WIP: Add create_function assist

crates/ra_assists/src/handlers/create_function.rs

@@ -0,0 +1,313 @@
+use ra_syntax::{
+    ast::{self, AstNode},
+    SmolStr, SyntaxKind, SyntaxNode, TextUnit,
+};
+
+use crate::{Assist, AssistCtx, AssistId};
+use ast::{CallExpr, Expr};
+use ra_fmt::leading_indent;
+
+// Assist: create_function
+//
+// Creates a stub function with a signature matching the function under the cursor.
+//
+// ```
+// fn foo() {
+//      bar<|>("", baz());
+// }
+//
+// ```
+// ->
+// ```
+// fn foo() {
+//      bar("", baz());
+// }
+//
+// fn bar(arg_1: &str, baz: Baz) {
+//      todo!();
+// }
+//
+// ```
+pub(crate) fn create_function(ctx: AssistCtx) -> Option<Assist> {
+    let path: ast::Path = ctx.find_node_at_offset()?;
+
+    if ctx.sema.resolve_path(&path).is_some() {
+        // The function call already resolves, no need to create a function
+        return None;
+    }
+
+    if path.qualifier().is_some() {
+        return None;
+    }
+
+    let call: ast::CallExpr = ctx.find_node_at_offset()?;
+
+    let (generated_fn, cursor_pos, text_start) = generate_fn(&ctx, &call)?;
+
+    ctx.add_assist(AssistId("create_function"), "Create function", |edit| {
+        edit.target(call.syntax().text_range());
+
+        edit.set_cursor(cursor_pos);
+        edit.insert(text_start, generated_fn);
+    })
+}
+
+/// Generates a function definition that will allow `call` to compile.
+/// The function name must match.
+/// The arguments must have valid, nonconflicting names.
+/// The arguments' types must match those being passed to `call`.
+/// TODO generics?
+fn generate_fn(ctx: &AssistCtx, call: &CallExpr) -> Option<(String, TextUnit, TextUnit)> {
+    let (start, indent) = next_space_for_fn(&call)?;
+    let indent = if let Some(i) = &indent { i.as_str() } else { "" };
+    let mut fn_buf = String::with_capacity(128);
+
+    fn_buf.push_str("\n\n");
+    fn_buf.push_str(indent);
+    fn_buf.push_str("fn ");
+
+    let fn_name = fn_name(&call)?;
+    fn_buf.push_str(&fn_name);
+
+    let fn_generics = fn_generics(&call)?;
+    fn_buf.push_str(&fn_generics);
+
+    let fn_args = fn_args()?;
+    fn_buf.push_str(&fn_args);
+
+    fn_buf.push_str(" {\n");
+    fn_buf.push_str(indent);
+    fn_buf.push_str("    ");
+
+    // We take the offset here to put the cursor in front of the `todo` body
+    let offset = TextUnit::of_str(&fn_buf);
+
+    fn_buf.push_str("todo!()\n");
+    fn_buf.push_str(indent);
+    fn_buf.push_str("}");
+
+    let cursor_pos = start + offset;
+    Some((fn_buf, cursor_pos, start))
+}
+
+fn fn_name(call: &CallExpr) -> Option<String> {
+    Some(call.expr()?.syntax().to_string())
+}
+
+fn fn_generics(_call: &CallExpr) -> Option<String> {
+    // TODO
+    Some("".into())
+}
+
+fn fn_args() -> Option<String> {
+    Some("()".into())
+}
+
+/// Returns the position inside the current mod or file
+/// directly after the current block
+/// We want to write the generated function directly after
+/// fns, impls or macro calls, but inside mods
+fn next_space_for_fn(expr: &CallExpr) -> Option<(TextUnit, Option<SmolStr>)> {
+    let mut ancestors = expr.syntax().ancestors().peekable();
+    let mut last_ancestor: Option<SyntaxNode> = None;
+    while let Some(next_ancestor) = ancestors.next() {
+        match next_ancestor.kind() {
+            SyntaxKind::SOURCE_FILE => {
+                break;
+            }
+            SyntaxKind::ITEM_LIST => {
+                if ancestors.peek().map(|a| a.kind()) == Some(SyntaxKind::MODULE) {
+                    break;
+                }
+            }
+            _ => {}
+        }
+        last_ancestor = Some(next_ancestor);
