8 files changed, 409 insertions, 247 deletions

diff --git a/crates/assists/src/handlers/add_turbo_fish.rs b/crates/assists/src/handlers/add_turbo_fish.rs
index f4f997d8e..e3d84d698 100644
--- a/crates/assists/src/handlers/add_turbo_fish.rs
+++ b/crates/assists/src/handlers/add_turbo_fish.rs
@@ -1,4 +1,4 @@
-use ide_db::defs::{classify_name_ref, Definition, NameRefClass};
+use ide_db::defs::{Definition, NameRefClass};
 use syntax::{ast, AstNode, SyntaxKind, T};
 use test_utils::mark;
 
@@ -39,7 +39,7 @@ pub(crate) fn add_turbo_fish(acc: &mut Assists, ctx: &AssistContext) -> Option<(
         return None;
     }
     let name_ref = ast::NameRef::cast(ident.parent())?;
-    let def = match classify_name_ref(&ctx.sema, &name_ref)? {
+    let def = match NameRefClass::classify(&ctx.sema, &name_ref)? {
         NameRefClass::Definition(def) => def,
         NameRefClass::ExternCrate(_) | NameRefClass::FieldShorthand { .. } => return None,
     };
diff --git a/crates/assists/src/handlers/auto_import.rs b/crates/assists/src/handlers/auto_import.rs
index d3ee98e5f..4a7059c83 100644
--- a/crates/assists/src/handlers/auto_import.rs
+++ b/crates/assists/src/handlers/auto_import.rs
@@ -1,23 +1,66 @@
-use std::collections::BTreeSet;
-
-use either::Either;
-use hir::{
-    AsAssocItem, AssocItemContainer, ModPath, Module, ModuleDef, PathResolution, Semantics, Trait,
-    Type,
-};
-use ide_db::{imports_locator, RootDatabase};
-use insert_use::ImportScope;
-use rustc_hash::FxHashSet;
-use syntax::{
-    ast::{self, AstNode},
-    SyntaxNode,
-};
+use syntax::ast;
 
 use crate::{
-    utils::insert_use, utils::mod_path_to_ast, AssistContext, AssistId, AssistKind, Assists,
-    GroupLabel,
+    utils::import_assets::{ImportAssets, ImportCandidate},
+    utils::{insert_use, mod_path_to_ast, ImportScope},
+    AssistContext, AssistId, AssistKind, Assists, GroupLabel,
 };
 
