7 files changed, 169 insertions, 4 deletions

diff --git a/crates/ra_analysis/src/lib.rs b/crates/ra_analysis/src/lib.rs
index b4c7db476..f6ceb7eb2 100644
--- a/crates/ra_analysis/src/lib.rs
+++ b/crates/ra_analysis/src/lib.rs
@@ -184,6 +184,12 @@ impl Analysis {
         let file = self.imp.file_syntax(file_id);
         SourceChange::from_local_edit(file_id, "join lines", ra_editor::join_lines(&file, range))
     }
+    pub fn on_enter(&self, file_id: FileId, offset: TextUnit) -> Option<SourceChange> {
+        let file = self.imp.file_syntax(file_id);
+        let edit = ra_editor::on_enter(&file, offset)?;
+        let res = SourceChange::from_local_edit(file_id, "on enter", edit);
+        Some(res)
+    }
     pub fn on_eq_typed(&self, file_id: FileId, offset: TextUnit) -> Option<SourceChange> {
         let file = self.imp.file_syntax(file_id);
         Some(SourceChange::from_local_edit(file_id, "add semicolon", ra_editor::on_eq_typed(&file, offset)?))
diff --git a/crates/ra_editor/src/lib.rs b/crates/ra_editor/src/lib.rs
index 2a801f7da..fe0045378 100644
--- a/crates/ra_editor/src/lib.rs
+++ b/crates/ra_editor/src/lib.rs
@@ -35,7 +35,7 @@ pub use self::{
         flip_comma, add_derive, add_impl,
         introduce_variable,
     },
-    typing::{join_lines, on_eq_typed},
+    typing::{join_lines, on_eq_typed, on_enter},
     completion::{scope_completion, CompletionItem},
     folding_ranges::{Fold, FoldKind, folding_ranges}
 };
diff --git a/crates/ra_editor/src/typing.rs b/crates/ra_editor/src/typing.rs
index 512076941..3384389d1 100644
--- a/crates/ra_editor/src/typing.rs
+++ b/crates/ra_editor/src/typing.rs
@@ -4,7 +4,7 @@ use ra_syntax::{
     TextUnit, TextRange, SyntaxNodeRef, File, AstNode, SyntaxKind,
     ast,
     algo::{
-        find_covering_node,
+        find_covering_node, find_leaf_at_offset, LeafAtOffset,
     },
     text_utils::{intersect, contains_offset_nonstrict},
     SyntaxKind::*,
@@ -56,6 +56,58 @@ pub fn join_lines(file: &File, range: TextRange) -> LocalEdit {
     }
 }
 
+pub fn on_enter(file: &File, offset: TextUnit) -> Option<LocalEdit> {
+    let comment = find_leaf_at_offset(file.syntax(), offset).left_biased().filter(|it| it.kind() == COMMENT)?;
+    let prefix = comment_preffix(comment)?;
+    if offset < comment.range().start() + TextUnit::of_str(prefix) {
+        return None;
+    }
+
+    let indent = node_indent(file, comment)?;
+    let inserted = format!("\n{}{}", indent, prefix);
+    let cursor_position = offset + TextUnit::of_str(&inserted);
+    let mut edit = EditBuilder::new();
+    edit.insert(offset, inserted);
+    Some(LocalEdit {
+        edit: edit.finish(),
+        cursor_position: Some(cursor_position),
+    })
+}
+
+fn comment_preffix(comment: SyntaxNodeRef) -> Option<&'static str> {
+    let text = comment.leaf_text().unwrap();
+    let res = if text.starts_with("///") {
+        "/// "
+    } else if text.starts_with("//!") {
+        "//! "
+    } else if text.starts_with("//") {
+        "// "
+    } else {
+        return None;
+    };
+    Some(res)
+}
+
+fn node_indent<'a>(file: &'a File, node: SyntaxNodeRef) -> Option<&'a str> {
+    let ws = match find_leaf_at_offset(file.syntax(), node.range().start()) {
+        LeafAtOffset::Between(l, r) => {
+            assert!(r == node);
+            l
+        }
+        LeafAtOffset::Single(n) => {
+            assert!(n == node);
+            return Some("")
+        }
+        LeafAtOffset::None => unreachable!(),
+    };
+    if ws.kind() != WHITESPACE {
+        return None;
+    }
+    let text = ws.leaf_text().unwrap();
+    let pos = text.as_str().rfind('\n').map(|it| it + 1).unwrap_or(0);
+    Some(&text[pos..])
+}
+
 pub fn on_eq_typed(file: &File, offset: TextUnit) -> Option<LocalEdit> {
     let let_stmt: ast::LetStmt = find_node_at_offset(file.syntax(), offset)?;
     if let_stmt.has_semi() {
@@ -187,7 +239,7 @@ fn compute_ws(left: SyntaxNodeRef, right: SyntaxNodeRef) -> &'static str {
 #[cfg(test)]
 mod tests {
     use super::*;
-    use test_utils::{check_action, extract_range, extract_offset};
+    use test_utils::{check_action, extract_range, extract_offset, add_cursor};
 
