut extern_source_roots = FxHashMap::default();
+
+        roots.extend(additional_out_dirs.iter().map(|(_, path)| {
+            let mut filter = RustPackageFilterBuilder::default().set_member(false);
+            for glob in exclude_globs.iter() {
+                filter = filter.exclude(glob.clone());
+            }
+            RootEntry::new(PathBuf::from(&path), filter.into_vfs_filter())
+        }));
+
         let (task_sender, task_receiver) = unbounded();
         let task_sender = Box::new(move |t| task_sender.send(t).unwrap());
         let (mut vfs, vfs_roots) = Vfs::new(roots, task_sender, watch);
@@ -108,6 +124,11 @@ impl WorldState {
             let is_local = folder_roots.iter().any(|it| vfs_root_path.starts_with(it));
             change.add_root(SourceRootId(r.0), is_local);
             change.set_debug_root_path(SourceRootId(r.0), vfs_root_path.display().to_string());
+
+            // FIXME: add path2root in vfs to simpily this logic
+            if extern_dirs.contains(&vfs_root_path) {
+                extern_source_roots.insert(vfs_root_path, ExternSourceId(r.0));
+            }
         }
 
         // FIXME: Read default cfgs from config
@@ -125,11 +146,20 @@ impl WorldState {
             vfs_file.map(|f| FileId(f.0))
         };
 
-        workspaces.iter().map(|ws| ws.to_crate_graph(&default_cfg_options, &mut load)).for_each(
-            |graph| {
+        let mut outdirs = FxHashMap::default();
+        for (name, path) in additional_out_dirs {
+            let path = PathBuf::from(&path);
+            if let Some(id) = extern_source_roots.get(&path) {
+                outdirs.insert(name, (id.clone(), path.to_string_lossy().replace("\\", "/")));
+            }
+        }
+
+        workspaces
+            .iter()
+            .map(|ws| ws.to_crate_graph(&default_cfg_options, &outdirs, &mut load))
+            .for_each(|graph| {
                 crate_graph.extend(graph);
-            },
-        );
+            });
         change.set_crate_graph(crate_graph);
 
         // FIXME: Figure out the multi-workspace situation
diff --git a/editors/code/package.json b/editors/code/package.json
index 512885454..1fe8e9f8a 100644
--- a/editors/code/package.json
+++ b/editors/code/package.json
@@ -224,6 +224,11 @@
                     "default": true,
                     "description": "Whether to ask for permission before downloading any files from the Internet"
                 },
+                "rust-analyzer.additionalOutDirs": {
+                    "type": "object",
+                    "default": {},
+                    "markdownDescription": "Fine grained controls for OUT_DIR `env!(\"OUT_DIR\")` variable. e.g. `{\"foo\":\"/path/to/foo\"}`, "
+                },
                 "rust-analyzer.serverPath": {
                     "type": [
                         "null",
diff --git a/editors/code/src/client.ts b/editors/code/src/client.ts
index 540f7c9ea..6ce3b9235 100644
--- a/editors/code/src/client.ts
+++ b/editors/code/src/client.ts
@@ -37,6 +37,7 @@ export async function createClient(config: Config, serverPath: string): Promise<
             excludeGlobs: config.excludeGlobs,
             useClientWatching: config.useClientWatching,
             featureFlags: config.featureFlags,
+            additionalOutDirs: config.additionalOutDirs,
             withSysroot: config.withSysroot,
             cargoFeatures: config.cargoFeatures,
             rustfmtArgs: config.rustfmtArgs,
diff --git a/editors/code/src/config.ts b/editors/code/src/config.ts
index b72206d3c..3ade7e900 100644
--- a/editors/code/src/config.ts
+++ b/editors/code/src/config.ts
@@ -154,6 +154,7 @@ export class Config {
     get excludeGlobs() { return this.cfg.get("excludeGlobs") as string[]; }
     get useClientWatching() { return this.cfg.get("useClientWatching") as boolean; }
     get featureFlags() { return this.cfg.get("featureFlags") as Record<string, boolean>; }
+    get additionalOutDirs() { return this.cfg.get("additionalOutDirs") as Record<string, string>; }
     get rustfmtArgs() { return this.cfg.get("rustfmtArgs") as string[]; }
 
     get cargoWatchOptions(): CargoWatchOptions {