+// Feature: Import Insertion
+//
+// Using the `auto-import` assist it is possible to insert missing imports for unresolved items.
+// When inserting an import it will do so in a structured manner by keeping imports grouped,
+// separated by a newline in the following order:
+//
+// - `std` and `core`
+// - External Crates
+// - Current Crate, paths prefixed by `crate`
+// - Current Module, paths prefixed by `self`
+// - Super Module, paths prefixed by `super`
+//
+// Example:
+// ```rust
+// use std::fs::File;
+//
+// use itertools::Itertools;
+// use syntax::ast;
+//
+// use crate::utils::insert_use;
+//
+// use self::auto_import;
+//
+// use super::AssistContext;
+// ```
+//
+// .Merge Behaviour
+//
+// It is possible to configure how use-trees are merged with the `importMergeBehaviour` setting.
+// It has the following configurations:
+//
+// - `full`: This setting will cause auto-import to always completely merge use-trees that share the
+//  same path prefix while also merging inner trees that share the same path-prefix. This kind of
+//  nesting is only supported in Rust versions later than 1.24.
+// - `last`: This setting will cause auto-import to merge use-trees as long as the resulting tree
+//  will only contain a nesting of single segment paths at the very end.
+// - `none`: This setting will cause auto-import to never merge use-trees keeping them as simple
+//  paths.
+//
+// In `VS Code` the configuration for this is `rust-analyzer.assist.importMergeBehaviour`.
+//
+// .Import Prefix
+//
+// The style of imports in the same crate is configurable through the `importPrefix` setting.
+// It has the following configurations:
+//
+// - `by_crate`: This setting will force paths to be always absolute, starting with the `crate`
+//  prefix, unless the item is defined outside of the current crate.
+// - `by_self`: This setting will force paths that are relative to the current module to always
+//  start with `self`. This will result in paths that always start with either `crate`, `self`,
+//  `super` or an extern crate identifier.
+// - `plain`: This setting does not impose any restrictions in imports.
+//
+// In `VS Code` the configuration for this is `rust-analyzer.assist.importPrefix`.
+
 // Assist: auto_import
 //
 // If the name is unresolved, provides all possible imports for it.
@@ -38,16 +81,24 @@ use crate::{
 // # pub mod std { pub mod collections { pub struct HashMap { } } }
 // ```
 pub(crate) fn auto_import(acc: &mut Assists, ctx: &AssistContext) -> Option<()> {
-    let auto_import_assets = AutoImportAssets::new(ctx)?;
-    let proposed_imports = auto_import_assets.search_for_imports(ctx);
+    let import_assets =
+        if let Some(path_under_caret) = ctx.find_node_at_offset_with_descend::<ast::Path>() {
+            ImportAssets::for_regular_path(path_under_caret, &ctx.sema)
+        } else if let Some(method_under_caret) =
+            ctx.find_node_at_offset_with_descend::<ast::MethodCallExpr>()
+        {
+            ImportAssets::for_method_call(method_under_caret, &ctx.sema)
+        } else {
+            None
+        }?;
+    let proposed_imports = import_assets.search_for_imports(&ctx.sema, &ctx.config.insert_use);
     if proposed_imports.is_empty() {
         return None;
     }
 
-    let range = ctx.sema.original_range(&auto_import_assets.syntax_under_caret).range;
-    let group = auto_import_assets.get_import_group_message();
-    let scope =
-        ImportScope::find_insert_use_container(&auto_import_assets.syntax_under_caret, ctx)?;
+    let range = ctx.sema.original_range(import_assets.syntax_under_caret()).range;
+    let group = import_group_message(import_assets.import_candidate());
+    let scope = ImportScope::find_insert_use_container(import_assets.syntax_under_caret(), ctx)?;
     let syntax = scope.as_syntax_node();
     for import in proposed_imports {
         acc.add_group(
@@ -65,227 +116,18 @@ pub(crate) fn auto_import(acc: &mut Assists, ctx: &AssistContext) -> Option<()>
     Some(())
 }
 
