Begin refactor of import insertion

1 files changed, 440 insertions, 468 deletions

diff --git a/crates/assists/src/utils/insert_use.rs b/crates/assists/src/utils/insert_use.rs
index 49096a67c..8ee5e0c9c 100644
--- a/crates/assists/src/utils/insert_use.rs
+++ b/crates/assists/src/utils/insert_use.rs
@@ -1,17 +1,13 @@
-//! Handle syntactic aspects of inserting a new `use`.
-// FIXME: rewrite according to the plan, outlined in
-// https://github.com/rust-analyzer/rust-analyzer/issues/3301#issuecomment-592931553
-
-use std::iter::successors;
+use std::iter::{self, successors};
 
+use algo::skip_trivia_token;
+use ast::{edit::AstNodeEdit, PathSegmentKind, VisibilityOwner};
 use either::Either;
 use syntax::{
-    ast::{self, NameOwner, VisibilityOwner},
-    AstNode, AstToken, Direction, SmolStr,
-    SyntaxKind::{PATH, PATH_SEGMENT},
-    SyntaxNode, SyntaxToken, T,
+    algo,
+    ast::{self, make, AstNode},
+    Direction, InsertPosition, SyntaxElement, SyntaxNode, T,
 };
-use text_edit::TextEditBuilder;
 
 use crate::assist_context::AssistContext;
 
@@ -22,525 +18,501 @@ pub(crate) fn find_insert_use_container(
 ) -> Option<Either<ast::ItemList, ast::SourceFile>> {
     ctx.sema.ancestors_with_macros(position.clone()).find_map(|n| {
         if let Some(module) = ast::Module::cast(n.clone()) {
-            return module.item_list().map(|it| Either::Left(it));
+            return module.item_list().map(Either::Left);
         }
         Some(Either::Right(ast::SourceFile::cast(n)?))
     })
 }
 
-/// Creates and inserts a use statement for the given path to import.
-/// The use statement is inserted in the scope most appropriate to the
-/// the cursor position given, additionally merged with the existing use imports.
 pub(crate) fn insert_use_statement(
     // Ideally the position of the cursor, used to
     position: &SyntaxNode,
     path_to_import: &str,
-    ctx: &AssistContext,
-    builder: &mut TextEditBuilder,
+    ctx: &crate::assist_context::AssistContext,
+    builder: &mut text_edit::TextEditBuilder,
 ) {
-    let target = path_to_import.split("::").map(SmolStr::new).collect::<Vec<_>>();
-    let container = find_insert_use_container(position, ctx);
+    insert_use(position.clone(), make::path_from_text(path_to_import), Some(MergeBehaviour::Full));
+}
+
+pub fn insert_use(
+    where_: SyntaxNode,
+    path: ast::Path,
+    merge_behaviour: Option<MergeBehaviour>,
+) -> SyntaxNode {
+    let use_item = make::use_(make::use_tree(path.clone(), None, None, false));
+    // merge into existing imports if possible
+    if let Some(mb) = merge_behaviour {
+        for existing_use in where_.children().filter_map(ast::Use::cast) {
+            if let Some(merged) = try_merge_imports(&existing_use, &use_item, mb) {
+                let to_delete: SyntaxElement = existing_use.syntax().clone().into();
+                let to_delete = to_delete.clone()..=to_delete;
+                let to_insert = iter::once(merged.syntax().clone().into());
+                return algo::replace_children(&where_, to_delete, to_insert);
+            }
+        }
+    }
+
+    // either we weren't allowed to merge or there is no import that fits the merge conditions
+    // so look for the place we have to insert to
+    let (insert_position, add_blank) = find_insert_position(&where_, path);
+
+    let to_insert: Vec<SyntaxElement> = {
+        let mut buf = Vec::new();
+
+        if add_blank == AddBlankLine::Before {
+            buf.push(make::tokens::single_newline().into());
+        }
+
+        buf.push(use_item.syntax().clone().into());
+
+        if add_blank == AddBlankLine::After {
+            buf.push(make::tokens::single_newline().into());
+        } else if add_blank == AddBlankLine::AfterTwice {
+            buf.push(make::tokens::single_newline().into());
+            buf.push(make::tokens::single_newline().into());
+        }
 
