6 files changed, 559 insertions, 6 deletions

diff --git a/crates/ra_hir_def/src/find_path.rs b/crates/ra_hir_def/src/find_path.rs
new file mode 100644
index 000000000..f7dc8acb7
--- /dev/null
+++ b/crates/ra_hir_def/src/find_path.rs
@@ -0,0 +1,455 @@
+//! An algorithm to find a path to refer to a certain item.
+
+use crate::{
+    db::DefDatabase,
+    item_scope::ItemInNs,
+    path::{ModPath, PathKind},
+    visibility::Visibility,
+    CrateId, ModuleDefId, ModuleId,
+};
+use hir_expand::name::Name;
+
+const MAX_PATH_LEN: usize = 15;
+
+// FIXME: handle local items
+
+/// Find a path that can be used to refer to a certain item. This can depend on
+/// *from where* you're referring to the item, hence the `from` parameter.
+pub fn find_path(db: &impl DefDatabase, item: ItemInNs, from: ModuleId) -> Option<ModPath> {
+    find_path_inner(db, item, from, MAX_PATH_LEN)
+}
+
+fn find_path_inner(
+    db: &impl DefDatabase,
+    item: ItemInNs,
+    from: ModuleId,
+    max_len: usize,
+) -> Option<ModPath> {
+    if max_len == 0 {
+        return None;
+    }
+
+    // Base cases:
+
+    // - if the item is already in scope, return the name under which it is
+    let def_map = db.crate_def_map(from.krate);
+    let from_scope: &crate::item_scope::ItemScope = &def_map.modules[from.local_id].scope;
+    if let Some((name, _)) = from_scope.name_of(item) {
+        return Some(ModPath::from_simple_segments(PathKind::Plain, vec![name.clone()]));
+    }
+
+    // - if the item is the crate root, return `crate`
+    if item
+        == ItemInNs::Types(ModuleDefId::ModuleId(ModuleId {
+            krate: from.krate,
+            local_id: def_map.root,
+        }))
+    {
+        return Some(ModPath::from_simple_segments(PathKind::Crate, Vec::new()));
+    }
+
+    // - if the item is the module we're in, use `self`
+    if item == ItemInNs::Types(from.into()) {
+        return Some(ModPath::from_simple_segments(PathKind::Super(0), Vec::new()));
+    }
+
+    // - if the item is the parent module, use `super` (this is not used recursively, since `super::super` is ugly)
+    if let Some(parent_id) = def_map.modules[from.local_id].parent {
+        if item
+            == ItemInNs::Types(ModuleDefId::ModuleId(ModuleId {
+                krate: from.krate,
+                local_id: parent_id,
+            }))
+        {
+            return Some(ModPath::from_simple_segments(PathKind::Super(1), Vec::new()));
+        }
+    }
+
+    // - if the item is the crate root of a dependency crate, return the name from the extern prelude
+    for (name, def_id) in &def_map.extern_prelude {
+        if item == ItemInNs::Types(*def_id) {
+            return Some(ModPath::from_simple_segments(PathKind::Plain, vec![name.clone()]));
+        }
+    }
+
+    // - if the item is in the prelude, return the name from there
+    if let Some(prelude_module) = def_map.prelude {
+        let prelude_def_map = db.crate_def_map(prelude_module.krate);
+        let prelude_scope: &crate::item_scope::ItemScope =
+            &prelude_def_map.modules[prelude_module.local_id].scope;
+        if let Some((name, vis)) = prelude_scope.name_of(item) {
+            if vis.is_visible_from(db, from) {
+                return Some(ModPath::from_simple_segments(PathKind::Plain, vec![name.clone()]));
+            }
+        }
+    }
+
+    // Recursive case:
+    // - if the item is an enum variant, refer to it via the enum
+    if let Some(ModuleDefId::EnumVariantId(variant)) = item.as_module_def_id() {
+        if let Some(mut path) = find_path(db, ItemInNs::Types(variant.parent.into()), from) {
+            let data = db.enum_data(variant.parent);
+            path.segments.push(data.variants[variant.local_id].name.clone());
+            return Some(path);
+        }
+        // If this doesn't work, it seems we have no way of referring to the
+        // enum; that's very weird, but there might still be a reexport of the
+        // variant somewhere
+    }
+
+    // - otherwise, look for modules containing (reexporting) it and import it from one of those
+    let importable_locations = find_importable_locations(db, item, from);
+    let mut best_path = None;
+    let mut best_path_len = max_len;
+    for (module_id, name) in importable_locations {
+        let mut path = match find_path_inner(
+            db,
+            ItemInNs::Types(ModuleDefId::ModuleId(module_id)),
+            from,
+            best_path_len - 1,
+        ) {
+            None => continue,
+            Some(path) => path,
+        };
+        path.segments.push(name);
+        if path_len(&path) < best_path_len {
+            best_path_len = path_len(&path);
+            best_path = Some(path);
+        }
+    }
+    best_path
+}
+