-#[derive(Debug)]
-struct AutoImportAssets {
-    import_candidate: ImportCandidate,
-    module_with_name_to_import: Module,
-    syntax_under_caret: SyntaxNode,
-}
-
-impl AutoImportAssets {
-    fn new(ctx: &AssistContext) -> Option<Self> {
-        if let Some(path_under_caret) = ctx.find_node_at_offset_with_descend::<ast::Path>() {
-            Self::for_regular_path(path_under_caret, &ctx)
-        } else {
-            Self::for_method_call(ctx.find_node_at_offset_with_descend()?, &ctx)
+fn import_group_message(import_candidate: &ImportCandidate) -> GroupLabel {
+    let name = match import_candidate {
+        ImportCandidate::UnqualifiedName(candidate)
+        | ImportCandidate::QualifierStart(candidate) => format!("Import {}", &candidate.name),
+        ImportCandidate::TraitAssocItem(candidate) => {
+            format!("Import a trait for item {}", &candidate.name)
         }
-    }
-
-    fn for_method_call(method_call: ast::MethodCallExpr, ctx: &AssistContext) -> Option<Self> {
-        let syntax_under_caret = method_call.syntax().to_owned();
-        let module_with_name_to_import = ctx.sema.scope(&syntax_under_caret).module()?;
-        Some(Self {
-            import_candidate: ImportCandidate::for_method_call(&ctx.sema, &method_call)?,
-            module_with_name_to_import,
-            syntax_under_caret,
-        })
-    }
-
-    fn for_regular_path(path_under_caret: ast::Path, ctx: &AssistContext) -> Option<Self> {
-        let syntax_under_caret = path_under_caret.syntax().to_owned();
-        if syntax_under_caret.ancestors().find_map(ast::Use::cast).is_some() {
-            return None;
+        ImportCandidate::TraitMethod(candidate) => {
+            format!("Import a trait for method {}", &candidate.name)
         }
-
-        let module_with_name_to_import = ctx.sema.scope(&syntax_under_caret).module()?;
-        Some(Self {
-            import_candidate: ImportCandidate::for_regular_path(&ctx.sema, &path_under_caret)?,
-            module_with_name_to_import,
-            syntax_under_caret,
-        })
-    }
-
-    fn get_search_query(&self) -> &str {
-        match &self.import_candidate {
-            ImportCandidate::UnqualifiedName(name) => name,
-            ImportCandidate::QualifierStart(qualifier_start) => qualifier_start,
-            ImportCandidate::TraitAssocItem(_, trait_assoc_item_name) => trait_assoc_item_name,
-            ImportCandidate::TraitMethod(_, trait_method_name) => trait_method_name,
-        }
-    }
-
-    fn get_import_group_message(&self) -> GroupLabel {
-        let name = match &self.import_candidate {
-            ImportCandidate::UnqualifiedName(name) => format!("Import {}", name),
-            ImportCandidate::QualifierStart(qualifier_start) => {
-                format!("Import {}", qualifier_start)
-            }
-            ImportCandidate::TraitAssocItem(_, trait_assoc_item_name) => {
-                format!("Import a trait for item {}", trait_assoc_item_name)
-            }
-            ImportCandidate::TraitMethod(_, trait_method_name) => {
-                format!("Import a trait for method {}", trait_method_name)
-            }
-        };
-        GroupLabel(name)
-    }
-
-    fn search_for_imports(&self, ctx: &AssistContext) -> BTreeSet<ModPath> {
-        let _p = profile::span("auto_import::search_for_imports");
-        let db = ctx.db();
-        let current_crate = self.module_with_name_to_import.krate();
-        imports_locator::find_imports(&ctx.sema, current_crate, &self.get_search_query())
-            .into_iter()
-            .filter_map(|candidate| match &self.import_candidate {
-                ImportCandidate::TraitAssocItem(assoc_item_type, _) => {
-                    let located_assoc_item = match candidate {
-                        Either::Left(ModuleDef::Function(located_function)) => located_function
-                            .as_assoc_item(db)
-                            .map(|assoc| assoc.container(db))
-                            .and_then(Self::assoc_to_trait),
-                        Either::Left(ModuleDef::Const(located_const)) => located_const
-                            .as_assoc_item(db)
-                            .map(|assoc| assoc.container(db))
-                            .and_then(Self::assoc_to_trait),
-                        _ => None,
-                    }?;
-
-                    let mut trait_candidates = FxHashSet::default();
-                    trait_candidates.insert(located_assoc_item.into());
-
-                    assoc_item_type
-                        .iterate_path_candidates(
-                            db,
-                            current_crate,
-                            &trait_candidates,
-                            None,
-                            |_, assoc| Self::assoc_to_trait(assoc.container(db)),
-                        )
-                        .map(ModuleDef::from)
-                        .map(Either::Left)
-                }
-                ImportCandidate::TraitMethod(function_callee, _) => {
-                    let located_assoc_item =
-                        if let Either::Left(ModuleDef::Function(located_function)) = candidate {
-                            located_function
-                                .as_assoc_item(db)
-                                .map(|assoc| assoc.container(db))
-                                .and_then(Self::assoc_to_trait)
-                        } else {
-                            None
-                        }?;
-
-                    let mut trait_candidates = FxHashSet::default();
-                    trait_candidates.insert(located_assoc_item.into());
-
-                    function_callee
-                        .iterate_method_candidates(
-                            db,
-                            current_crate,
-                            &trait_candidates,
-                            None,
-                            |_, function| {
-                                Self::assoc_to_trait(function.as_assoc_item(db)?.container(db))
-                            },
-                        )
-                        .map(ModuleDef::from)
-                        .map(Either::Left)
-                }
-                _ => Some(candidate),
-            })
-            .filter_map(|candidate| match candidate {
-                Either::Left(module_def) => self.module_with_name_to_import.find_use_path_prefixed(
-                    db,
-                    module_def,
-                    ctx.config.insert_use.prefix_kind,
-                ),
-                Either::Right(macro_def) => self.module_with_name_to_import.find_use_path_prefixed(
-                    db,
-                    macro_def,
-                    ctx.config.insert_use.prefix_kind,
-                ),
-            })
-            .filter(|use_path| !use_path.segments.is_empty())
-            .take(20)
-            .collect::<BTreeSet<_>>()
-    }
-
-    fn assoc_to_trait(assoc: AssocItemContainer) -> Option<Trait> {
-        if let AssocItemContainer::Trait(extracted_trait) = assoc {
-            Some(extracted_trait)
-        } else {
-            None
-        }
-    }
-}
-
-#[derive(Debug)]
-enum ImportCandidate {
-    /// Simple name like 'HashMap'
-    UnqualifiedName(String),
-    /// First part of the qualified name.
-    /// For 'std::collections::HashMap', that will be 'std'.
-    QualifierStart(String),
-    /// A trait associated function (with no self parameter) or associated constant.
-    /// For 'test_mod::TestEnum::test_function', `Type` is the `test_mod::TestEnum` expression type
-    /// and `String` is the `test_function`
-    TraitAssocItem(Type, String),
-    /// A trait method with self parameter.
-    /// For 'test_enum.test_method()', `Type` is the `test_enum` expression type
-    /// and `String` is the `test_method`
-    TraitMethod(Type, String),
-}
-
-impl ImportCandidate {
-    fn for_method_call(
-        sema: &Semantics<RootDatabase>,
-        method_call: &ast::MethodCallExpr,
-    ) -> Option<Self> {
-        if sema.resolve_method_call(method_call).is_some() {
-            return None;
-        }
-        Some(Self::TraitMethod(
-            sema.type_of_expr(&method_call.receiver()?)?,
-            method_call.name_ref()?.syntax().to_string(),
-        ))
-    }
-
-    fn for_regular_path(
-        sema: &Semantics<RootDatabase>,
-        path_under_caret: &ast::Path,
-    ) -> Option<Self> {
-        if sema.resolve_path(path_under_caret).is_some() {
-            return None;
-        }
-
-        let segment = path_under_caret.segment()?;
-        if let Some(qualifier) = path_under_caret.qualifier() {
-            let qualifier_start = qualifier.syntax().descendants().find_map(ast::NameRef::cast)?;
-            let qualifier_start_path =
-                qualifier_start.syntax().ancestors().find_map(ast::Path::cast)?;
-            if let Some(qualifier_start_resolution) = sema.resolve_path(&qualifier_start_path) {
-                let qualifier_resolution = if qualifier_start_path == qualifier {
-                    qualifier_start_resolution
-                } else {
-                    sema.resolve_path(&qualifier)?
-                };
-                if let PathResolution::Def(ModuleDef::Adt(assoc_item_path)) = qualifier_resolution {
-                    Some(ImportCandidate::TraitAssocItem(
-                        assoc_item_path.ty(sema.db),
-                        segment.syntax().to_string(),
-                    ))
-                } else {
-                    None
-                }
-            } else {
-                Some(ImportCandidate::QualifierStart(qualifier_start.syntax().to_string()))
-            }
-        } else {
-            Some(ImportCandidate::UnqualifiedName(
-                segment.syntax().descendants().find_map(ast::NameRef::cast)?.syntax().to_string(),
-            ))
-        }
-    }
+    };
+    GroupLabel(name)
 }
 
