6 files changed, 238 insertions, 3 deletions

diff --git a/crates/ra_assists/Cargo.toml b/crates/ra_assists/Cargo.toml
index d4056a349..29d9ceb59 100644
--- a/crates/ra_assists/Cargo.toml
+++ b/crates/ra_assists/Cargo.toml
@@ -5,8 +5,10 @@ version = "0.1.0"
 authors = ["rust-analyzer developers"]
 
 [dependencies]
+lazy_static = "1.3.0"
 join_to_string = "0.1.3"
 itertools = "0.8.0"
+arrayvec = "0.4.10"
 
 ra_syntax = { path = "../ra_syntax" }
 ra_text_edit = { path = "../ra_text_edit" }
diff --git a/crates/ra_assists/src/ast_editor.rs b/crates/ra_assists/src/ast_editor.rs
new file mode 100644
index 000000000..7347c1738
--- /dev/null
+++ b/crates/ra_assists/src/ast_editor.rs
@@ -0,0 +1,153 @@
+use arrayvec::ArrayVec;
+use ra_text_edit::{TextEdit, TextEditBuilder};
+use ra_syntax::{AstNode, TreeArc, ast, SyntaxKind::*, SyntaxElement, SourceFile, InsertPosition, Direction};
+
+pub struct AstEditor<N: AstNode> {
+    original_ast: TreeArc<N>,
+    ast: TreeArc<N>,
+}
+
+impl<N: AstNode> AstEditor<N> {
+    pub fn new(node: &N) -> AstEditor<N> {
+        AstEditor { original_ast: node.to_owned(), ast: node.to_owned() }
+    }
+
+    pub fn into_text_edit(self) -> TextEdit {
+        // FIXME: compute a more fine-grained diff here.
+        // If *you* know a nice algorithm to compute diff between two syntax
+        // tree, tell me about it!
+        let mut builder = TextEditBuilder::default();
+        builder.replace(self.original_ast.syntax().range(), self.ast().syntax().text().to_string());
+        builder.finish()
+    }
+
+    pub fn ast(&self) -> &N {
+        &*self.ast
+    }
+}
+
+impl AstEditor<ast::NamedFieldList> {
+    pub fn append_field(&mut self, field: &ast::NamedField) {
+        self.insert_field(InsertPosition::Last, field)
+    }
+
+    pub fn insert_field(
+        &mut self,
+        position: InsertPosition<&'_ ast::NamedField>,
+        field: &ast::NamedField,
+    ) {
+        let mut to_insert: ArrayVec<[SyntaxElement; 2]> =
+            [field.syntax().into(), tokens::comma().into()].into();
+        let position = match position {
+            InsertPosition::First => {
+                let anchor = match self
+                    .ast()
+                    .syntax()
+                    .children_with_tokens()
+                    .find(|it| it.kind() == L_CURLY)
+                {
+                    Some(it) => it,
+                    None => return,
+                };
+                InsertPosition::After(anchor)
+            }
+            InsertPosition::Last => {
+                let anchor = match self
+                    .ast()
+                    .syntax()
+                    .children_with_tokens()
+                    .find(|it| it.kind() == R_CURLY)
+                {
+                    Some(it) => it,
+                    None => return,
+                };
+                InsertPosition::Before(anchor)
+            }
+            InsertPosition::Before(anchor) => InsertPosition::Before(anchor.syntax().into()),
+            InsertPosition::After(anchor) => {
+                if let Some(comma) = anchor
+                    .syntax()
+                    .siblings_with_tokens(Direction::Next)
+                    .find(|it| it.kind() == COMMA)
+                {
+                    InsertPosition::After(comma)
+                } else {
+                    to_insert.insert(0, tokens::comma().into());
+                    InsertPosition::After(anchor.syntax().into())
+                }
+            }
+        };
+        self.ast = insert_children_into_ast(self.ast(), position, to_insert.iter().cloned());
+    }
+}
+
+fn insert_children_into_ast<'a, N: AstNode>(
+    node: &N,
+    position: InsertPosition<SyntaxElement<'_>>,
+    to_insert: impl Iterator<Item = SyntaxElement<'a>>,
+) -> TreeArc<N> {
+    let new_syntax = node.syntax().insert_children(position, to_insert);
+    N::cast(&new_syntax).unwrap().to_owned()
+}
+
+pub struct AstBuilder<N: AstNode> {
+    _phantom: std::marker::PhantomData<N>,
+}
+
+impl AstBuilder<ast::NamedField> {
+    pub fn from_text(text: &str) -> TreeArc<ast::NamedField> {
+        ast_node_from_file_text(&format!("fn f() {{ S {{ {}, }} }}", text))
+    }
+}
+
+fn ast_node_from_file_text<N: AstNode>(text: &str) -> TreeArc<N> {
+    let file = SourceFile::parse(text);
+    let res = file.syntax().descendants().find_map(N::cast).unwrap().to_owned();
+    res
+}
+
+mod tokens {
+    use lazy_static::lazy_static;
+    use ra_syntax::{AstNode, SourceFile, TreeArc, SyntaxToken, SyntaxKind::*};
+
+    lazy_static! {
+        static ref SOURCE_FILE: TreeArc<SourceFile> = SourceFile::parse(",");
+    }
+
+    pub(crate) fn comma() -> SyntaxToken<'static> {
+        SOURCE_FILE
+            .syntax()
+            .descendants_with_tokens()
+            .filter_map(|it| it.as_token())
+            .find(|it| it.kind() == COMMA)
+            .unwrap()
+    }
+}
+
+#[cfg(test)]
+mod tests {
+    use super::*;
+
+    use ra_syntax::SourceFile;
+
+    #[test]
+    fn structure_editing() {
+        let file = SourceFile::parse(
+            "\
+fn foo() {
+    let s = S {
+        original: 92,
+    }
+}
+",
+        );
+        let field_list = file.syntax().descendants().find_map(ast::NamedFieldList::cast).unwrap();
+        let mut editor = AstEditor::new(field_list);
+
+        let field = AstBuilder::<ast::NamedField>::from_text("first_inserted: 1");
+        editor.append_field(&field);
+        let field = AstBuilder::<ast::NamedField>::from_text("second_inserted: 2");
+        editor.append_field(&field);
+        eprintln!("{}", editor.ast().syntax());
+    }
+}
diff --git a/crates/ra_assists/src/lib.rs b/crates/ra_assists/src/lib.rs
index ded401b63..3151b1c44 100644
--- a/crates/ra_assists/src/lib.rs
+++ b/crates/ra_assists/src/lib.rs
@@ -7,6 +7,7 @@
 
