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diff --git a/crates/ra_assists/src/assist_context.rs b/crates/ra_assists/src/assist_context.rs
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index 000000000..a680f752b
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+++ b/crates/ra_assists/src/assist_context.rs
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+//! See `AssistContext`
+
+use algo::find_covering_element;
+use hir::Semantics;
+use ra_db::{FileId, FileRange};
+use ra_fmt::{leading_indent, reindent};
+use ra_ide_db::{
+    source_change::{SingleFileChange, SourceChange},
+    RootDatabase,
+};
+use ra_syntax::{
+    algo::{self, find_node_at_offset, SyntaxRewriter},
+    AstNode, SourceFile, SyntaxElement, SyntaxKind, SyntaxNode, SyntaxToken, TextRange, TextSize,
+    TokenAtOffset,
+};
+use ra_text_edit::TextEditBuilder;
+
+use crate::{Assist, AssistId, GroupLabel, ResolvedAssist};
+
+/// `AssistContext` allows to apply an assist or check if it could be applied.
+///
+/// Assists use a somewhat over-engineered approach, given the current needs.
+/// The assists workflow consists of two phases. In the first phase, a user asks
+/// for the list of available assists. In the second phase, the user picks a
+/// particular assist and it gets applied.
+///
+/// There are two peculiarities here:
+///
+/// * first, we ideally avoid computing more things then necessary to answer "is
+///   assist applicable" in the first phase.
+/// * second, when we are applying assist, we don't have a guarantee that there
+///   weren't any changes between the point when user asked for assists and when
+///   they applied a particular assist. So, when applying assist, we need to do
+///   all the checks from scratch.
+///
+/// To avoid repeating the same code twice for both "check" and "apply"
+/// functions, we use an approach reminiscent of that of Django's function based
+/// views dealing with forms. Each assist receives a runtime parameter,
+/// `resolve`. It first check if an edit is applicable (potentially computing
+/// info required to compute the actual edit). If it is applicable, and
+/// `resolve` is `true`, it then computes the actual edit.
+///
+/// So, to implement the original assists workflow, we can first apply each edit
+/// with `resolve = false`, and then applying the selected edit again, with
+/// `resolve = true` this time.
+///
+/// Note, however, that we don't actually use such two-phase logic at the
+/// moment, because the LSP API is pretty awkward in this place, and it's much
+/// easier to just compute the edit eagerly :-)
+pub(crate) struct AssistContext<'a> {
+    pub(crate) sema: Semantics<'a, RootDatabase>,
+    pub(crate) db: &'a RootDatabase,
+    pub(crate) frange: FileRange,
+    source_file: SourceFile,
+}
+
+impl<'a> AssistContext<'a> {
+    pub fn new(sema: Semantics<'a, RootDatabase>, frange: FileRange) -> AssistContext<'a> {
+        let source_file = sema.parse(frange.file_id);
+        let db = sema.db;
+        AssistContext { sema, db, frange, source_file }
+    }
+
+    // NB, this ignores active selection.
+    pub(crate) fn offset(&self) -> TextSize {
+        self.frange.range.start()
+    }
+
+    pub(crate) fn token_at_offset(&self) -> TokenAtOffset<SyntaxToken> {
+        self.source_file.syntax().token_at_offset(self.offset())
+    }
+    pub(crate) fn find_token_at_offset(&self, kind: SyntaxKind) -> Option<SyntaxToken> {
+        self.token_at_offset().find(|it| it.kind() == kind)
+    }
+    pub(crate) fn find_node_at_offset<N: AstNode>(&self) -> Option<N> {
+        find_node_at_offset(self.source_file.syntax(), self.offset())
+    }
+    pub(crate) fn find_node_at_offset_with_descend<N: AstNode>(&self) -> Option<N> {
+        self.sema.find_node_at_offset_with_descend(self.source_file.syntax(), self.offset())
+    }
+    pub(crate) fn covering_element(&self) -> SyntaxElement {
+        find_covering_element(self.source_file.syntax(), self.frange.range)
+    }
+    // FIXME: remove
+    pub(crate) fn covering_node_for_range(&self, range: TextRange) -> SyntaxElement {
+        find_covering_element(self.source_file.syntax(), range)
+    }
+}
+
+pub(crate) struct Assists {
+    resolve: bool,
+    file: FileId,
+    buf: Vec<(Assist, Option<SourceChange>)>,
+}
+
+impl Assists {
+    pub(crate) fn new_resolved(ctx: &AssistContext) -> Assists {
+        Assists { resolve: true, file: ctx.frange.file_id, buf: Vec::new() }
+    }
+    pub(crate) fn new_unresolved(ctx: &AssistContext) -> Assists {
+        Assists { resolve: false, file: ctx.frange.file_id, buf: Vec::new() }