 #[cfg(test)]
diff --git a/crates/assists/src/handlers/expand_glob_import.rs b/crates/assists/src/handlers/expand_glob_import.rs
index d1adff972..316a58d88 100644
--- a/crates/assists/src/handlers/expand_glob_import.rs
+++ b/crates/assists/src/handlers/expand_glob_import.rs
@@ -1,7 +1,7 @@
 use either::Either;
 use hir::{AssocItem, MacroDef, Module, ModuleDef, Name, PathResolution, ScopeDef};
 use ide_db::{
-    defs::{classify_name_ref, Definition, NameRefClass},
+    defs::{Definition, NameRefClass},
     search::SearchScope,
 };
 use syntax::{
@@ -217,7 +217,7 @@ fn find_imported_defs(ctx: &AssistContext, star: SyntaxToken) -> Option<Vec<Def>
             .flatten()
             .filter_map(|n| Some(n.descendants().filter_map(ast::NameRef::cast)))
             .flatten()
-            .filter_map(|r| match classify_name_ref(&ctx.sema, &r)? {
+            .filter_map(|r| match NameRefClass::classify(&ctx.sema, &r)? {
                 NameRefClass::Definition(Definition::ModuleDef(def)) => Some(Def::ModuleDef(def)),
                 NameRefClass::Definition(Definition::Macro(def)) => Some(Def::MacroDef(def)),
                 _ => None,
diff --git a/crates/assists/src/handlers/merge_imports.rs b/crates/assists/src/handlers/merge_imports.rs
index fe33cee53..fd9c9e03c 100644
--- a/crates/assists/src/handlers/merge_imports.rs
+++ b/crates/assists/src/handlers/merge_imports.rs
@@ -73,6 +73,20 @@ mod tests {
     use super::*;
 