-    if let Some(container) = container {
-        let syntax = container.either(|l| l.syntax().clone(), |r| r.syntax().clone());
-        let action = best_action_for_target(syntax, position.clone(), &target);
-        make_assist(&action, &target, builder);
+        buf
+    };
+
+    algo::insert_children(&where_, insert_position, to_insert)
+}
+
+fn try_merge_imports(
+    old: &ast::Use,
+    new: &ast::Use,
+    merge_behaviour: MergeBehaviour,
+) -> Option<ast::Use> {
+    // dont merge into re-exports
+    if old.visibility().map(|vis| vis.pub_token()).is_some() {
+        return None;
+    }
+    let old_tree = old.use_tree()?;
+    let new_tree = new.use_tree()?;
+    let merged = try_merge_trees(&old_tree, &new_tree, merge_behaviour)?;
+    Some(old.with_use_tree(merged))
+}
+
+/// Simple function that checks if a UseTreeList is deeper than one level
+fn use_tree_list_is_nested(tl: &ast::UseTreeList) -> bool {
+    tl.use_trees().any(|use_tree| {
+        use_tree.use_tree_list().is_some() || use_tree.path().and_then(|p| p.qualifier()).is_some()
+    })
+}
+
+pub fn try_merge_trees(
+    old: &ast::UseTree,
+    new: &ast::UseTree,
+    merge_behaviour: MergeBehaviour,
+) -> Option<ast::UseTree> {
+    let lhs_path = old.path()?;
+    let rhs_path = new.path()?;
+
+    let (lhs_prefix, rhs_prefix) = common_prefix(&lhs_path, &rhs_path)?;
+    let lhs = old.split_prefix(&lhs_prefix);
+    let rhs = new.split_prefix(&rhs_prefix);
+    let lhs_tl = lhs.use_tree_list()?;
+    let rhs_tl = rhs.use_tree_list()?;
+
+    // if we are only allowed to merge the last level check if the paths are only one level deep
+    // FIXME: This shouldn't work yet i think
+    if merge_behaviour == MergeBehaviour::Last && use_tree_list_is_nested(&lhs_tl)
+        || use_tree_list_is_nested(&rhs_tl)
+    {
+        return None;
     }
+
+    let should_insert_comma = lhs_tl
+        .r_curly_token()
+        .and_then(|it| skip_trivia_token(it.prev_token()?, Direction::Prev))
+        .map(|it| it.kind() != T![,])
+        .unwrap_or(true);
+    let mut to_insert: Vec<SyntaxElement> = Vec::new();
+    if should_insert_comma {
+        to_insert.push(make::token(T![,]).into());
+        to_insert.push(make::tokens::single_space().into());
+    }
+    to_insert.extend(
+        rhs_tl
+            .syntax()
+            .children_with_tokens()
+            .filter(|it| it.kind() != T!['{'] && it.kind() != T!['}']),
+    );
+    let pos = InsertPosition::Before(lhs_tl.r_curly_token()?.into());
+    let use_tree_list = lhs_tl.insert_children(pos, to_insert);
+    Some(lhs.with_use_tree_list(use_tree_list))
 }
 
-fn collect_path_segments_raw(
-    segments: &mut Vec<ast::PathSegment>,
-    mut path: ast::Path,
-) -> Option<usize> {
-    let oldlen = segments.len();
+/// Traverses both paths until they differ, returning the common prefix of both.
+fn common_prefix(lhs: &ast::Path, rhs: &ast::Path) -> Option<(ast::Path, ast::Path)> {
+    let mut res = None;
+    let mut lhs_curr = first_path(&lhs);
+    let mut rhs_curr = first_path(&rhs);
     loop {
-        let mut children = path.syntax().children_with_tokens();
-        let (first, second, third) = (
-            children.next().map(|n| (n.clone(), n.kind())),
-            children.next().map(|n| (n.clone(), n.kind())),
-            children.next().map(|n| (n.clone(), n.kind())),
-        );
-        match (first, second, third) {
-            (Some((subpath, PATH)), Some((_, T![::])), Some((segment, PATH_SEGMENT))) => {
-                path = ast::Path::cast(subpath.as_node()?.clone())?;
-                segments.push(ast::PathSegment::cast(segment.as_node()?.clone())?);
-            }
-            (Some((segment, PATH_SEGMENT)), _, _) => {
-                segments.push(ast::PathSegment::cast(segment.as_node()?.clone())?);
-                break;
+        match (lhs_curr.segment(), rhs_curr.segment()) {
+            (Some(lhs), Some(rhs)) if lhs.syntax().text() == rhs.syntax().text() => (),
+            _ => break,
+        }
+        res = Some((lhs_curr.clone(), rhs_curr.clone()));
+
+        match lhs_curr.parent_path().zip(rhs_curr.parent_path()) {
+            Some((lhs, rhs)) => {
+                lhs_curr = lhs;
+                rhs_curr = rhs;
             }
-            (_, _, _) => return None,
+            _ => break,
         }
     }
-    // We need to reverse only the new added segments
-    let only_new_segments = segments.split_at_mut(oldlen).1;
-    only_new_segments.reverse();
-    Some(segments.len() - oldlen)
+
+    res
 }
 