+    }
+
+    pub(crate) fn finish_unresolved(self) -> Vec<Assist> {
+        assert!(!self.resolve);
+        self.finish()
+            .into_iter()
+            .map(|(label, edit)| {
+                assert!(edit.is_none());
+                label
+            })
+            .collect()
+    }
+
+    pub(crate) fn finish_resolved(self) -> Vec<ResolvedAssist> {
+        assert!(self.resolve);
+        self.finish()
+            .into_iter()
+            .map(|(label, edit)| ResolvedAssist { assist: label, source_change: edit.unwrap() })
+            .collect()
+    }
+
+    pub(crate) fn add(
+        &mut self,
+        id: AssistId,
+        label: impl Into<String>,
+        target: TextRange,
+        f: impl FnOnce(&mut AssistBuilder),
+    ) -> Option<()> {
+        let label = Assist::new(id, label.into(), None, target);
+        self.add_impl(label, f)
+    }
+    pub(crate) fn add_group(
+        &mut self,
+        group: &GroupLabel,
+        id: AssistId,
+        label: impl Into<String>,
+        target: TextRange,
+        f: impl FnOnce(&mut AssistBuilder),
+    ) -> Option<()> {
+        let label = Assist::new(id, label.into(), Some(group.clone()), target);
+        self.add_impl(label, f)
+    }
+    fn add_impl(&mut self, label: Assist, f: impl FnOnce(&mut AssistBuilder)) -> Option<()> {
+        let change_label = label.label.clone();
+        let source_change = if self.resolve {
+            let mut builder = AssistBuilder::new(self.file);
+            f(&mut builder);
+            Some(builder.finish(change_label))
+        } else {
+            None
+        };
+
+        self.buf.push((label, source_change));
+        Some(())
+    }
+
+    fn finish(mut self) -> Vec<(Assist, Option<SourceChange>)> {
+        self.buf.sort_by_key(|(label, _edit)| label.target.len());
+        self.buf
+    }
+}
+
+pub(crate) struct AssistBuilder {
+    edit: TextEditBuilder,
+    cursor_position: Option<TextSize>,
+    file: FileId,
+}
+
+impl AssistBuilder {
+    pub(crate) fn new(file: FileId) -> AssistBuilder {
+        AssistBuilder { edit: TextEditBuilder::default(), cursor_position: None, file }
+    }
+
+    /// Remove specified `range` of text.
+    pub(crate) fn delete(&mut self, range: TextRange) {
+        self.edit.delete(range)
+    }
+    /// Append specified `text` at the given `offset`
+    pub(crate) fn insert(&mut self, offset: TextSize, text: impl Into<String>) {
+        self.edit.insert(offset, text.into())
+    }
+    /// Replaces specified `range` of text with a given string.
+    pub(crate) fn replace(&mut self, range: TextRange, replace_with: impl Into<String>) {
+        self.edit.replace(range, replace_with.into())
+    }
+    pub(crate) fn replace_ast<N: AstNode>(&mut self, old: N, new: N) {
+        algo::diff(old.syntax(), new.syntax()).into_text_edit(&mut self.edit)
+    }
+    /// Replaces specified `node` of text with a given string, reindenting the
+    /// string to maintain `node`'s existing indent.
+    // FIXME: remove in favor of ra_syntax::edit::IndentLevel::increase_indent
+    pub(crate) fn replace_node_and_indent(
+        &mut self,
+        node: &SyntaxNode,
+        replace_with: impl Into<String>,
+    ) {
+        let mut replace_with = replace_with.into();
+        if let Some(indent) = leading_indent(node) {
+            replace_with = reindent(&replace_with, &indent)
+        }
+        self.replace(node.text_range(), replace_with)
+    }
+    pub(crate) fn rewrite(&mut self, rewriter: SyntaxRewriter) {
+        let node = rewriter.rewrite_root().unwrap();
+        let new = rewriter.rewrite(&node);
+        algo::diff(&node, &new).into_text_edit(&mut self.edit)
+    }
+
+    /// Specify desired position of the cursor after the assist is applied.
+    pub(crate) fn set_cursor(&mut self, offset: TextSize) {
+        self.cursor_position = Some(offset)
+    }
+    // FIXME: better API
+    pub(crate) fn set_file(&mut self, assist_file: FileId) {
+        self.file = assist_file;
+    }
+
+    // FIXME: kill this API
+    /// Get access to the raw `TextEditBuilder`.
+    pub(crate) fn text_edit_builder(&mut self) -> &mut TextEditBuilder {
+        &mut self.edit
+    }
+
+    fn finish(self, change_label: String) -> SourceChange {
+        let edit = self.edit.finish();
+        if edit.is_empty() && self.cursor_position.is_none() {
+            panic!("Only call `add_assist` if the assist can be applied")
+        }
+        SingleFileChange { label: change_label, edit, cursor_position: self.cursor_position }
+            .into_source_change(self.file)
+    }
+}