19 files changed, 995 insertions, 85 deletions

diff --git a/Cargo.lock b/Cargo.lock
index ba9a201b9..90c505f40 100644
--- a/Cargo.lock
+++ b/Cargo.lock
@@ -872,6 +872,7 @@ dependencies = [
  "ra_hir 0.1.0",
  "ra_syntax 0.1.0",
  "ra_text_edit 0.1.0",
+ "rustc-hash 1.0.1 (registry+https://github.com/rust-lang/crates.io-index)",
  "test_utils 0.1.0",
 ]
 
diff --git a/crates/ra_assists/Cargo.toml b/crates/ra_assists/Cargo.toml
index 434e6656c..50be8d9bc 100644
--- a/crates/ra_assists/Cargo.toml
+++ b/crates/ra_assists/Cargo.toml
@@ -10,6 +10,7 @@ doctest = false
 [dependencies]
 format-buf = "1.0.0"
 join_to_string = "0.1.3"
+rustc-hash = "1.0"
 itertools = "0.8.0"
 
 ra_syntax = { path = "../ra_syntax" }
diff --git a/crates/ra_assists/src/assists/add_missing_impl_members.rs b/crates/ra_assists/src/assists/add_missing_impl_members.rs
index bc49e71fe..bf1136193 100644
--- a/crates/ra_assists/src/assists/add_missing_impl_members.rs
+++ b/crates/ra_assists/src/assists/add_missing_impl_members.rs
@@ -1,12 +1,13 @@
-use std::collections::HashMap;
-
-use hir::{db::HirDatabase, HasSource};
+use hir::{db::HirDatabase, HasSource, InFile};
 use ra_syntax::{
     ast::{self, edit, make, AstNode, NameOwner},
     SmolStr,
 };
 
-use crate::{Assist, AssistCtx, AssistId};
+use crate::{
+    ast_transform::{self, AstTransform, QualifyPaths, SubstituteTypeParams},
+    Assist, AssistCtx, AssistId,
+};
 
 #[derive(PartialEq)]
 enum AddMissingImplMembersMode {
@@ -134,25 +135,23 @@ fn add_missing_impl_members_inner(
         return None;
     }
 
-    let file_id = ctx.frange.file_id;
     let db = ctx.db;
+    let file_id = ctx.frange.file_id;
+    let trait_file_id = trait_.source(db).file_id;
 
     ctx.add_assist(AssistId(assist_id), label, |edit| {
         let n_existing_items = impl_item_list.impl_items().count();
-        let substs = get_syntactic_substs(impl_node).unwrap_or_default();
-        let generic_def: hir::GenericDef = trait_.into();
-        let substs_by_param: HashMap<_, _> = generic_def
-            .params(db)
-            .into_iter()
-            // this is a trait impl, so we need to skip the first type parameter -- this is a bit hacky
-            .skip(1)
-            .zip(substs.into_iter())
-            .collect();
+        let module = hir::SourceAnalyzer::new(
+            db,
+            hir::InFile::new(file_id.into(), impl_node.syntax()),
+            None,
+        )
+        .module();
+        let ast_transform = QualifyPaths::new(db, module)
+            .or(SubstituteTypeParams::for_trait_impl(db, trait_, impl_node));
         let items = missing_items
             .into_iter()
-            .map(|it| {
-                substitute_type_params(db, hir::InFile::new(file_id.into(), it), &substs_by_param)
-            })
+            .map(|it| ast_transform::apply(&*ast_transform, InFile::new(trait_file_id, it)))
             .map(|it| match it {
                 ast::ImplItem::FnDef(def) => ast::ImplItem::FnDef(add_body(def)),
                 _ => it,
@@ -177,56 +176,6 @@ fn add_body(fn_def: ast::FnDef) -> ast::FnDef {
     }
 }
 