     #[test]
+    fn test_merge_equal() {
+        check_assist(
+            merge_imports,
+            r"
+use std::fmt<|>::{Display, Debug};
+use std::fmt::{Display, Debug};
+",
+            r"
+use std::fmt::{Debug, Display};
+",
+        )
+    }
+
+    #[test]
     fn test_merge_first() {
         check_assist(
             merge_imports,
diff --git a/crates/assists/src/handlers/replace_qualified_name_with_use.rs b/crates/assists/src/handlers/replace_qualified_name_with_use.rs
index e95b971a4..c50bc7604 100644
--- a/crates/assists/src/handlers/replace_qualified_name_with_use.rs
+++ b/crates/assists/src/handlers/replace_qualified_name_with_use.rs
@@ -124,6 +124,23 @@ mod tests {
     use super::*;
 
     #[test]
+    fn test_replace_already_imported() {
+        check_assist(
+            replace_qualified_name_with_use,
+            r"use std::fs;
+
+fn main() {
+    std::f<|>s::Path
+}",
+            r"use std::fs;
+
+fn main() {
+    fs::Path
+}",
+        )
+    }
+
+    #[test]
     fn test_replace_add_use_no_anchor() {
         check_assist(
             replace_qualified_name_with_use,
diff --git a/crates/assists/src/utils.rs b/crates/assists/src/utils.rs
index c1847f601..b37b0d2b6 100644
--- a/crates/assists/src/utils.rs
+++ b/crates/assists/src/utils.rs
@@ -1,5 +1,6 @@
 //! Assorted functions shared by several assists.
 pub(crate) mod insert_use;
+pub(crate) mod import_assets;
 
 use std::{iter, ops};
 