     fn check_join_lines(before: &str, after: &str) {
         check_action(before, after, |file, offset| {
@@ -344,4 +396,49 @@ fn foo() {
         // }
         // ");
     }
+
+    #[test]
+    fn test_on_enter() {
+        fn apply_on_enter(before: &str) -> Option<String> {
+            let (offset, before) = extract_offset(before);
+            let file = File::parse(&before);
+            let result = on_enter(&file, offset)?;
+            let actual = result.edit.apply(&before);
+            let actual = add_cursor(&actual, result.cursor_position.unwrap());
+            Some(actual)
+        }
+
+        fn do_check(before: &str, after: &str) {
+            let actual = apply_on_enter(before).unwrap();
+            assert_eq_text!(after, &actual);
+        }
+
+        fn do_check_noop(text: &str) {
+            assert!(apply_on_enter(text).is_none())
+        }
+
+        do_check(r"
+/// Some docs<|>
+fn foo() {
+}
+", r"
+/// Some docs
+/// <|>
+fn foo() {
+}
+");
+        do_check(r"
+impl S {
+    /// Some<|> docs.
+    fn foo() {}
+}
+", r"
+impl S {
+    /// Some
+    /// <|> docs.
+    fn foo() {}
+}
+");
+        do_check_noop(r"<|>//! docz");
+    }
 }
diff --git a/crates/ra_lsp_server/src/conv.rs b/crates/ra_lsp_server/src/conv.rs
index 759e5e914..08a656569 100644
--- a/crates/ra_lsp_server/src/conv.rs
+++ b/crates/ra_lsp_server/src/conv.rs
@@ -190,9 +190,13 @@ impl TryConvWith for SourceChange {
             None => None,
             Some(pos) => {
                 let line_index = world.analysis().file_line_index(pos.file_id);
+                let edits = self.source_file_edits.iter().find(|it| it.file_id == pos.file_id)
+                    .map(|it| it.edits.as_slice()).unwrap_or(&[]);
+                let line_col = translate_offset_with_edit(&*line_index, pos.offset, edits);
+                let position = Position::new(line_col.line as u64, u32::from(line_col.col) as u64);
                 Some(TextDocumentPositionParams {
                     text_document: TextDocumentIdentifier::new(pos.file_id.try_conv_with(world)?),
-                    position: pos.offset.conv_with(&line_index),
+                    position,
                 })
             }
         };
@@ -207,6 +211,41 @@ impl TryConvWith for SourceChange {
     }
 }
 