+fn path_len(path: &ModPath) -> usize {
+    path.segments.len()
+        + match path.kind {
+            PathKind::Plain => 0,
+            PathKind::Super(i) => i as usize,
+            PathKind::Crate => 1,
+            PathKind::Abs => 0,
+            PathKind::DollarCrate(_) => 1,
+        }
+}
+
+fn find_importable_locations(
+    db: &impl DefDatabase,
+    item: ItemInNs,
+    from: ModuleId,
+) -> Vec<(ModuleId, Name)> {
+    let crate_graph = db.crate_graph();
+    let mut result = Vec::new();
+    // We only look in the crate from which we are importing, and the direct
+    // dependencies. We cannot refer to names from transitive dependencies
+    // directly (only through reexports in direct dependencies).
+    for krate in Some(from.krate)
+        .into_iter()
+        .chain(crate_graph.dependencies(from.krate).map(|dep| dep.crate_id))
+    {
+        result.extend(
+            importable_locations_in_crate(db, item, krate)
+                .iter()
+                .filter(|(_, _, vis)| vis.is_visible_from(db, from))
+                .map(|(m, n, _)| (*m, n.clone())),
+        );
+    }
+    result
+}
+
+/// Collects all locations from which we might import the item in a particular
+/// crate. These include the original definition of the item, and any
+/// non-private `use`s.
+///
+/// Note that the crate doesn't need to be the one in which the item is defined;
+/// it might be re-exported in other crates.
+fn importable_locations_in_crate(
+    db: &impl DefDatabase,
+    item: ItemInNs,
+    krate: CrateId,
+) -> Vec<(ModuleId, Name, Visibility)> {
+    let def_map = db.crate_def_map(krate);
+    let mut result = Vec::new();
+    for (local_id, data) in def_map.modules.iter() {
+        if let Some((name, vis)) = data.scope.name_of(item) {
+            let is_private = if let Visibility::Module(private_to) = vis {
+                private_to.local_id == local_id
+            } else {
+                false
+            };
+            let is_original_def = if let Some(module_def_id) = item.as_module_def_id() {
+                data.scope.declarations().any(|it| it == module_def_id)
+            } else {
+                false
+            };
+            if is_private && !is_original_def {
+                // Ignore private imports. these could be used if we are
+                // in a submodule of this module, but that's usually not
+                // what the user wants; and if this module can import
+                // the item and we're a submodule of it, so can we.
+                // Also this keeps the cached data smaller.
+                continue;
+            }
+            result.push((ModuleId { krate, local_id }, name.clone(), vis));
+        }
+    }
+    result
+}
+
+#[cfg(test)]
+mod tests {
+    use super::*;
+    use crate::test_db::TestDB;
+    use hir_expand::hygiene::Hygiene;
+    use ra_db::fixture::WithFixture;
+    use ra_syntax::ast::AstNode;
+
+    /// `code` needs to contain a cursor marker; checks that `find_path` for the
+    /// item the `path` refers to returns that same path when called from the
+    /// module the cursor is in.
+    fn check_found_path(code: &str, path: &str) {
+        let (db, pos) = TestDB::with_position(code);
+        let module = db.module_for_file(pos.file_id);
+        let parsed_path_file = ra_syntax::SourceFile::parse(&format!("use {};", path));
+        let ast_path = parsed_path_file
+            .syntax_node()
+            .descendants()
+            .find_map(ra_syntax::ast::Path::cast)
+            .unwrap();
+        let mod_path = ModPath::from_src(ast_path, &Hygiene::new_unhygienic()).unwrap();
+
+        let crate_def_map = db.crate_def_map(module.krate);
+        let resolved = crate_def_map
+            .resolve_path(
+                &db,
+                module.local_id,
+                &mod_path,
+                crate::item_scope::BuiltinShadowMode::Module,
+            )
+            .0
+            .take_types()
+            .unwrap();
+
+        let found_path = find_path(&db, ItemInNs::Types(resolved), module);
+
+        assert_eq!(found_path, Some(mod_path));
+    }
+
+    #[test]
+    fn same_module() {
+        let code = r#"
+            //- /main.rs
+            struct S;
+            <|>
+        "#;
+        check_found_path(code, "S");
+    }
+
+    #[test]
+    fn enum_variant() {
+        let code = r#"
+            //- /main.rs
+            enum E { A }
+            <|>
+        "#;
+        check_found_path(code, "E::A");
+    }
+
+    #[test]
+    fn sub_module() {
+        let code = r#"
+            //- /main.rs
+            mod foo {
+                pub struct S;
+            }
+            <|>
+        "#;
+        check_found_path(code, "foo::S");