-// FIXME: It would probably be nicer if we could get this via HIR (i.e. get the
-// trait ref, and then go from the types in the substs back to the syntax)
-// FIXME: This should be a general utility (not even just for assists)
-fn get_syntactic_substs(impl_block: ast::ImplBlock) -> Option<Vec<ast::TypeRef>> {
-    let target_trait = impl_block.target_trait()?;
-    let path_type = match target_trait {
-        ast::TypeRef::PathType(path) => path,
-        _ => return None,
-    };
-    let type_arg_list = path_type.path()?.segment()?.type_arg_list()?;
-    let mut result = Vec::new();
-    for type_arg in type_arg_list.type_args() {
-        let type_arg: ast::TypeArg = type_arg;
-        result.push(type_arg.type_ref()?);
-    }
-    Some(result)
-}
-
-// FIXME: This should be a general utility (not even just for assists)
-fn substitute_type_params<N: AstNode>(
-    db: &impl HirDatabase,
-    node: hir::InFile<N>,
-    substs: &HashMap<hir::TypeParam, ast::TypeRef>,
-) -> N {
-    let type_param_replacements = node
-        .value
-        .syntax()
-        .descendants()
-        .filter_map(ast::TypeRef::cast)
-        .filter_map(|n| {
-            let path = match &n {
-                ast::TypeRef::PathType(path_type) => path_type.path()?,
-                _ => return None,
-            };
-            let analyzer = hir::SourceAnalyzer::new(db, node.with_value(n.syntax()), None);
-            let resolution = analyzer.resolve_path(db, &path)?;
-            match resolution {
-                hir::PathResolution::TypeParam(tp) => Some((n, substs.get(&tp)?.clone())),
-                _ => None,
-            }
-        })
-        .collect::<Vec<_>>();
-
-    if type_param_replacements.is_empty() {
-        node.value
-    } else {
-        edit::replace_descendants(&node.value, type_param_replacements.into_iter())
-    }
-}
-
 /// Given an `ast::ImplBlock`, resolves the target trait (the one being
 /// implemented) to a `ast::TraitDef`.
 fn resolve_target_trait_def(
@@ -401,6 +350,174 @@ impl Foo for S {
     }
 