diff --git a/crates/assists/src/utils/import_assets.rs b/crates/assists/src/utils/import_assets.rs
new file mode 100644
index 000000000..601f51098
--- /dev/null
+++ b/crates/assists/src/utils/import_assets.rs
@@ -0,0 +1,268 @@
+//! Look up accessible paths for items.
+use std::collections::BTreeSet;
+
+use either::Either;
+use hir::{AsAssocItem, AssocItemContainer, ModuleDef, Semantics};
+use ide_db::{imports_locator, RootDatabase};
+use rustc_hash::FxHashSet;
+use syntax::{ast, AstNode, SyntaxNode};
+
+use crate::assist_config::InsertUseConfig;
+
+#[derive(Debug)]
+pub(crate) enum ImportCandidate {
+    /// Simple name like 'HashMap'
+    UnqualifiedName(PathImportCandidate),
+    /// First part of the qualified name.
+    /// For 'std::collections::HashMap', that will be 'std'.
+    QualifierStart(PathImportCandidate),
+    /// A trait associated function (with no self parameter) or associated constant.
+    /// For 'test_mod::TestEnum::test_function', `ty` is the `test_mod::TestEnum` expression type
+    /// and `name` is the `test_function`
+    TraitAssocItem(TraitImportCandidate),
+    /// A trait method with self parameter.
+    /// For 'test_enum.test_method()', `ty` is the `test_enum` expression type
+    /// and `name` is the `test_method`
+    TraitMethod(TraitImportCandidate),
+}
+
+#[derive(Debug)]
+pub(crate) struct TraitImportCandidate {
+    pub ty: hir::Type,
+    pub name: String,
+}
+
+#[derive(Debug)]
+pub(crate) struct PathImportCandidate {
+    pub name: String,
+}
+
+#[derive(Debug)]
+pub(crate) struct ImportAssets {
+    import_candidate: ImportCandidate,
+    module_with_name_to_import: hir::Module,
+    syntax_under_caret: SyntaxNode,
+}
+
+impl ImportAssets {
+    pub(crate) fn for_method_call(
+        method_call: ast::MethodCallExpr,
+        sema: &Semantics<RootDatabase>,
+    ) -> Option<Self> {
+        let syntax_under_caret = method_call.syntax().to_owned();
+        let module_with_name_to_import = sema.scope(&syntax_under_caret).module()?;
+        Some(Self {
+            import_candidate: ImportCandidate::for_method_call(sema, &method_call)?,
+            module_with_name_to_import,
+            syntax_under_caret,
+        })
+    }
+
+    pub(crate) fn for_regular_path(
+        path_under_caret: ast::Path,
+        sema: &Semantics<RootDatabase>,
+    ) -> Option<Self> {
+        let syntax_under_caret = path_under_caret.syntax().to_owned();
+        if syntax_under_caret.ancestors().find_map(ast::Use::cast).is_some() {
+            return None;
+        }
+
+        let module_with_name_to_import = sema.scope(&syntax_under_caret).module()?;
+        Some(Self {
+            import_candidate: ImportCandidate::for_regular_path(sema, &path_under_caret)?,
+            module_with_name_to_import,
+            syntax_under_caret,
+        })
+    }
+
+    pub(crate) fn syntax_under_caret(&self) -> &SyntaxNode {
+        &self.syntax_under_caret
+    }
+
+    pub(crate) fn import_candidate(&self) -> &ImportCandidate {
+        &self.import_candidate
+    }
+
+    fn get_search_query(&self) -> &str {
+        match &self.import_candidate {
+            ImportCandidate::UnqualifiedName(candidate)
+            | ImportCandidate::QualifierStart(candidate) => &candidate.name,
+            ImportCandidate::TraitAssocItem(candidate)
+            | ImportCandidate::TraitMethod(candidate) => &candidate.name,
+        }
+    }
+
+    pub(crate) fn search_for_imports(
+        &self,
+        sema: &Semantics<RootDatabase>,
+        config: &InsertUseConfig,
+    ) -> BTreeSet<hir::ModPath> {
+        let _p = profile::span("import_assists::search_for_imports");
+        self.search_for(sema, Some(config.prefix_kind))
+    }
+
+    /// This may return non-absolute paths if a part of the returned path is already imported into scope.
+    #[allow(dead_code)]
+    pub(crate) fn search_for_relative_paths(
+        &self,
+        sema: &Semantics<RootDatabase>,
+    ) -> BTreeSet<hir::ModPath> {
+        let _p = profile::span("import_assists::search_for_relative_paths");
+        self.search_for(sema, None)
+    }
+
+    fn search_for(
+        &self,
+        sema: &Semantics<RootDatabase>,
+        prefixed: Option<hir::PrefixKind>,
+    ) -> BTreeSet<hir::ModPath> {
+        let db = sema.db;
+        let mut trait_candidates = FxHashSet::default();
+        let current_crate = self.module_with_name_to_import.krate();
+
+        let filter = |candidate: Either<hir::ModuleDef, hir::MacroDef>| {
+            trait_candidates.clear();
+            match &self.import_candidate {
+                ImportCandidate::TraitAssocItem(trait_candidate) => {
+                    let located_assoc_item = match candidate {
+                        Either::Left(ModuleDef::Function(located_function)) => {
+                            located_function.as_assoc_item(db)
+                        }
+                        Either::Left(ModuleDef::Const(located_const)) => {
+                            located_const.as_assoc_item(db)
+                        }
+                        _ => None,
+                    }
+                    .map(|assoc| assoc.container(db))
+                    .and_then(Self::assoc_to_trait)?;
+
+                    trait_candidates.insert(located_assoc_item.into());
+
+                    trait_candidate
+                        .ty