-fn fmt_segments_raw(segments: &[SmolStr], buf: &mut String) {
-    let mut iter = segments.iter();
-    if let Some(s) = iter.next() {
-        buf.push_str(s);
-    }
-    for s in iter {
-        buf.push_str("::");
-        buf.push_str(s);
-    }
+/// What type of merges are allowed.
+#[derive(Copy, Clone, PartialEq, Eq)]
+pub enum MergeBehaviour {
+    /// Merge everything together creating deeply nested imports.
+    Full,
+    /// Only merge the last import level, doesn't allow import nesting.
+    Last,
 }
 
-/// Returns the number of common segments.
-fn compare_path_segments(left: &[SmolStr], right: &[ast::PathSegment]) -> usize {
-    left.iter().zip(right).take_while(|(l, r)| compare_path_segment(l, r)).count()
+#[derive(Eq, PartialEq, PartialOrd, Ord)]
+enum ImportGroup {
+    // the order here defines the order of new group inserts
+    Std,
+    ExternCrate,
+    ThisCrate,
+    ThisModule,
+    SuperModule,
 }
 
-fn compare_path_segment(a: &SmolStr, b: &ast::PathSegment) -> bool {
-    if let Some(kb) = b.kind() {
-        match kb {
-            ast::PathSegmentKind::Name(nameref_b) => a == nameref_b.text(),
-            ast::PathSegmentKind::SelfKw => a == "self",
-            ast::PathSegmentKind::SuperKw => a == "super",
-            ast::PathSegmentKind::CrateKw => a == "crate",
-            ast::PathSegmentKind::Type { .. } => false, // not allowed in imports
+impl ImportGroup {
+    fn new(path: &ast::Path) -> ImportGroup {
+        let default = ImportGroup::ExternCrate;
+
+        let first_segment = match first_segment(path) {
+            Some(it) => it,
+            None => return default,
+        };
+
+        let kind = first_segment.kind().unwrap_or(PathSegmentKind::SelfKw);
+        match kind {
+            PathSegmentKind::SelfKw => ImportGroup::ThisModule,
+            PathSegmentKind::SuperKw => ImportGroup::SuperModule,
+            PathSegmentKind::CrateKw => ImportGroup::ThisCrate,
+            PathSegmentKind::Name(name) => match name.text().as_str() {
+                "std" => ImportGroup::Std,
+                "core" => ImportGroup::Std,
+                // FIXME: can be ThisModule as well
+                _ => ImportGroup::ExternCrate,
+            },
+            PathSegmentKind::Type { .. } => unreachable!(),
         }
-    } else {
-        false
     }
 }
 
-fn compare_path_segment_with_name(a: &SmolStr, b: &ast::Name) -> bool {
-    a == b.text()
+fn first_segment(path: &ast::Path) -> Option<ast::PathSegment> {
+    first_path(path).segment()
 }
 
-#[derive(Clone, Debug)]
-enum ImportAction {
-    Nothing,
-    // Add a brand new use statement.
-    AddNewUse {
-        anchor: Option<SyntaxNode>, // anchor node
-        add_after_anchor: bool,
-    },
-
-    // To split an existing use statement creating a nested import.
-    AddNestedImport {
-        // how may segments matched with the target path
-        common_segments: usize,
-        path_to_split: ast::Path,
-        // the first segment of path_to_split we want to add into the new nested list
-        first_segment_to_split: Option<ast::PathSegment>,
-        // Wether to add 'self' in addition to the target path
-        add_self: bool,
-    },
-    // To add the target path to an existing nested import tree list.
-    AddInTreeList {
-        common_segments: usize,
-        // The UseTreeList where to add the target path
-        tree_list: ast::UseTreeList,
-        add_self: bool,
-    },
+fn first_path(path: &ast::Path) -> ast::Path {
+    successors(Some(path.clone()), ast::Path::qualifier).last().unwrap()
 }
 