     #[test]
+    fn test_qualify_path_1() {
+        check_assist(
+            add_missing_impl_members,
+            "
+mod foo {
+    pub struct Bar;
+    trait Foo { fn foo(&self, bar: Bar); }
+}
+struct S;
+impl foo::Foo for S { <|> }",
+            "
+mod foo {
+    pub struct Bar;
+    trait Foo { fn foo(&self, bar: Bar); }
+}
+struct S;
+impl foo::Foo for S {
+    <|>fn foo(&self, bar: foo::Bar) { unimplemented!() }
+}",
+        );
+    }
+
+    #[test]
+    fn test_qualify_path_generic() {
+        check_assist(
+            add_missing_impl_members,
+            "
+mod foo {
+    pub struct Bar<T>;
+    trait Foo { fn foo(&self, bar: Bar<u32>); }
+}
+struct S;
+impl foo::Foo for S { <|> }",
+            "
+mod foo {
+    pub struct Bar<T>;
+    trait Foo { fn foo(&self, bar: Bar<u32>); }
+}
+struct S;
+impl foo::Foo for S {
+    <|>fn foo(&self, bar: foo::Bar<u32>) { unimplemented!() }
+}",
+        );
+    }
+
+    #[test]
+    fn test_qualify_path_and_substitute_param() {
+        check_assist(
+            add_missing_impl_members,
+            "
+mod foo {
+    pub struct Bar<T>;
+    trait Foo<T> { fn foo(&self, bar: Bar<T>); }
+}
+struct S;
+impl foo::Foo<u32> for S { <|> }",
+            "
+mod foo {
+    pub struct Bar<T>;
+    trait Foo<T> { fn foo(&self, bar: Bar<T>); }
+}
+struct S;
+impl foo::Foo<u32> for S {
+    <|>fn foo(&self, bar: foo::Bar<u32>) { unimplemented!() }
+}",
+        );
+    }
+
+    #[test]
+    fn test_substitute_param_no_qualify() {
+        // when substituting params, the substituted param should not be qualified!
+        check_assist(
+            add_missing_impl_members,
+            "
+mod foo {
+    trait Foo<T> { fn foo(&self, bar: T); }
+    pub struct Param;
+}
+struct Param;
+struct S;
+impl foo::Foo<Param> for S { <|> }",
+            "
+mod foo {
+    trait Foo<T> { fn foo(&self, bar: T); }
+    pub struct Param;
+}
+struct Param;
+struct S;
+impl foo::Foo<Param> for S {
+    <|>fn foo(&self, bar: Param) { unimplemented!() }
+}",
+        );
+    }
+
+    #[test]
+    fn test_qualify_path_associated_item() {
+        check_assist(
+            add_missing_impl_members,
+            "
+mod foo {
+    pub struct Bar<T>;
+    impl Bar<T> { type Assoc = u32; }
+    trait Foo { fn foo(&self, bar: Bar<u32>::Assoc); }
+}
+struct S;
+impl foo::Foo for S { <|> }",
+            "
+mod foo {
+    pub struct Bar<T>;
+    impl Bar<T> { type Assoc = u32; }
+    trait Foo { fn foo(&self, bar: Bar<u32>::Assoc); }
+}
+struct S;
+impl foo::Foo for S {
+    <|>fn foo(&self, bar: foo::Bar<u32>::Assoc) { unimplemented!() }
+}",
+        );
+    }
+
+    #[test]
+    fn test_qualify_path_nested() {
+        check_assist(
+            add_missing_impl_members,
+            "
+mod foo {
+    pub struct Bar<T>;
+    pub struct Baz;
+    trait Foo { fn foo(&self, bar: Bar<Baz>); }
+}
+struct S;
+impl foo::Foo for S { <|> }",
+            "
+mod foo {
+    pub struct Bar<T>;
+    pub struct Baz;
+    trait Foo { fn foo(&self, bar: Bar<Baz>); }
+}
+struct S;
+impl foo::Foo for S {
+    <|>fn foo(&self, bar: foo::Bar<foo::Baz>) { unimplemented!() }
+}",
+        );
+    }
+
+    #[test]
+    fn test_qualify_path_fn_trait_notation() {
+        check_assist(
+            add_missing_impl_members,
+            "
+mod foo {
+    pub trait Fn<Args> { type Output; }
+    trait Foo { fn foo(&self, bar: dyn Fn(u32) -> i32); }
+}
+struct S;
+impl foo::Foo for S { <|> }",
+            "
+mod foo {
+    pub trait Fn<Args> { type Output; }
+    trait Foo { fn foo(&self, bar: dyn Fn(u32) -> i32); }
+}
+struct S;
+impl foo::Foo for S {
+    <|>fn foo(&self, bar: dyn Fn(u32) -> i32) { unimplemented!() }
+}",
+        );
+    }
+
+    #[test]
     fn test_empty_trait() {
         check_assist_not_applicable(
             add_missing_impl_members,
diff --git a/crates/ra_assists/src/ast_transform.rs b/crates/ra_assists/src/ast_transform.rs
new file mode 100644
index 000000000..cbddc50ac
--- /dev/null
+++ b/crates/ra_assists/src/ast_transform.rs
@@ -0,0 +1,179 @@
+//! `AstTransformer`s are functions that replace nodes in an AST and can be easily combined.
+use rustc_hash::FxHashMap;
+
+use hir::{db::HirDatabase, InFile, PathResolution};
+use ra_syntax::ast::{self, make, AstNode};
+
+pub trait AstTransform<'a> {
+    fn get_substitution(
+        &self,
+        node: InFile<&ra_syntax::SyntaxNode>,
+    ) -> Option<ra_syntax::SyntaxNode>;
+
+    fn chain_before(self, other: Box<dyn AstTransform<'a> + 'a>) -> Box<dyn AstTransform<'a> + 'a>;
+    fn or<T: AstTransform<'a> + 'a>(self, other: T) -> Box<dyn AstTransform<'a> + 'a>
+    where
+        Self: Sized + 'a,
+    {
+        self.chain_before(Box::new(other))
+    }
+}
+
+struct NullTransformer;
+
+impl<'a> AstTransform<'a> for NullTransformer {
+    fn get_substitution(
+        &self,
+        _node: InFile<&ra_syntax::SyntaxNode>,
+    ) -> Option<ra_syntax::SyntaxNode> {
+        None
+    }
+    fn chain_before(self, other: Box<dyn AstTransform<'a> + 'a>) -> Box<dyn AstTransform<'a> + 'a> {
+        other
+    }
+}
+
+pub struct SubstituteTypeParams<'a, DB: HirDatabase> {
+    db: &'a DB,
+    substs: FxHashMap<hir::TypeParam, ast::TypeRef>,
+    previous: Box<dyn AstTransform<'a> + 'a>,
+}
+
+impl<'a, DB: HirDatabase> SubstituteTypeParams<'a, DB> {
+    pub fn for_trait_impl(
+        db: &'a DB,
+        trait_: hir::Trait,
+        impl_block: ast::ImplBlock,
+    ) -> SubstituteTypeParams<'a, DB> {
+        let substs = get_syntactic_substs(impl_block).unwrap_or_default();