+// HACK: we should translate offset to line/column using linde_index *with edits applied*.
+// A naive version of this function would be to apply `edits` to the original text,
+// construct a new line index and use that, but it would be slow.
+//
+// Writing fast & correct version is issue #105, let's use a quick hack in the meantime
+fn translate_offset_with_edit(
+    pre_edit_index: &LineIndex,
+    offset: TextUnit,
+    edits: &[AtomEdit],
+) -> LineCol {
+    let fallback = pre_edit_index.line_col(offset);
+    let edit = match edits.first() {
+        None => return fallback,
+        Some(edit) => edit
+    };
+    let end_offset = edit.delete.start() + TextUnit::of_str(&edit.insert);
+    if !(edit.delete.start() <= offset && offset <= end_offset) {
+        return fallback
+    }
+    let rel_offset = offset - edit.delete.start();
+    let in_edit_line_col = LineIndex::new(&edit.insert).line_col(rel_offset);
+    let edit_line_col = pre_edit_index.line_col(edit.delete.start());
+    if in_edit_line_col.line == 0 {
+        LineCol {
+            line: edit_line_col.line,
+            col: edit_line_col.col + in_edit_line_col.col,
+        }
+    } else {
+        LineCol {
+            line: edit_line_col.line + in_edit_line_col.line,
+            col: in_edit_line_col.col,
+        }
+    }
+}
+
 impl TryConvWith for SourceFileEdit {
     type Ctx = ServerWorld;
     type Output = TextDocumentEdit;
diff --git a/crates/ra_lsp_server/src/main_loop/handlers.rs b/crates/ra_lsp_server/src/main_loop/handlers.rs
index 79a54183e..725036cc7 100644
--- a/crates/ra_lsp_server/src/main_loop/handlers.rs
+++ b/crates/ra_lsp_server/src/main_loop/handlers.rs
@@ -77,6 +77,20 @@ pub fn handle_join_lines(
         .try_conv_with(&world)
 }
 
+pub fn handle_on_enter(
+    world: ServerWorld,
+    params: req::TextDocumentPositionParams,
+    _token: JobToken,
+) -> Result<Option<req::SourceChange>> {
+    let file_id = params.text_document.try_conv_with(&world)?;
+    let line_index = world.analysis().file_line_index(file_id);
+    let offset = params.position.conv_with(&line_index);
+    match world.analysis().on_enter(file_id, offset) {
+        None => Ok(None),
+        Some(edit) => Ok(Some(edit.try_conv_with(&world)?))
+    }
+}
+
 pub fn handle_on_type_formatting(
     world: ServerWorld,
     params: req::DocumentOnTypeFormattingParams,
diff --git a/crates/ra_lsp_server/src/main_loop/mod.rs b/crates/ra_lsp_server/src/main_loop/mod.rs
index 47a9b202e..53c6f1dff 100644
--- a/crates/ra_lsp_server/src/main_loop/mod.rs
+++ b/crates/ra_lsp_server/src/main_loop/mod.rs
@@ -244,6 +244,7 @@ fn on_request(
         .on::<req::ExtendSelection>(handlers::handle_extend_selection)?
         .on::<req::FindMatchingBrace>(handlers::handle_find_matching_brace)?
         .on::<req::JoinLines>(handlers::handle_join_lines)?
+        .on::<req::OnEnter>(handlers::handle_on_enter)?
         .on::<req::OnTypeFormatting>(handlers::handle_on_type_formatting)?
         .on::<req::DocumentSymbolRequest>(handlers::handle_document_symbol)?
         .on::<req::WorkspaceSymbol>(handlers::handle_workspace_symbol)?
diff --git a/crates/ra_lsp_server/src/req.rs b/crates/ra_lsp_server/src/req.rs
index 4af61dbbd..458c79ea9 100644
--- a/crates/ra_lsp_server/src/req.rs
+++ b/crates/ra_lsp_server/src/req.rs
@@ -119,6 +119,14 @@ pub struct JoinLinesParams {
     pub range: Range,
 }
 
+pub enum OnEnter {}
+
+impl Request for OnEnter {
+    type Params = TextDocumentPositionParams;
+    type Result = Option<SourceChange>;
+    const METHOD: &'static str = "m/onEnter";
+}
+
 pub enum Runnables {}
 
 impl Request for Runnables {