-impl ImportAction {
-    fn add_new_use(anchor: Option<SyntaxNode>, add_after_anchor: bool) -> Self {
-        ImportAction::AddNewUse { anchor, add_after_anchor }
+fn segment_iter(path: &ast::Path) -> impl Iterator<Item = ast::PathSegment> + Clone {
+    path.syntax().children().flat_map(ast::PathSegment::cast)
+}
+
+#[derive(PartialEq, Eq)]
+enum AddBlankLine {
+    Before,
+    After,
+    AfterTwice,
+}
+
+fn find_insert_position(
+    scope: &SyntaxNode,
+    insert_path: ast::Path,
+) -> (InsertPosition<SyntaxElement>, AddBlankLine) {
+    let group = ImportGroup::new(&insert_path);
+    let path_node_iter = scope
+        .children()
+        .filter_map(|node| ast::Use::cast(node.clone()).zip(Some(node)))
+        .flat_map(|(use_, node)| use_.use_tree().and_then(|tree| tree.path()).zip(Some(node)));
+    // Iterator that discards anything thats not in the required grouping
+    // This implementation allows the user to rearrange their import groups as this only takes the first group that fits
+    let group_iter = path_node_iter
+        .clone()
+        .skip_while(|(path, _)| ImportGroup::new(path) != group)
+        .take_while(|(path, _)| ImportGroup::new(path) == group);
+
+    let segments = segment_iter(&insert_path);
+    // track the last element we iterated over, if this is still None after the iteration then that means we never iterated in the first place
+    let mut last = None;
+    // find the element that would come directly after our new import
+    let post_insert =
+        group_iter.inspect(|(_, node)| last = Some(node.clone())).find(|(path, _)| {
+            let check_segments = segment_iter(&path);
+            segments
+                .clone()
+                .zip(check_segments)
+                .flat_map(|(seg, seg2)| seg.name_ref().zip(seg2.name_ref()))
+                .all(|(l, r)| l.text() <= r.text())
+        });
+    match post_insert {
+        // insert our import before that element
+        Some((_, node)) => (InsertPosition::Before(node.into()), AddBlankLine::After),
+        // there is no element after our new import, so append it to the end of the group
+        None => match last {
+            Some(node) => (InsertPosition::After(node.into()), AddBlankLine::Before),
+            // the group we were looking for actually doesnt exist, so insert
+            None => {
+                // similar concept here to the `last` from above
+                let mut last = None;
+                // find the group that comes after where we want to insert
+                let post_group = path_node_iter
+                    .inspect(|(_, node)| last = Some(node.clone()))
+                    .find(|(p, _)| ImportGroup::new(p) > group);
+                match post_group {
+                    Some((_, node)) => {
+                        (InsertPosition::Before(node.into()), AddBlankLine::AfterTwice)
+                    }
+                    // there is no such group, so append after the last one
+                    None => match last {
+                        Some(node) => (InsertPosition::After(node.into()), AddBlankLine::Before),
+                        // there are no imports in this file at all
+                        None => (InsertPosition::First, AddBlankLine::AfterTwice),
+                    },
+                }
+            }
+        },
     }
+}
 
-    fn add_nested_import(
-        common_segments: usize,
-        path_to_split: ast::Path,
-        first_segment_to_split: Option<ast::PathSegment>,
-        add_self: bool,
-    ) -> Self {
-        ImportAction::AddNestedImport {
-            common_segments,
-            path_to_split,
-            first_segment_to_split,
-            add_self,
-        }
+#[cfg(test)]
+mod tests {
+    use super::*;
+
+    use test_utils::assert_eq_text;
+
+    #[test]
+    fn insert_start() {
+        check_none(
+            "std::bar::A",
+            r"use std::bar::B;
+use std::bar::D;
+use std::bar::F;
+use std::bar::G;",
+            r"use std::bar::A;
+use std::bar::B;
+use std::bar::D;
+use std::bar::F;
+use std::bar::G;",
+        )
     }
 
-    fn add_in_tree_list(
-        common_segments: usize,
-        tree_list: ast::UseTreeList,
-        add_self: bool,
-    ) -> Self {
-        ImportAction::AddInTreeList { common_segments, tree_list, add_self }
+    #[test]
+    fn insert_middle() {
+        check_none(
+            "std::bar::E",
+            r"use std::bar::A;
+use std::bar::D;
+use std::bar::F;
+use std::bar::G;",
+            r"use std::bar::A;
+use std::bar::D;
+use std::bar::E;
+use std::bar::F;
+use std::bar::G;",
+        )
     }
 