+        let generic_def: hir::GenericDef = trait_.into();
+        let substs_by_param: FxHashMap<_, _> = generic_def
+            .params(db)
+            .into_iter()
+            // this is a trait impl, so we need to skip the first type parameter -- this is a bit hacky
+            .skip(1)
+            .zip(substs.into_iter())
+            .collect();
+        return SubstituteTypeParams {
+            db,
+            substs: substs_by_param,
+            previous: Box::new(NullTransformer),
+        };
+
+        // FIXME: It would probably be nicer if we could get this via HIR (i.e. get the
+        // trait ref, and then go from the types in the substs back to the syntax)
+        fn get_syntactic_substs(impl_block: ast::ImplBlock) -> Option<Vec<ast::TypeRef>> {
+            let target_trait = impl_block.target_trait()?;
+            let path_type = match target_trait {
+                ast::TypeRef::PathType(path) => path,
+                _ => return None,
+            };
+            let type_arg_list = path_type.path()?.segment()?.type_arg_list()?;
+            let mut result = Vec::new();
+            for type_arg in type_arg_list.type_args() {
+                let type_arg: ast::TypeArg = type_arg;
+                result.push(type_arg.type_ref()?);
+            }
+            Some(result)
+        }
+    }
+    fn get_substitution_inner(
+        &self,
+        node: InFile<&ra_syntax::SyntaxNode>,
+    ) -> Option<ra_syntax::SyntaxNode> {
+        let type_ref = ast::TypeRef::cast(node.value.clone())?;
+        let path = match &type_ref {
+            ast::TypeRef::PathType(path_type) => path_type.path()?,
+            _ => return None,
+        };
+        let analyzer = hir::SourceAnalyzer::new(self.db, node, None);
+        let resolution = analyzer.resolve_path(self.db, &path)?;
+        match resolution {
+            hir::PathResolution::TypeParam(tp) => Some(self.substs.get(&tp)?.syntax().clone()),
+            _ => None,
+        }
+    }
+}
+
+impl<'a, DB: HirDatabase> AstTransform<'a> for SubstituteTypeParams<'a, DB> {
+    fn get_substitution(
+        &self,
+        node: InFile<&ra_syntax::SyntaxNode>,
+    ) -> Option<ra_syntax::SyntaxNode> {
+        self.get_substitution_inner(node).or_else(|| self.previous.get_substitution(node))
+    }
+    fn chain_before(self, other: Box<dyn AstTransform<'a> + 'a>) -> Box<dyn AstTransform<'a> + 'a> {
+        Box::new(SubstituteTypeParams { previous: other, ..self })
+    }
+}
+
+pub struct QualifyPaths<'a, DB: HirDatabase> {
+    db: &'a DB,
+    from: Option<hir::Module>,
+    previous: Box<dyn AstTransform<'a> + 'a>,
+}
+
+impl<'a, DB: HirDatabase> QualifyPaths<'a, DB> {
+    pub fn new(db: &'a DB, from: Option<hir::Module>) -> Self {
+        Self { db, from, previous: Box::new(NullTransformer) }
+    }
+
+    fn get_substitution_inner(
+        &self,
+        node: InFile<&ra_syntax::SyntaxNode>,
+    ) -> Option<ra_syntax::SyntaxNode> {
+        // FIXME handle value ns?
+        let from = self.from?;
+        let p = ast::Path::cast(node.value.clone())?;
+        if p.segment().and_then(|s| s.param_list()).is_some() {
+            // don't try to qualify `Fn(Foo) -> Bar` paths, they are in prelude anyway
+            return None;
+        }
+        let analyzer = hir::SourceAnalyzer::new(self.db, node, None);
+        let resolution = analyzer.resolve_path(self.db, &p)?;
+        match resolution {
+            PathResolution::Def(def) => {
+                let found_path = from.find_use_path(self.db, def)?;
+                let args = p
+                    .segment()
+                    .and_then(|s| s.type_arg_list())
+                    .map(|arg_list| apply(self, node.with_value(arg_list)));
+                Some(make::path_with_type_arg_list(path_to_ast(found_path), args).syntax().clone())
+            }
+            PathResolution::Local(_)
+            | PathResolution::TypeParam(_)
+            | PathResolution::SelfType(_) => None,
+            PathResolution::Macro(_) => None,
+            PathResolution::AssocItem(_) => None,
+        }
+    }
+}
+
+pub fn apply<'a, N: AstNode>(transformer: &dyn AstTransform<'a>, node: InFile<N>) -> N {
+    let syntax = node.value.syntax();
+    let result = ra_syntax::algo::replace_descendants(syntax, &|element| match element {
+        ra_syntax::SyntaxElement::Node(n) => {
+            let replacement = transformer.get_substitution(node.with_value(&n))?;
+            Some(replacement.into())
+        }
+        _ => None,
+    });
+    N::cast(result).unwrap()
+}
+
+impl<'a, DB: HirDatabase> AstTransform<'a> for QualifyPaths<'a, DB> {
+    fn get_substitution(
+        &self,
+        node: InFile<&ra_syntax::SyntaxNode>,
+    ) -> Option<ra_syntax::SyntaxNode> {
+        self.get_substitution_inner(node).or_else(|| self.previous.get_substitution(node))
+    }
+    fn chain_before(self, other: Box<dyn AstTransform<'a> + 'a>) -> Box<dyn AstTransform<'a> + 'a> {
+        Box::new(QualifyPaths { previous: other, ..self })
+    }
+}
+
+fn path_to_ast(path: hir::ModPath) -> ast::Path {
+    let parse = ast::SourceFile::parse(&path.to_string());
+    parse.tree().syntax().descendants().find_map(ast::Path::cast).unwrap()
+}
diff --git a/crates/ra_assists/src/lib.rs b/crates/ra_assists/src/lib.rs
index 98fb20b22..712ff6f6a 100644
--- a/crates/ra_assists/src/lib.rs
+++ b/crates/ra_assists/src/lib.rs
@@ -11,6 +11,7 @@ mod marks;
 mod doc_tests;
 #[cfg(test)]
 mod test_db;
+pub mod ast_transform;
 
