1 files changed, 547 insertions, 0 deletions

diff --git a/crates/assists/src/utils/insert_use.rs b/crates/assists/src/utils/insert_use.rs
new file mode 100644
index 000000000..50a62ee82
--- /dev/null
+++ b/crates/assists/src/utils/insert_use.rs
@@ -0,0 +1,547 @@
+//! 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 either::Either;
+use hir::{self, ModPath};
+use syntax::{
+    ast::{self, NameOwner, VisibilityOwner},
+    AstNode, AstToken, Direction, SmolStr,
+    SyntaxKind::{PATH, PATH_SEGMENT},
+    SyntaxNode, SyntaxToken, T,
+};
+use text_edit::TextEditBuilder;
+
+use crate::assist_context::AssistContext;
+
+/// Determines the containing syntax node in which to insert a `use` statement affecting `position`.
+pub(crate) fn find_insert_use_container(
+    position: &SyntaxNode,
+    ctx: &AssistContext,
+) -> 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));
+        }
+        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: &ModPath,
+    ctx: &AssistContext,
+    builder: &mut TextEditBuilder,
+) {
+    let target = path_to_import.to_string().split("::").map(SmolStr::new).collect::<Vec<_>>();
+    let container = find_insert_use_container(position, ctx);
+
+    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);
+    }
+}
+
+fn collect_path_segments_raw(
+    segments: &mut Vec<ast::PathSegment>,
+    mut path: ast::Path,
+) -> Option<usize> {
+    let oldlen = segments.len();
+    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;
+            }
+            (_, _, _) => return None,
+        }
+    }
+    // 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)
+}
+
+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);
+    }
+}
+
+/// 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()
+}
+
+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
+        }
+    } else {
+        false
+    }
+}
+
+fn compare_path_segment_with_name(a: &SmolStr, b: &ast::Name) -> bool {
+    a == b.text()
+}
+
+#[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,
+    },
+}
+
+impl ImportAction {
+    fn add_new_use(anchor: Option<SyntaxNode>, add_after_anchor: bool) -> Self {
+        ImportAction::AddNewUse { anchor, add_after_anchor }
+    }
+
+    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,
+        }
+    }
+
+    fn add_in_tree_list(
+        common_segments: usize,
+        tree_list: ast::UseTreeList,
+        add_self: bool,
+    ) -> Self {
+        ImportAction::AddInTreeList { common_segments, tree_list, add_self }
+    }
+
+    fn better(left: ImportAction, right: ImportAction) -> ImportAction {
+        if left.is_better(&right) {
+            left
+        } else {
+            right
+        }
+    }
+
+    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,
+        }
+    }
+}
+
+// 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,
+            );
+        }
+    };
+
+    // 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;
+        }
+    }
+
+    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!(),
+    };
+
+    // 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,
+    };
+
+    // We remove the segments added
+    current_path_segments.truncate(prev_len);
+    action
+}
+
+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),
+        });
+
+    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));
+
+            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)
+        }
+    }
+}
+
+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,
+            )
+        }
+        _ => {}
+    }
+}
+
+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);
+    }
+}
+
+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 {
+    }
+}
+
+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();
+
+        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());
+    }
+}
+
+/// 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;
+        }
+    }
+    return None;
+    fn prev_tokens(token: SyntaxToken) -> impl Iterator<Item = SyntaxToken> {
+        successors(token.prev_token(), |token| token.prev_token())
+    }
+}