-    fn better(left: ImportAction, right: ImportAction) -> ImportAction {
-        if left.is_better(&right) {
-            left
-        } else {
-            right
-        }
+    #[test]
+    fn insert_end() {
+        check_none(
+            "std::bar::Z",
+            r"use std::bar::A;
+use std::bar::D;
+use std::bar::F;
+use std::bar::G;",
+            r"use std::bar::A;
+use std::bar::D;
+use std::bar::F;
+use std::bar::G;
+use std::bar::Z;",
+        )
     }
 
-    fn is_better(&self, other: &ImportAction) -> bool {
-        match (self, other) {
-            (ImportAction::Nothing, _) => true,
-            (ImportAction::AddInTreeList { .. }, ImportAction::Nothing) => false,
-            (
-                ImportAction::AddNestedImport { common_segments: n, .. },
-                ImportAction::AddInTreeList { common_segments: m, .. },
-            )
-            | (
-                ImportAction::AddInTreeList { common_segments: n, .. },
-                ImportAction::AddNestedImport { common_segments: m, .. },
-            )
-            | (
-                ImportAction::AddInTreeList { common_segments: n, .. },
-                ImportAction::AddInTreeList { common_segments: m, .. },
-            )
-            | (
-                ImportAction::AddNestedImport { common_segments: n, .. },
-                ImportAction::AddNestedImport { common_segments: m, .. },
-            ) => n > m,
-            (ImportAction::AddInTreeList { .. }, _) => true,
-            (ImportAction::AddNestedImport { .. }, ImportAction::Nothing) => false,
-            (ImportAction::AddNestedImport { .. }, _) => true,
-            (ImportAction::AddNewUse { .. }, _) => false,
-        }
+    #[test]
+    fn insert_middle_pnested() {
+        check_none(
+            "std::bar::E",
+            r"use std::bar::A;
+use std::bar::{D, Z}; // example of weird imports due to user
+use std::bar::F;
+use std::bar::G;",
+            r"use std::bar::A;
+use std::bar::E;
+use std::bar::{D, Z}; // example of weird imports due to user
+use std::bar::F;
+use std::bar::G;",
+        )
     }
-}
 
-// Find out the best ImportAction to import target path against current_use_tree.
-// If current_use_tree has a nested import the function gets called recursively on every UseTree inside a UseTreeList.
-fn walk_use_tree_for_best_action(
-    current_path_segments: &mut Vec<ast::PathSegment>, // buffer containing path segments
-    current_parent_use_tree_list: Option<ast::UseTreeList>, // will be Some value if we are in a nested import
-    current_use_tree: ast::UseTree, // the use tree we are currently examinating
-    target: &[SmolStr],             // the path we want to import
-) -> ImportAction {
-    // We save the number of segments in the buffer so we can restore the correct segments
-    // before returning. Recursive call will add segments so we need to delete them.
-    let prev_len = current_path_segments.len();
-
-    let tree_list = current_use_tree.use_tree_list();
-    let alias = current_use_tree.rename();
-
-    let path = match current_use_tree.path() {
-        Some(path) => path,
-        None => {
-            // If the use item don't have a path, it means it's broken (syntax error)
-            return ImportAction::add_new_use(
-                current_use_tree
-                    .syntax()
-                    .ancestors()
-                    .find_map(ast::Use::cast)
-                    .map(|it| it.syntax().clone()),
-                true,
-            );
-        }
-    };
+    #[test]
+    fn insert_middle_groups() {
+        check_none(
+            "foo::bar::G",
+            r"use std::bar::A;
+use std::bar::D;
+
+use foo::bar::F;
+use foo::bar::H;",
+            r"use std::bar::A;
+use std::bar::D;
+
+use foo::bar::F;
+use foo::bar::G;
+use foo::bar::H;",
+        )
+    }
 
-    // This can happen only if current_use_tree is a direct child of a UseItem
-    if let Some(name) = alias.and_then(|it| it.name()) {
-        if compare_path_segment_with_name(&target[0], &name) {
-            return ImportAction::Nothing;
-        }
+    #[test]
+    fn insert_first_matching_group() {
+        check_none(
+            "foo::bar::G",
+            r"use foo::bar::A;
+use foo::bar::D;
+
+use std;
+
+use foo::bar::F;
+use foo::bar::H;",
+            r"use foo::bar::A;
+use foo::bar::D;
+use foo::bar::G;
+
+use std;
+
+use foo::bar::F;
+use foo::bar::H;",
+        )
     }
 