 use hir::db::HirDatabase;
 use ra_db::FileRange;
diff --git a/crates/ra_hir/src/code_model.rs b/crates/ra_hir/src/code_model.rs
index cc42068a1..df9c151e5 100644
--- a/crates/ra_hir/src/code_model.rs
+++ b/crates/ra_hir/src/code_model.rs
@@ -227,6 +227,21 @@ impl Module {
     pub(crate) fn with_module_id(self, module_id: LocalModuleId) -> Module {
         Module::new(self.krate(), module_id)
     }
+
+    /// Finds a path that can be used to refer to the given item from within
+    /// this module, if possible.
+    pub fn find_use_path(
+        self,
+        db: &impl DefDatabase,
+        item: ModuleDef,
+    ) -> Option<hir_def::path::ModPath> {
+        // FIXME expose namespace choice
+        hir_def::find_path::find_path(
+            db,
+            hir_def::item_scope::ItemInNs::Types(item.into()),
+            self.into(),
+        )
+    }
 }
 
 #[derive(Debug, Clone, Copy, PartialEq, Eq, Hash)]
diff --git a/crates/ra_hir/src/from_id.rs b/crates/ra_hir/src/from_id.rs
index 75a1a7772..c16c17072 100644
--- a/crates/ra_hir/src/from_id.rs
+++ b/crates/ra_hir/src/from_id.rs
@@ -91,6 +91,22 @@ impl From<ModuleDefId> for ModuleDef {
     }
 }
 