 mod assist_ctx;
 mod marks;
+pub mod ast_editor;
 
 use itertools::Itertools;
 
diff --git a/crates/ra_syntax/src/lib.rs b/crates/ra_syntax/src/lib.rs
index a6ce14f06..9cb66b76b 100644
--- a/crates/ra_syntax/src/lib.rs
+++ b/crates/ra_syntax/src/lib.rs
@@ -38,7 +38,7 @@ pub use crate::{
     ast::AstNode,
     syntax_error::{SyntaxError, SyntaxErrorKind, Location},
     syntax_text::SyntaxText,
-    syntax_node::{Direction,  SyntaxNode, WalkEvent, TreeArc, SyntaxTreeBuilder, SyntaxElement, SyntaxToken},
+    syntax_node::{Direction,  SyntaxNode, WalkEvent, TreeArc, SyntaxTreeBuilder, SyntaxElement, SyntaxToken, InsertPosition},
     ptr::{SyntaxNodePtr, AstPtr},
     parsing::{tokenize, classify_literal, Token},
 };
diff --git a/crates/ra_syntax/src/ptr.rs b/crates/ra_syntax/src/ptr.rs
index 15a8b94cd..b0816b135 100644
--- a/crates/ra_syntax/src/ptr.rs
+++ b/crates/ra_syntax/src/ptr.rs
@@ -10,7 +10,7 @@ use crate::{
 /// specific node across reparses of the same file.
 #[derive(Debug, Clone, Copy, PartialEq, Eq, Hash)]
 pub struct SyntaxNodePtr {
-    range: TextRange,
+    pub(crate) range: TextRange,
     kind: SyntaxKind,
 }
 
diff --git a/crates/ra_syntax/src/syntax_node.rs b/crates/ra_syntax/src/syntax_node.rs
index dc2352c76..d9591781d 100644
--- a/crates/ra_syntax/src/syntax_node.rs
+++ b/crates/ra_syntax/src/syntax_node.rs
@@ -17,13 +17,20 @@ use ra_parser::ParseError;
 use rowan::{TransparentNewType, GreenNodeBuilder};
 
 use crate::{
-    SmolStr, SyntaxKind, TextUnit, TextRange, SyntaxText, SourceFile, AstNode,
+    SmolStr, SyntaxKind, TextUnit, TextRange, SyntaxText, SourceFile, AstNode, SyntaxNodePtr,
     syntax_error::{SyntaxError, SyntaxErrorKind},
 };
 
 pub use rowan::WalkEvent;
 pub(crate) use rowan::{GreenNode, GreenToken};
 