-    collect_path_segments_raw(current_path_segments, path.clone());
-
-    // We compare only the new segments added in the line just above.
-    // The first prev_len segments were already compared in 'parent' recursive calls.
-    let left = target.split_at(prev_len).1;
-    let right = current_path_segments.split_at(prev_len).1;
-    let common = compare_path_segments(left, &right);
-    let mut action = match common {
-        0 => ImportAction::add_new_use(
-            // e.g: target is std::fmt and we can have
-            // use foo::bar
-            // We add a brand new use statement
-            current_use_tree
-                .syntax()
-                .ancestors()
-                .find_map(ast::Use::cast)
-                .map(|it| it.syntax().clone()),
-            true,
-        ),
-        common if common == left.len() && left.len() == right.len() => {
-            // e.g: target is std::fmt and we can have
-            // 1- use std::fmt;
-            // 2- use std::fmt::{ ... }
-            if let Some(list) = tree_list {
-                // In case 2 we need to add self to the nested list
-                // unless it's already there
-                let has_self = list.use_trees().map(|it| it.path()).any(|p| {
-                    p.and_then(|it| it.segment())
-                        .and_then(|it| it.kind())
-                        .filter(|k| *k == ast::PathSegmentKind::SelfKw)
-                        .is_some()
-                });
-
-                if has_self {
-                    ImportAction::Nothing
-                } else {
-                    ImportAction::add_in_tree_list(current_path_segments.len(), list, true)
-                }
-            } else {
-                // Case 1
-                ImportAction::Nothing
-            }
-        }
-        common if common != left.len() && left.len() == right.len() => {
-            // e.g: target is std::fmt and we have
-            // use std::io;
-            // We need to split.
-            let segments_to_split = current_path_segments.split_at(prev_len + common).1;
-            ImportAction::add_nested_import(
-                prev_len + common,
-                path,
-                Some(segments_to_split[0].clone()),
-                false,
-            )
-        }
-        common if common == right.len() && left.len() > right.len() => {
-            // e.g: target is std::fmt and we can have
-            // 1- use std;
-            // 2- use std::{ ... };
-
-            // fallback action
-            let mut better_action = ImportAction::add_new_use(
-                current_use_tree
-                    .syntax()
-                    .ancestors()
-                    .find_map(ast::Use::cast)
-                    .map(|it| it.syntax().clone()),
-                true,
-            );
-            if let Some(list) = tree_list {
-                // Case 2, check recursively if the path is already imported in the nested list
-                for u in list.use_trees() {
-                    let child_action = walk_use_tree_for_best_action(
-                        current_path_segments,
-                        Some(list.clone()),
-                        u,
-                        target,
-                    );
-                    if child_action.is_better(&better_action) {
-                        better_action = child_action;
-                        if let ImportAction::Nothing = better_action {
-                            return better_action;
-                        }
-                    }
-                }
-            } else {
-                // Case 1, split adding self
-                better_action = ImportAction::add_nested_import(prev_len + common, path, None, true)
-            }
-            better_action
-        }
-        common if common == left.len() && left.len() < right.len() => {
-            // e.g: target is std::fmt and we can have
-            // use std::fmt::Debug;
-            let segments_to_split = current_path_segments.split_at(prev_len + common).1;
-            ImportAction::add_nested_import(
-                prev_len + common,
-                path,
-                Some(segments_to_split[0].clone()),
-                true,
-            )
-        }
-        common if common < left.len() && common < right.len() => {
-            // e.g: target is std::fmt::nested::Debug
-            // use std::fmt::Display
-            let segments_to_split = current_path_segments.split_at(prev_len + common).1;
-            ImportAction::add_nested_import(
-                prev_len + common,
-                path,
-                Some(segments_to_split[0].clone()),
-                false,
-            )
-        }
-        _ => unreachable!(),
-    };
+    #[test]
+    fn insert_missing_group() {
+        check_none(
+            "std::fmt",
+            r"use foo::bar::A;
+use foo::bar::D;",
+            r"use std::fmt;
+
+use foo::bar::A;
+use foo::bar::D;",
+        )
+    }
 
-    // If we are inside a UseTreeList adding a use statement become adding to the existing
-    // tree list.
-    action = match (current_parent_use_tree_list, action.clone()) {
-        (Some(use_tree_list), ImportAction::AddNewUse { .. }) => {
-            ImportAction::add_in_tree_list(prev_len, use_tree_list, false)
-        }
-        (_, _) => action,
-    };
+    #[test]
+    fn insert_no_imports() {
+        check_full(
+            "foo::bar",
+            "fn main() {}",
+            r"use foo::bar;
 