+                        .iterate_path_candidates(
+                            db,
+                            current_crate,
+                            &trait_candidates,
+                            None,
+                            |_, assoc| Self::assoc_to_trait(assoc.container(db)),
+                        )
+                        .map(ModuleDef::from)
+                        .map(Either::Left)
+                }
+                ImportCandidate::TraitMethod(trait_candidate) => {
+                    let located_assoc_item =
+                        if let Either::Left(ModuleDef::Function(located_function)) = candidate {
+                            located_function
+                                .as_assoc_item(db)
+                                .map(|assoc| assoc.container(db))
+                                .and_then(Self::assoc_to_trait)
+                        } else {
+                            None
+                        }?;
+
+                    trait_candidates.insert(located_assoc_item.into());
+
+                    trait_candidate
+                        .ty
+                        .iterate_method_candidates(
+                            db,
+                            current_crate,
+                            &trait_candidates,
+                            None,
+                            |_, function| {
+                                Self::assoc_to_trait(function.as_assoc_item(db)?.container(db))
+                            },
+                        )
+                        .map(ModuleDef::from)
+                        .map(Either::Left)
+                }
+                _ => Some(candidate),
+            }
+        };
+
+        imports_locator::find_imports(sema, current_crate, &self.get_search_query())
+            .into_iter()
+            .filter_map(filter)
+            .filter_map(|candidate| {
+                let item: hir::ItemInNs = candidate.either(Into::into, Into::into);
+                if let Some(prefix_kind) = prefixed {
+                    self.module_with_name_to_import.find_use_path_prefixed(db, item, prefix_kind)
+                } else {
+                    self.module_with_name_to_import.find_use_path(db, item)
+                }
+            })
+            .filter(|use_path| !use_path.segments.is_empty())
+            .take(20)
+            .collect::<BTreeSet<_>>()
+    }
+
+    fn assoc_to_trait(assoc: AssocItemContainer) -> Option<hir::Trait> {
+        if let AssocItemContainer::Trait(extracted_trait) = assoc {
+            Some(extracted_trait)
+        } else {
+            None
+        }
+    }
+}
+
+impl ImportCandidate {
+    fn for_method_call(
+        sema: &Semantics<RootDatabase>,
+        method_call: &ast::MethodCallExpr,
+    ) -> Option<Self> {
+        match sema.resolve_method_call(method_call) {
+            Some(_) => None,
+            None => Some(Self::TraitMethod(TraitImportCandidate {
+                ty: sema.type_of_expr(&method_call.receiver()?)?,
+                name: method_call.name_ref()?.syntax().to_string(),
+            })),
+        }
+    }
+
+    fn for_regular_path(
+        sema: &Semantics<RootDatabase>,
+        path_under_caret: &ast::Path,
+    ) -> Option<Self> {
+        if sema.resolve_path(path_under_caret).is_some() {
+            return None;
+        }
+
+        let segment = path_under_caret.segment()?;
+        let candidate = if let Some(qualifier) = path_under_caret.qualifier() {
+            let qualifier_start = qualifier.syntax().descendants().find_map(ast::NameRef::cast)?;
+            let qualifier_start_path =
+                qualifier_start.syntax().ancestors().find_map(ast::Path::cast)?;
+            if let Some(qualifier_start_resolution) = sema.resolve_path(&qualifier_start_path) {
+                let qualifier_resolution = if qualifier_start_path == qualifier {
+                    qualifier_start_resolution
+                } else {
+                    sema.resolve_path(&qualifier)?
+                };
+                match qualifier_resolution {
+                    hir::PathResolution::Def(hir::ModuleDef::Adt(assoc_item_path)) => {
+                        ImportCandidate::TraitAssocItem(TraitImportCandidate {
+                            ty: assoc_item_path.ty(sema.db),
+                            name: segment.syntax().to_string(),
+                        })
+                    }
+                    _ => return None,
+                }
+            } else {
+                ImportCandidate::QualifierStart(PathImportCandidate {
+                    name: qualifier_start.syntax().to_string(),
+                })
+            }
+        } else {
+            ImportCandidate::UnqualifiedName(PathImportCandidate {
+                name: segment
+                    .syntax()
+                    .descendants()
+                    .find_map(ast::NameRef::cast)?
+                    .syntax()
+                    .to_string(),
+            })
+        };
+        Some(candidate)
+    }
+}
diff --git a/crates/assists/src/utils/insert_use.rs b/crates/assists/src/utils/insert_use.rs
index bfd457d18..409985b3b 100644
--- a/crates/assists/src/utils/insert_use.rs
+++ b/crates/assists/src/utils/insert_use.rs
@@ -173,8 +173,15 @@ pub(crate) fn try_merge_trees(
     let rhs_path = rhs.path()?;
 