+    }
+
+    #[test]
+    fn super_module() {
+        let code = r#"
+            //- /main.rs
+            mod foo;
+            //- /foo.rs
+            mod bar;
+            struct S;
+            //- /foo/bar.rs
+            <|>
+        "#;
+        check_found_path(code, "super::S");
+    }
+
+    #[test]
+    fn self_module() {
+        let code = r#"
+            //- /main.rs
+            mod foo;
+            //- /foo.rs
+            <|>
+        "#;
+        check_found_path(code, "self");
+    }
+
+    #[test]
+    fn crate_root() {
+        let code = r#"
+            //- /main.rs
+            mod foo;
+            //- /foo.rs
+            <|>
+        "#;
+        check_found_path(code, "crate");
+    }
+
+    #[test]
+    fn same_crate() {
+        let code = r#"
+            //- /main.rs
+            mod foo;
+            struct S;
+            //- /foo.rs
+            <|>
+        "#;
+        check_found_path(code, "crate::S");
+    }
+
+    #[test]
+    fn different_crate() {
+        let code = r#"
+            //- /main.rs crate:main deps:std
+            <|>
+            //- /std.rs crate:std
+            pub struct S;
+        "#;
+        check_found_path(code, "std::S");
+    }
+
+    #[test]
+    fn different_crate_renamed() {
+        let code = r#"
+            //- /main.rs crate:main deps:std
+            extern crate std as std_renamed;
+            <|>
+            //- /std.rs crate:std
+            pub struct S;
+        "#;
+        check_found_path(code, "std_renamed::S");
+    }
+
+    #[test]
+    fn same_crate_reexport() {
+        let code = r#"
+            //- /main.rs
+            mod bar {
+                mod foo { pub(super) struct S; }
+                pub(crate) use foo::*;
+            }
+            <|>
+        "#;
+        check_found_path(code, "bar::S");
+    }
+
+    #[test]
+    fn same_crate_reexport_rename() {
+        let code = r#"
+            //- /main.rs
+            mod bar {
+                mod foo { pub(super) struct S; }
+                pub(crate) use foo::S as U;
+            }
+            <|>
+        "#;
+        check_found_path(code, "bar::U");
+    }
+
+    #[test]
+    fn different_crate_reexport() {
+        let code = r#"
+            //- /main.rs crate:main deps:std
+            <|>
+            //- /std.rs crate:std deps:core
+            pub use core::S;
+            //- /core.rs crate:core
+            pub struct S;
+        "#;
+        check_found_path(code, "std::S");
+    }
+
+    #[test]
+    fn prelude() {
+        let code = r#"
+            //- /main.rs crate:main deps:std
+            <|>
+            //- /std.rs crate:std
+            pub mod prelude { pub struct S; }
+            #[prelude_import]
+            pub use prelude::*;
+        "#;
+        check_found_path(code, "S");
+    }
+
+    #[test]
+    fn enum_variant_from_prelude() {
+        let code = r#"
+            //- /main.rs crate:main deps:std
+            <|>
+            //- /std.rs crate:std
+            pub mod prelude {
+                pub enum Option<T> { Some(T), None }
+                pub use Option::*;
+            }
+            #[prelude_import]
+            pub use prelude::*;
+        "#;
+        check_found_path(code, "None");
+        check_found_path(code, "Some");
+    }
+
+    #[test]
+    fn shortest_path() {
+        let code = r#"
+            //- /main.rs
+            pub mod foo;
+            pub mod baz;
+            struct S;
+            <|>
+            //- /foo.rs
+            pub mod bar { pub struct S; }
+            //- /baz.rs
+            pub use crate::foo::bar::S;
+        "#;
+        check_found_path(code, "baz::S");
+    }
+
+    #[test]
+    fn discount_private_imports() {
+        let code = r#"
+            //- /main.rs
+            mod foo;
+            pub mod bar { pub struct S; }
+            use bar::S;
+            //- /foo.rs
+            <|>
+        "#;
+        // crate::S would be shorter, but using private imports seems wrong
+        check_found_path(code, "crate::bar::S");
+    }
+
+    #[test]
+    fn import_cycle() {
+        let code = r#"
+            //- /main.rs
+            pub mod foo;
+            pub mod bar;
+            pub mod baz;
+            //- /bar.rs
+            <|>
+            //- /foo.rs
+            pub use super::baz;
+            pub struct S;
+            //- /baz.rs
+            pub use super::foo;
+        "#;
+        check_found_path(code, "crate::foo::S");
+    }
+}
diff --git a/crates/ra_hir_def/src/item_scope.rs b/crates/ra_hir_def/src/item_scope.rs
index fe7bb9779..d74a1cef2 100644
--- a/crates/ra_hir_def/src/item_scope.rs
+++ b/crates/ra_hir_def/src/item_scope.rs
@@ -104,6 +104,15 @@ impl ItemScope {
         }
     }
 