-    // We remove the segments added
-    current_path_segments.truncate(prev_len);
-    action
-}
+fn main() {}",
+        )
+    }
 
-fn best_action_for_target(
-    container: SyntaxNode,
-    anchor: SyntaxNode,
-    target: &[SmolStr],
-) -> ImportAction {
-    let mut storage = Vec::with_capacity(16); // this should be the only allocation
-    let best_action = container
-        .children()
-        .filter_map(ast::Use::cast)
-        .filter(|u| u.visibility().is_none())
-        .filter_map(|it| it.use_tree())
-        .map(|u| walk_use_tree_for_best_action(&mut storage, None, u, target))
-        .fold(None, |best, a| match best {
-            Some(best) => Some(ImportAction::better(best, a)),
-            None => Some(a),
-        });
+    #[test]
+    fn insert_empty_file() {
+        // empty files will get two trailing newlines
+        // this is due to the test case insert_no_imports above
+        check_full(
+            "foo::bar",
+            "",
+            r"use foo::bar;
+
+",
+        )
+    }
 
-    match best_action {
-        Some(action) => action,
-        None => {
-            // We have no action and no UseItem was found in container so we find
-            // another item and we use it as anchor.
-            // If there are no items above, we choose the target path itself as anchor.
-            // todo: we should include even whitespace blocks as anchor candidates
-            let anchor = container.children().next().or_else(|| Some(anchor));
+    #[test]
+    fn adds_std_group() {
+        check_full(
+            "std::fmt::Debug",
+            r"use stdx;",
+            r"use std::fmt::Debug;
 
-            let add_after_anchor = anchor
-                .clone()
-                .and_then(ast::Attr::cast)
-                .map(|attr| attr.kind() == ast::AttrKind::Inner)
-                .unwrap_or(false);
-            ImportAction::add_new_use(anchor, add_after_anchor)
-        }
+use stdx;",
+        )
     }
-}
 
-fn make_assist(action: &ImportAction, target: &[SmolStr], edit: &mut TextEditBuilder) {
-    match action {
-        ImportAction::AddNewUse { anchor, add_after_anchor } => {
-            make_assist_add_new_use(anchor, *add_after_anchor, target, edit)
-        }
-        ImportAction::AddInTreeList { common_segments, tree_list, add_self } => {
-            // We know that the fist n segments already exists in the use statement we want
-            // to modify, so we want to add only the last target.len() - n segments.
-            let segments_to_add = target.split_at(*common_segments).1;
-            make_assist_add_in_tree_list(tree_list, segments_to_add, *add_self, edit)
-        }
-        ImportAction::AddNestedImport {
-            common_segments,
-            path_to_split,
-            first_segment_to_split,
-            add_self,
-        } => {
-            let segments_to_add = target.split_at(*common_segments).1;
-            make_assist_add_nested_import(
-                path_to_split,
-                first_segment_to_split,
-                segments_to_add,
-                *add_self,
-                edit,
-            )
-        }
-        _ => {}
+    #[test]
+    fn merges_groups() {
+        check_last("std::io", r"use std::fmt;", r"use std::{fmt, io};")
     }
-}
 
-fn make_assist_add_new_use(
-    anchor: &Option<SyntaxNode>,
-    after: bool,
-    target: &[SmolStr],
-    edit: &mut TextEditBuilder,
-) {
-    if let Some(anchor) = anchor {
-        let indent = leading_indent(anchor);
-        let mut buf = String::new();
-        if after {
-            buf.push_str("\n");
-            if let Some(spaces) = &indent {
-                buf.push_str(spaces);
-            }
-        }
-        buf.push_str("use ");
-        fmt_segments_raw(target, &mut buf);
-        buf.push_str(";");
-        if !after {
-            buf.push_str("\n\n");
-            if let Some(spaces) = &indent {
-                buf.push_str(&spaces);
-            }
-        }
-        let position = if after { anchor.text_range().end() } else { anchor.text_range().start() };
-        edit.insert(position, buf);
+    #[test]
+    fn merges_groups_last() {
+        check_last(
+            "std::io",
+            r"use std::fmt::{Result, Display};",
+            r"use std::fmt::{Result, Display};
+use std::io;",
+        )
     }
-}
 