     let (lhs_prefix, rhs_prefix) = common_prefix(&lhs_path, &rhs_path)?;
-    let lhs = lhs.split_prefix(&lhs_prefix);
-    let rhs = rhs.split_prefix(&rhs_prefix);
+    let (lhs, rhs) = if is_simple_path(lhs)
+        && is_simple_path(rhs)
+        && lhs_path == lhs_prefix
+        && rhs_path == rhs_prefix
+    {
+        (lhs.clone(), rhs.clone())
+    } else {
+        (lhs.split_prefix(&lhs_prefix), rhs.split_prefix(&rhs_prefix))
+    };
     recursive_merge(&lhs, &rhs, merge)
 }
 
@@ -250,6 +257,10 @@ fn recursive_merge(
                         use_trees.insert(idx, make::glob_use_tree());
                         continue;
                     }
+
+                    if lhs_t.use_tree_list().is_none() && rhs_t.use_tree_list().is_none() {
+                        continue;
+                    }
                 }
                 let lhs = lhs_t.split_prefix(&lhs_prefix);
                 let rhs = rhs_t.split_prefix(&rhs_prefix);
@@ -295,6 +306,10 @@ fn common_prefix(lhs: &ast::Path, rhs: &ast::Path) -> Option<(ast::Path, ast::Pa
     }
 }
 
+fn is_simple_path(use_tree: &ast::UseTree) -> bool {
+    use_tree.use_tree_list().is_none() && use_tree.star_token().is_none()
+}
+
 fn path_is_self(path: &ast::Path) -> bool {
     path.segment().and_then(|seg| seg.self_token()).is_some() && path.qualifier().is_none()
 }
@@ -524,6 +539,11 @@ mod tests {
     use test_utils::assert_eq_text;
 
     #[test]
+    fn insert_existing() {
+        check_full("std::fs", "use std::fs;", "use std::fs;")
+    }
+
+    #[test]
     fn insert_start() {
         check_none(
             "std::bar::AA",