+    pub(crate) fn name_of(&self, item: ItemInNs) -> Option<(&Name, Visibility)> {
+        for (name, per_ns) in &self.visible {
+            if let Some(vis) = item.match_with(*per_ns) {
+                return Some((name, vis));
+            }
+        }
+        None
+    }
+
     pub(crate) fn traits<'a>(&'a self) -> impl Iterator<Item = TraitId> + 'a {
         self.visible.values().filter_map(|def| match def.take_types() {
             Some(ModuleDefId::TraitId(t)) => Some(t),
@@ -173,3 +182,33 @@ impl PerNs {
         }
     }
 }
+
+#[derive(Clone, Copy, PartialEq, Eq, Debug, Hash)]
+pub enum ItemInNs {
+    Types(ModuleDefId),
+    Values(ModuleDefId),
+    Macros(MacroDefId),
+}
+
+impl ItemInNs {
+    fn match_with(self, per_ns: PerNs) -> Option<Visibility> {
+        match self {
+            ItemInNs::Types(def) => {
+                per_ns.types.filter(|(other_def, _)| *other_def == def).map(|(_, vis)| vis)
+            }
+            ItemInNs::Values(def) => {
+                per_ns.values.filter(|(other_def, _)| *other_def == def).map(|(_, vis)| vis)
+            }
+            ItemInNs::Macros(def) => {
+                per_ns.macros.filter(|(other_def, _)| *other_def == def).map(|(_, vis)| vis)
+            }
+        }
+    }
+
+    pub fn as_module_def_id(self) -> Option<ModuleDefId> {
+        match self {
+            ItemInNs::Types(id) | ItemInNs::Values(id) => Some(id),
+            ItemInNs::Macros(_) => None,
+        }
+    }
+}
diff --git a/crates/ra_hir_def/src/lib.rs b/crates/ra_hir_def/src/lib.rs
index 61f044ecf..ebc12e891 100644
--- a/crates/ra_hir_def/src/lib.rs
+++ b/crates/ra_hir_def/src/lib.rs
@@ -37,6 +37,7 @@ pub mod src;
 pub mod child_by_source;
 
