1 files changed, 153 insertions, 2 deletions

diff --git a/crates/ra_editor/src/assists.rs b/crates/ra_editor/src/assists.rs
index b6e6dd628..cc40ee4c8 100644
--- a/crates/ra_editor/src/assists.rs
+++ b/crates/ra_editor/src/assists.rs
@@ -9,8 +9,13 @@ mod add_impl;
 mod introduce_variable;
 mod change_visibility;
 
-use ra_text_edit::TextEdit;
-use ra_syntax::{Direction, SyntaxNodeRef, TextUnit};
+use ra_text_edit::{TextEdit, TextEditBuilder};
+use ra_syntax::{
+    Direction, SyntaxNodeRef, TextUnit, TextRange,SourceFileNode, AstNode,
+    algo::{find_leaf_at_offset, find_covering_node, LeafAtOffset},
+};
+
+use crate::find_node_at_offset;
 
 pub use self::{
     flip_comma::flip_comma,
@@ -20,6 +25,21 @@ pub use self::{
     change_visibility::change_visibility,
 };
 
+/// Return all the assists applicable at the given position.
+pub fn assists(file: &SourceFileNode, range: TextRange) -> Vec<LocalEdit> {
+    let ctx = AssistCtx::new(file, range);
+    [
+        flip_comma,
+        add_derive,
+        add_impl,
+        introduce_variable,
+        change_visibility,
+    ]
+    .iter()
+    .filter_map(|&assist| ctx.clone().apply(assist))
+    .collect()
+}
+
 #[derive(Debug)]
 pub struct LocalEdit {
     pub label: String,
@@ -32,3 +52,134 @@ fn non_trivia_sibling(node: SyntaxNodeRef, direction: Direction) -> Option<Synta
         .skip(1)
         .find(|node| !node.kind().is_trivia())
 }
+
+/// `AssistCtx` allows to apply an assist or check if it could be applied.
+///
+/// Assists use a somewhat overengeneered 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 neccessary to answer
+///   "is assist applicable" in the first phase.
+/// * second, when we are appling assist, we don't have a gurantee 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 remeniscent of that of Django's function based
+/// views dealing with forms. Each assist receives a runtime parameter,
+/// `should_compute_edit`. It first check if an edit is applicable (potentially
+/// computing info required to compute the actual edit). If it is applicable,
+/// and `should_compute_edit` is `true`, it then computes the actual edit.
+///
+/// So, to implement the original assists workflow, we can first apply each edit
+/// with `should_compute_edit = false`, and then applying the selected edit
+/// again, with `should_compute_edit = 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 eagarly :-)
+#[derive(Debug, Clone)]
+pub struct AssistCtx<'a> {
+    source_file: &'a SourceFileNode,
+    range: TextRange,
+    should_compute_edit: bool,
+}
+
+#[derive(Debug)]
+pub enum Assist {
+    Applicable,
+    Edit(LocalEdit),
+}
+
+#[derive(Default)]
+struct AssistBuilder {
+    edit: TextEditBuilder,
+    cursor_position: Option<TextUnit>,
+}
+
+impl<'a> AssistCtx<'a> {
+    pub fn new(source_file: &'a SourceFileNode, range: TextRange) -> AssistCtx {
+        AssistCtx {
+            source_file,
+            range,
+            should_compute_edit: false,
+        }
+    }
+
+    pub fn apply(mut self, assist: fn(AssistCtx) -> Option<Assist>) -> Option<LocalEdit> {
+        self.should_compute_edit = true;
+        match assist(self) {
+            None => None,
+            Some(Assist::Edit(e)) => Some(e),
+            Some(Assist::Applicable) => unreachable!(),
+        }
+    }
+
+    pub fn check(mut self, assist: fn(AssistCtx) -> Option<Assist>) -> bool {
+        self.should_compute_edit = false;
+        match assist(self) {
+            None => false,
+            Some(Assist::Edit(_)) => unreachable!(),
+            Some(Assist::Applicable) => true,
+        }
+    }
+
+    fn build(self, label: impl Into<String>, f: impl FnOnce(&mut AssistBuilder)) -> Option<Assist> {
+        if !self.should_compute_edit {
+            return Some(Assist::Applicable);
+        }
+        let mut edit = AssistBuilder::default();
+        f(&mut edit);
+        Some(Assist::Edit(LocalEdit {
+            label: label.into(),
+            edit: edit.edit.finish(),
+            cursor_position: edit.cursor_position,
+        }))
+    }
+
+    pub(crate) fn leaf_at_offset(&self) -> LeafAtOffset<SyntaxNodeRef<'a>> {
+        find_leaf_at_offset(self.source_file.syntax(), self.range.start())
+    }
+    pub(crate) fn node_at_offset<N: AstNode<'a>>(&self) -> Option<N> {
+        find_node_at_offset(self.source_file.syntax(), self.range.start())
+    }
+    pub(crate) fn covering_node(&self) -> SyntaxNodeRef<'a> {
+        find_covering_node(self.source_file.syntax(), self.range)
+    }
+}
+
+impl AssistBuilder {
+    fn replace(&mut self, range: TextRange, replace_with: impl Into<String>) {
+        self.edit.replace(range, replace_with.into())
+    }
+    #[allow(unused)]
+    fn delete(&mut self, range: TextRange) {
+        self.edit.delete(range)
+    }
+    fn insert(&mut self, offset: TextUnit, text: impl Into<String>) {
+        self.edit.insert(offset, text.into())
+    }
+    fn set_cursor(&mut self, offset: TextUnit) {
+        self.cursor_position = Some(offset)
+    }
+}
+
+#[cfg(test)]
+fn check_assist(assist: fn(AssistCtx) -> Option<Assist>, before: &str, after: &str) {
+    crate::test_utils::check_action(before, after, |file, off| {
+        let range = TextRange::offset_len(off, 0.into());
+        AssistCtx::new(file, range).apply(assist)
+    })
+}
+
+#[cfg(test)]
+fn check_assist_range(assist: fn(AssistCtx) -> Option<Assist>, before: &str, after: &str) {
+    crate::test_utils::check_action_range(before, after, |file, range| {
+        AssistCtx::new(file, range).apply(assist)
+    })
+}