+impl From<ModuleDef> for ModuleDefId {
+    fn from(id: ModuleDef) -> Self {
+        match id {
+            ModuleDef::Module(it) => ModuleDefId::ModuleId(it.into()),
+            ModuleDef::Function(it) => ModuleDefId::FunctionId(it.into()),
+            ModuleDef::Adt(it) => ModuleDefId::AdtId(it.into()),
+            ModuleDef::EnumVariant(it) => ModuleDefId::EnumVariantId(it.into()),
+            ModuleDef::Const(it) => ModuleDefId::ConstId(it.into()),
+            ModuleDef::Static(it) => ModuleDefId::StaticId(it.into()),
+            ModuleDef::Trait(it) => ModuleDefId::TraitId(it.into()),
+            ModuleDef::TypeAlias(it) => ModuleDefId::TypeAliasId(it.into()),
+            ModuleDef::BuiltinType(it) => ModuleDefId::BuiltinType(it),
+        }
+    }
+}
+
 impl From<DefWithBody> for DefWithBodyId {
     fn from(def: DefWithBody) -> Self {
         match def {
diff --git a/crates/ra_hir/src/source_binder.rs b/crates/ra_hir/src/source_binder.rs
index 2c422af8b..a2a9d968c 100644
--- a/crates/ra_hir/src/source_binder.rs
+++ b/crates/ra_hir/src/source_binder.rs
@@ -205,6 +205,10 @@ impl SourceAnalyzer {
         }
     }
 
+    pub fn module(&self) -> Option<crate::code_model::Module> {
+        Some(crate::code_model::Module { id: self.resolver.module()? })
+    }
+
     fn expr_id(&self, expr: &ast::Expr) -> Option<ExprId> {
         let src = InFile { file_id: self.file_id, value: expr };
         self.body_source_map.as_ref()?.node_expr(src)
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();
diff --git a/crates/ra_hir_expand/src/lib.rs b/crates/ra_hir_expand/src/lib.rs
index 2fa5d5140..51c5f9623 100644
--- a/crates/ra_hir_expand/src/lib.rs
+++ b/crates/ra_hir_expand/src/lib.rs
@@ -322,3 +322,13 @@ impl InFile<SyntaxNode> {
         })
     }
 }
+
+impl<N: AstNode> InFile<N> {
+    pub fn descendants<T: AstNode>(self) -> impl Iterator<Item = InFile<T>> {
+        self.value.syntax().descendants().filter_map(T::cast).map(move |n| self.with_value(n))
+    }
+
+    pub fn syntax(&self) -> InFile<&SyntaxNode> {
+        self.with_value(self.value.syntax())
+    }
+}
diff --git a/crates/ra_ide/src/expand_macro.rs b/crates/ra_ide/src/expand_macro.rs
index bdbc31704..0f7b6e875 100644
--- a/crates/ra_ide/src/expand_macro.rs
+++ b/crates/ra_ide/src/expand_macro.rs
@@ -7,8 +7,7 @@ use rustc_hash::FxHashMap;
 
 use ra_syntax::{
     algo::{find_node_at_offset, replace_descendants},
-    ast::{self},
-    AstNode, NodeOrToken, SyntaxKind, SyntaxNode, WalkEvent, T,
+    ast, AstNode, NodeOrToken, SyntaxElement, SyntaxKind, SyntaxNode, WalkEvent, T,
 };
 
 pub struct ExpandedMacro {
@@ -43,7 +42,7 @@ fn expand_macro_recur(
     let mut expanded: SyntaxNode = db.parse_or_expand(macro_file_id)?;
 
     let children = expanded.descendants().filter_map(ast::MacroCall::cast);
-    let mut replaces = FxHashMap::default();
+    let mut replaces: FxHashMap<SyntaxElement, SyntaxElement> = FxHashMap::default();
 
     for child in children.into_iter() {
         let node = hir::InFile::new(macro_file_id, &child);
@@ -59,7 +58,7 @@ fn expand_macro_recur(
         }
     }
 
-    Some(replace_descendants(&expanded, &replaces))
+    Some(replace_descendants(&expanded, &|n| replaces.get(n).cloned()))
 }
 