 pub mod visibility;
+pub mod find_path;
 
 #[cfg(test)]
 mod test_db;
diff --git a/crates/ra_hir_def/src/path.rs b/crates/ra_hir_def/src/path.rs
index 82cfa67a9..9f93a5424 100644
--- a/crates/ra_hir_def/src/path.rs
+++ b/crates/ra_hir_def/src/path.rs
@@ -1,7 +1,11 @@
 //! A desugared representation of paths like `crate::foo` or `<Type as Trait>::bar`.
 mod lower;
 
-use std::{iter, sync::Arc};
+use std::{
+    fmt::{self, Display},
+    iter,
+    sync::Arc,
+};
 
 use hir_expand::{
     hygiene::Hygiene,
@@ -248,6 +252,42 @@ impl From<Name> for ModPath {
     }
 }
 
+impl Display for ModPath {
+    fn fmt(&self, f: &mut fmt::Formatter<'_>) -> fmt::Result {
+        let mut first_segment = true;
+        let mut add_segment = |s| {
+            if !first_segment {
+                f.write_str("::")?;
+            }
+            first_segment = false;
+            f.write_str(s)?;
+            Ok(())
+        };
+        match self.kind {
+            PathKind::Plain => {}
+            PathKind::Super(n) => {
+                if n == 0 {
+                    add_segment("self")?;
+                }
+                for _ in 0..n {
+                    add_segment("super")?;
+                }
+            }
+            PathKind::Crate => add_segment("crate")?,
+            PathKind::Abs => add_segment("")?,
+            PathKind::DollarCrate(_) => add_segment("$crate")?,
+        }
+        for segment in &self.segments {
+            if !first_segment {
+                f.write_str("::")?;
+            }
+            first_segment = false;
+            write!(f, "{}", segment)?;
+        }
+        Ok(())
+    }
+}
+
 pub use hir_expand::name as __name;
 
 #[macro_export]
diff --git a/crates/ra_hir_def/src/resolver.rs b/crates/ra_hir_def/src/resolver.rs
index 5d16dd087..f7bac5801 100644
--- a/crates/ra_hir_def/src/resolver.rs
+++ b/crates/ra_hir_def/src/resolver.rs
@@ -128,7 +128,7 @@ impl Resolver {
         path: &ModPath,
         shadow: BuiltinShadowMode,
     ) -> PerNs {
-        let (item_map, module) = match self.module() {
+        let (item_map, module) = match self.module_scope() {
             Some(it) => it,
             None => return PerNs::none(),
         };
@@ -239,7 +239,7 @@ impl Resolver {
     ) -> Option<Visibility> {
         match visibility {
             RawVisibility::Module(_) => {
-                let (item_map, module) = match self.module() {
+                let (item_map, module) = match self.module_scope() {
                     Some(it) => it,
                     None => return None,
                 };
@@ -379,7 +379,7 @@ impl Resolver {
         db: &impl DefDatabase,
         path: &ModPath,
     ) -> Option<MacroDefId> {
-        let (item_map, module) = self.module()?;
+        let (item_map, module) = self.module_scope()?;
         item_map.resolve_path(db, module, &path, BuiltinShadowMode::Other).0.take_macros()
     }
 
@@ -403,7 +403,7 @@ impl Resolver {
         traits
     }
 
-    fn module(&self) -> Option<(&CrateDefMap, LocalModuleId)> {
+    fn module_scope(&self) -> Option<(&CrateDefMap, LocalModuleId)> {
         self.scopes.iter().rev().find_map(|scope| match scope {
             Scope::ModuleScope(m) => Some((&*m.crate_def_map, m.module_id)),
 
@@ -411,8 +411,13 @@ impl Resolver {
         })
     }
 
+    pub fn module(&self) -> Option<ModuleId> {
+        let (def_map, local_id) = self.module_scope()?;
+        Some(ModuleId { krate: def_map.krate, local_id })
+    }
+
     pub fn krate(&self) -> Option<CrateId> {
-        self.module().map(|t| t.0.krate)
+        self.module_scope().map(|t| t.0.krate)
     }
 
     pub fn where_predicates_in_scope<'a>(
diff --git a/crates/ra_hir_def/src/test_db.rs b/crates/ra_hir_def/src/test_db.rs
index 54e3a84bd..1568820e9 100644
--- a/crates/ra_hir_def/src/test_db.rs
+++ b/crates/ra_hir_def/src/test_db.rs
@@ -5,6 +5,7 @@ use std::{
     sync::{Arc, Mutex},
 };
 
+use crate::db::DefDatabase;
 use ra_db::{salsa, CrateId, FileId, FileLoader, FileLoaderDelegate, RelativePath};
 
 #[salsa::database(
@@ -54,6 +55,18 @@ impl FileLoader for TestDB {
 }
 
 impl TestDB {
+    pub fn module_for_file(&self, file_id: FileId) -> crate::ModuleId {
+        for &krate in self.relevant_crates(file_id).iter() {
+            let crate_def_map = self.crate_def_map(krate);
+            for (local_id, data) in crate_def_map.modules.iter() {
+                if data.origin.file_id() == Some(file_id) {
+                    return crate::ModuleId { krate, local_id };
+                }
+            }
+        }
+        panic!("Can't find module for file")
+    }
+
     pub fn log(&self, f: impl FnOnce()) -> Vec<salsa::Event<TestDB>> {
         *self.events.lock().unwrap() = Some(Vec::new());
         f();