-fn make_assist_add_in_tree_list(
-    tree_list: &ast::UseTreeList,
-    target: &[SmolStr],
-    add_self: bool,
-    edit: &mut TextEditBuilder,
-) {
-    let last = tree_list.use_trees().last();
-    if let Some(last) = last {
-        let mut buf = String::new();
-        let comma = last.syntax().siblings(Direction::Next).find(|n| n.kind() == T![,]);
-        let offset = if let Some(comma) = comma {
-            comma.text_range().end()
-        } else {
-            buf.push_str(",");
-            last.syntax().text_range().end()
-        };
-        if add_self {
-            buf.push_str(" self")
-        } else {
-            buf.push_str(" ");
-        }
-        fmt_segments_raw(target, &mut buf);
-        edit.insert(offset, buf);
-    } else {
+    #[test]
+    fn merges_groups2() {
+        check_full(
+            "std::io",
+            r"use std::fmt::{Result, Display};",
+            r"use std::{fmt::{Result, Display}, io};",
+        )
     }
-}
 
-fn make_assist_add_nested_import(
-    path: &ast::Path,
-    first_segment_to_split: &Option<ast::PathSegment>,
-    target: &[SmolStr],
-    add_self: bool,
-    edit: &mut TextEditBuilder,
-) {
-    let use_tree = path.syntax().ancestors().find_map(ast::UseTree::cast);
-    if let Some(use_tree) = use_tree {
-        let (start, add_colon_colon) = if let Some(first_segment_to_split) = first_segment_to_split
-        {
-            (first_segment_to_split.syntax().text_range().start(), false)
-        } else {
-            (use_tree.syntax().text_range().end(), true)
-        };
-        let end = use_tree.syntax().text_range().end();
+    #[test]
+    fn skip_merges_groups_pub() {
+        check_full(
+            "std::io",
+            r"pub use std::fmt::{Result, Display};",
+            r"pub use std::fmt::{Result, Display};
+use std::io;",
+        )
+    }
 
-        let mut buf = String::new();
-        if add_colon_colon {
-            buf.push_str("::");
-        }
-        buf.push_str("{");
-        if add_self {
-            buf.push_str("self, ");
-        }
-        fmt_segments_raw(target, &mut buf);
-        if !target.is_empty() {
-            buf.push_str(", ");
-        }
-        edit.insert(start, buf);
-        edit.insert(end, "}".to_string());
+    #[test]
+    fn merges_groups_self() {
+        check_full("std::fmt::Debug", r"use std::fmt;", r"use std::fmt::{self, Debug};")
     }
-}
 
-/// If the node is on the beginning of the line, calculate indent.
-fn leading_indent(node: &SyntaxNode) -> Option<SmolStr> {
-    for token in prev_tokens(node.first_token()?) {
-        if let Some(ws) = ast::Whitespace::cast(token.clone()) {
-            let ws_text = ws.text();
-            if let Some(pos) = ws_text.rfind('\n') {
-                return Some(ws_text[pos + 1..].into());
-            }
-        }
-        if token.text().contains('\n') {
-            break;
-        }
+    fn check(
+        path: &str,
+        ra_fixture_before: &str,
+        ra_fixture_after: &str,
+        mb: Option<MergeBehaviour>,
+    ) {
+        let file = ast::SourceFile::parse(ra_fixture_before).tree().syntax().clone();
+        let path = ast::SourceFile::parse(&format!("use {};", path))
+            .tree()
+            .syntax()
+            .descendants()
+            .find_map(ast::Path::cast)
+            .unwrap();
+
+        let result = insert_use(file, path, mb).to_string();
+        assert_eq_text!(&result, ra_fixture_after);
+    }
+
+    fn check_full(path: &str, ra_fixture_before: &str, ra_fixture_after: &str) {
+        check(path, ra_fixture_before, ra_fixture_after, Some(MergeBehaviour::Full))
     }
-    return None;
-    fn prev_tokens(token: SyntaxToken) -> impl Iterator<Item = SyntaxToken> {
-        successors(token.prev_token(), |token| token.prev_token())
+
+    fn check_last(path: &str, ra_fixture_before: &str, ra_fixture_after: &str) {
+        check(path, ra_fixture_before, ra_fixture_after, Some(MergeBehaviour::Last))
+    }
+
+    fn check_none(path: &str, ra_fixture_before: &str, ra_fixture_after: &str) {
+        check(path, ra_fixture_before, ra_fixture_after, None)
     }
 }