+    }
+    last_ancestor.map(|a| (a.text_range().end(), leading_indent(&a)))
+}
+
+#[cfg(test)]
+mod tests {
+    use crate::helpers::{check_assist, check_assist_not_applicable};
+
+    use super::*;
+
+    #[test]
+    fn create_function_with_no_args() {
+        check_assist(
+            create_function,
+            r"
+fn foo() {
+    bar<|>();
+}
+",
+            r"
+fn foo() {
+    bar();
+}
+
+fn bar() {
+    <|>todo!()
+}
+",
+        )
+    }
+
+    #[test]
+    fn create_function_from_method() {
+        // This ensures that the function is correctly generated
+        // in the next outer mod or file
+        check_assist(
+            create_function,
+            r"
+impl Foo {
+    fn foo() {
+        bar<|>();
+    }
+}
+",
+            r"
+impl Foo {
+    fn foo() {
+        bar();
+    }
+}
+
+fn bar() {
+    <|>todo!()
+}
+",
+        )
+    }
+
+    #[test]
+    fn create_function_directly_after_current_block() {
+        // The new fn should not be created at the end of the file or module
+        check_assist(
+            create_function,
+            r"
+fn foo1() {
+    bar<|>();
+}
+
+fn foo2() {}
+",
+            r"
+fn foo1() {
+    bar();
+}
+
+fn bar() {
+    <|>todo!()
+}
+
+fn foo2() {}
+",
+        )
+    }
+
+    #[test]
+    fn create_function_with_no_args_in_same_module() {
+        check_assist(
+            create_function,
+            r"
+mod baz {
+    fn foo() {
+        bar<|>();
+    }
+}
+",
+            r"
+mod baz {
+    fn foo() {
+        bar();
+    }
+
+    fn bar() {
+        <|>todo!()
+    }
+}
+",
+        )
+    }
+
+    #[test]
+    fn create_function_with_function_call_arg() {
+        check_assist(
+            create_function,
+            r"
+fn baz() -> Baz { todo!() }
+fn foo() {
+    bar<|>(baz());
+}
+",
+            r"
+fn baz() -> Baz { todo!() }
+fn foo() {
+    bar(baz());
+}
+
+fn bar(baz: Baz) {
+    <|>todo!()
+}
+            ",
+        )
+    }
+
+    #[test]
+    fn create_function_not_applicable_if_function_already_exists() {
+        check_assist_not_applicable(
+            create_function,
+            r"
+fn foo() {
+    bar<|>();
+}
+
+fn bar() {}
+",
+        )
+    }
+
+    #[test]
+    fn create_function_not_applicable_if_function_path_not_singleton() {
+        // In the future this assist could be extended to generate functions
+        // if the path is in the same crate (or even the same workspace).
+        // For the beginning, I think this is fine.
+        check_assist_not_applicable(
+            create_function,
+            r"
+fn foo() {
+    other_crate::bar<|>();
+}
+        ",
+        )
+    }
+
+    #[test]
+    #[ignore]
+    fn create_method_with_no_args() {
+        check_assist(
+            create_function,
+            r"
+struct Foo;
+impl Foo {
+    fn foo(&self) {
+        self.bar()<|>;
+    }
+}
+        ",
+            r"
+struct Foo;
+impl Foo {
+    fn foo(&self) {
+        self.bar();
+    }
+    fn bar(&self) {
+        todo!();
+    }
+}
+        ",
+        )
+    }
+}

crates/ra_assists/src/lib.rs

@@ -102,6 +102,7 @@ mod handlers {
     mod apply_demorgan;
     mod auto_import;
     mod change_visibility;
+    mod create_function;
     mod early_return;
     mod fill_match_arms;
     mod flip_binexpr;
@@ -134,6 +135,7 @@ mod handlers {
             apply_demorgan::apply_demorgan,
             auto_import::auto_import,
             change_visibility::change_visibility,
+            create_function::create_function,
             early_return::convert_to_guarded_return,
             fill_match_arms::fill_match_arms,
             flip_binexpr::flip_binexpr,