 // FIXME: It would also be cool to share logic here and in the mbe tests,
diff --git a/crates/ra_syntax/src/algo.rs b/crates/ra_syntax/src/algo.rs
index 2b2b295f9..30a479f01 100644
--- a/crates/ra_syntax/src/algo.rs
+++ b/crates/ra_syntax/src/algo.rs
@@ -184,17 +184,17 @@ pub fn replace_children(
 /// to create a type-safe abstraction on top of it instead.
 pub fn replace_descendants(
     parent: &SyntaxNode,
-    map: &FxHashMap<SyntaxElement, SyntaxElement>,
+    map: &impl Fn(&SyntaxElement) -> Option<SyntaxElement>,
 ) -> SyntaxNode {
     //  FIXME: this could be made much faster.
     let new_children = parent.children_with_tokens().map(|it| go(map, it)).collect::<Vec<_>>();
     return with_children(parent, new_children);
 
     fn go(
-        map: &FxHashMap<SyntaxElement, SyntaxElement>,
+        map: &impl Fn(&SyntaxElement) -> Option<SyntaxElement>,
         element: SyntaxElement,
     ) -> NodeOrToken<rowan::GreenNode, rowan::GreenToken> {
-        if let Some(replacement) = map.get(&element) {
+        if let Some(replacement) = map(&element) {
             return match replacement {
                 NodeOrToken::Node(it) => NodeOrToken::Node(it.green().clone()),
                 NodeOrToken::Token(it) => NodeOrToken::Token(it.green().clone()),
diff --git a/crates/ra_syntax/src/ast/edit.rs b/crates/ra_syntax/src/ast/edit.rs
index ae5d63927..b736098ac 100644
--- a/crates/ra_syntax/src/ast/edit.rs
+++ b/crates/ra_syntax/src/ast/edit.rs
@@ -236,8 +236,8 @@ pub fn replace_descendants<N: AstNode, D: AstNode>(
 ) -> N {
     let map = replacement_map
         .map(|(from, to)| (from.syntax().clone().into(), to.syntax().clone().into()))
-        .collect::<FxHashMap<_, _>>();
-    let new_syntax = algo::replace_descendants(parent.syntax(), &map);
+        .collect::<FxHashMap<SyntaxElement, _>>();
+    let new_syntax = algo::replace_descendants(parent.syntax(), &|n| map.get(n).cloned());
     N::cast(new_syntax).unwrap()
 }
 
@@ -292,7 +292,7 @@ impl IndentLevel {
                 )
             })
             .collect();
-        algo::replace_descendants(&node, &replacements)
+        algo::replace_descendants(&node, &|n| replacements.get(n).cloned())
     }
 
     pub fn decrease_indent<N: AstNode>(self, node: N) -> N {
@@ -320,7 +320,7 @@ impl IndentLevel {
                 )
             })
             .collect();
-        algo::replace_descendants(&node, &replacements)
+        algo::replace_descendants(&node, &|n| replacements.get(n).cloned())
     }
 }
 
diff --git a/crates/ra_syntax/src/ast/make.rs b/crates/ra_syntax/src/ast/make.rs
index 04a5408fe..9781b748f 100644
--- a/crates/ra_syntax/src/ast/make.rs
+++ b/crates/ra_syntax/src/ast/make.rs
@@ -2,7 +2,7 @@
 //! of smaller pieces.
 use itertools::Itertools;
 
-use crate::{ast, AstNode, SourceFile};
+use crate::{algo, ast, AstNode, SourceFile};
 
 pub fn name(text: &str) -> ast::Name {
     ast_from_text(&format!("mod {};", text))
@@ -21,6 +21,20 @@ pub fn path_qualified(qual: ast::Path, name_ref: ast::NameRef) -> ast::Path {
 fn path_from_text(text: &str) -> ast::Path {
     ast_from_text(text)
 }
+pub fn path_with_type_arg_list(path: ast::Path, args: Option<ast::TypeArgList>) -> ast::Path {
+    if let Some(args) = args {
+        let syntax = path.syntax();
+        // FIXME: remove existing type args
+        let new_syntax = algo::insert_children(
+            syntax,
+            crate::algo::InsertPosition::Last,
+            &mut Some(args).into_iter().map(|n| n.syntax().clone().into()),
+        );
+        ast::Path::cast(new_syntax).unwrap()
+    } else {
+        path
+    }
+}
 
 pub fn record_field(name: ast::NameRef, expr: Option<ast::Expr>) -> ast::RecordField {
     return match expr {