+pub enum InsertPosition<T> {
+    First,
+    Last,
+    Before(T),
+    After(T),
+}
+
 /// Marker trait for CST and AST nodes
 pub trait SyntaxNodeWrapper: TransparentNewType<Repr = rowan::SyntaxNode> {}
 impl<T: TransparentNewType<Repr = rowan::SyntaxNode>> SyntaxNodeWrapper for T {}
@@ -309,6 +316,71 @@ impl SyntaxNode {
     pub(crate) fn replace_with(&self, replacement: GreenNode) -> GreenNode {
         self.0.replace_with(replacement)
     }
+
+    /// Adds specified children (tokens or nodes) to the current node at the
+    /// specific position.
+    ///
+    /// This is a type-unsafe low-level editing API, if you need to use it,
+    /// prefer to create a type-safe abstraction on top of it instead.
+    ///
+    ///
+    pub fn insert_children<'a>(
+        &self,
+        position: InsertPosition<SyntaxElement<'_>>,
+        to_insert: impl Iterator<Item = SyntaxElement<'a>>,
+    ) -> TreeArc<SyntaxNode> {
+        let mut delta = TextUnit::default();
+        let to_insert = to_insert.map(|element| {
+            delta += element.text_len();
+            to_green_element(element)
+        });
+
+        let old_children = self.0.green().children();
+
+        let get_anchor_pos = |anchor: SyntaxElement| -> usize {
+            self.children_with_tokens()
+                .position(|it| it == anchor)
+                .expect("anchor is not a child of current element")
+        };
+
+        let new_children = match position {
+            InsertPosition::First => {
+                to_insert.chain(old_children.iter().cloned()).collect::<Box<[_]>>()
+            }
+            InsertPosition::Last => {
+                old_children.iter().cloned().chain(to_insert).collect::<Box<[_]>>()
+            }
+            InsertPosition::Before(anchor) | InsertPosition::After(anchor) => {
+                let take_anchor = if let InsertPosition::After(_) = position { 1 } else { 0 };
+                let (before, after) = old_children.split_at(get_anchor_pos(anchor) + take_anchor);
+                before
+                    .iter()
+                    .cloned()
+                    .chain(to_insert)
+                    .chain(after.iter().cloned())
+                    .collect::<Box<[_]>>()
+            }
+        };
+
+        let new_node = GreenNode::new(rowan::SyntaxKind(self.kind() as u16), new_children);
+        let new_file_node = self.replace_with(new_node);
+        let file = SourceFile::new(new_file_node, Vec::new());
+
+        // FIXME: use a more elegant way to re-fetch the node (#1185), make
+        // `range` private afterwards
+        let mut ptr = SyntaxNodePtr::new(self);
+        ptr.range = TextRange::from_to(ptr.range().start(), ptr.range().end() + delta);
+        return ptr.to_node(&file).to_owned();
+
+        fn to_green_element(element: SyntaxElement) -> rowan::GreenElement {
+            match element {
+                SyntaxElement::Node(node) => node.0.green().clone().into(),
+                SyntaxElement::Token(tok) => {
+                    GreenToken::new(rowan::SyntaxKind(tok.kind() as u16), tok.text().clone()).into()
+                }
+            }
+        }
+    }
 }
 
 #[derive(Clone, Copy, PartialEq, Eq, Hash)]
@@ -451,6 +523,13 @@ impl<'a> SyntaxElement<'a> {
         }
         .ancestors()
     }
+
+    fn text_len(&self) -> TextUnit {
+        match self {
+            SyntaxElement::Node(node) => node.0.green().text_len(),
+            SyntaxElement::Token(token) => TextUnit::of_str(token.0.text()),
+        }
+    }
 }
 
 impl<'a> From<rowan::SyntaxElement<'a>> for SyntaxElement<'a> {