initial version of extract function assist

there are a few currently limitations:
* no modifications of function body
* does not handle mutability and references
* no method support
* may produce incorrect results

crates/assists/src/handlers/extract_function.rs

@@ -0,0 +1,819 @@
+use either::Either;
+use hir::{HirDisplay, Local};
+use ide_db::defs::{Definition, NameRefClass};
+use rustc_hash::FxHashSet;
+use stdx::format_to;
+use syntax::{
+    ast::{
+        self,
+        edit::{AstNodeEdit, IndentLevel},
+        AstNode, NameOwner,
+    },
+    Direction, SyntaxElement,
+    SyntaxKind::{self, BLOCK_EXPR, BREAK_EXPR, COMMENT, PATH_EXPR, RETURN_EXPR},
+    SyntaxNode, TextRange,
+};
+use test_utils::mark;
+
+use crate::{
+    assist_context::{AssistContext, Assists},
+    AssistId,
+};
+
+// Assist: extract_function
+//
+// Extracts selected statements into new function.
+//
+// ```
+// fn main() {
+//     let n = 1;
+//     $0let m = n + 2;
+//     let k = m + n;$0
+//     let g = 3;
+// }
+// ```
+// ->
+// ```
+// fn main() {
+//     let n = 1;
+//     fun_name(n);
+//     let g = 3;
+// }
+//
+// fn $0fun_name(n: i32) {
+//     let m = n + 2;
+//     let k = m + n;
+// }
+// ```
+pub(crate) fn extract_function(acc: &mut Assists, ctx: &AssistContext) -> Option<()> {
+    if ctx.frange.range.is_empty() {
+        return None;
+    }
+
+    let node = ctx.covering_element();
+    if node.kind() == COMMENT {
+        mark::hit!(extract_function_in_comment_is_not_applicable);
+        return None;
+    }
+
+    let node = match node {
+        syntax::NodeOrToken::Node(n) => n,
+        syntax::NodeOrToken::Token(t) => t.parent(),
+    };
+
+    let mut body = None;
+    if node.text_range() == ctx.frange.range {
+        body = FunctionBody::from_whole_node(node.clone());
+    }
+    if body.is_none() && node.kind() == BLOCK_EXPR {
+        body = FunctionBody::from_range(&node, ctx.frange.range);
+    }
+    if body.is_none() {
+        body = FunctionBody::from_whole_node(node.clone());
+    }
+    if body.is_none() {
+        body = node.ancestors().find_map(FunctionBody::from_whole_node);
+    }
+    let body = body?;
+
+    let insert_after = body.scope_for_fn_insertion()?;
+
+    let module = ctx.sema.scope(&insert_after).module()?;
+
+    let expr = body.tail_expr();
+    let ret_ty = match expr {
+        Some(expr) => {
+            // TODO: can we do assist when type is unknown?
+            //       We can insert something like `-> ()`
+            let ty = ctx.sema.type_of_expr(&expr)?;
+            Some(ty.display_source_code(ctx.db(), module.into()).ok()?)
+        }
+        None => None,
+    };
+
+    let target_range = match &body {
+        FunctionBody::Expr(expr) => expr.syntax().text_range(),
+        FunctionBody::Span { .. } => ctx.frange.range,
+    };
+
+    let mut params = local_variables(&body, &ctx)
+        .into_iter()
+        .map(|node| node.source(ctx.db()))
+        .filter(|src| src.file_id.original_file(ctx.db()) == ctx.frange.file_id)
+        .map(|src| match src.value {
+            Either::Left(pat) => {
+                (pat.syntax().clone(), pat.name(), ctx.sema.type_of_pat(&pat.into()))
+            }
+            Either::Right(it) => (it.syntax().clone(), it.name(), ctx.sema.type_of_self(&it)),
+        })
+        .filter(|(node, _, _)| !body.contains_node(node))
+        .map(|(_, name, ty)| {
+            let ty = ty
+                .and_then(|ty| ty.display_source_code(ctx.db(), module.into()).ok())
+                .unwrap_or_else(|| "()".to_string());
+
+            let name = name.unwrap().to_string();
+
+            Param { name, ty }
+        })
+        .collect::<Vec<_>>();
+    deduplicate_params(&mut params);
+
+    acc.add(
+        AssistId("extract_function", crate::AssistKind::RefactorExtract),
+        "Extract into function",
+        target_range,
+        move |builder| {
+
+            let fun = Function { name: "fun_name".to_string(), params, ret_ty, body };
+
+            builder.replace(target_range, format_replacement(&fun));
+
+            let indent = IndentLevel::from_node(&insert_after);
+
+            let fn_def = format_function(&fun, indent);
+            let insert_offset = insert_after.text_range().end();
+            builder.insert(insert_offset, fn_def);
+        },
+    )
+}
+
+fn format_replacement(fun: &Function) -> String {
+    let mut buf = String::new();
+    format_to!(buf, "{}(", fun.name);
+    {
+        let mut it = fun.params.iter();
+        if let Some(param) = it.next() {
+            format_to!(buf, "{}", param.name);
+        }
+        for param in it {
+            format_to!(buf, ", {}", param.name);
+        }
+    }
+    format_to!(buf, ")");
+
+    if fun.has_unit_ret() {
+        format_to!(buf, ";");
+    }
+
+    buf
+}
+
+struct Function {
+    name: String,
+    params: Vec<Param>,
+    ret_ty: Option<String>,
+    body: FunctionBody,
+}
+
+impl Function {
+    fn has_unit_ret(&self) -> bool {
+        match &self.ret_ty {
+            Some(ty) => ty == "()",
+            None => true,
+        }
+    }
+}
+
+#[derive(Debug)]
+struct Param {
+    name: String,
+    ty: String,
+}
+
+fn format_function(fun: &Function, indent: IndentLevel) -> String {
+    let mut fn_def = String::new();
+    format_to!(fn_def, "\n\n{}fn $0{}(", indent, fun.name);
+    {
+        let mut it = fun.params.iter();
+        if let Some(param) = it.next() {
+            format_to!(fn_def, "{}: {}", param.name, param.ty);
+        }
+        for param in it {
+            format_to!(fn_def, ", {}: {}", param.name, param.ty);
+        }
+    }
+
+    format_to!(fn_def, ")");
+    if !fun.has_unit_ret() {
+        if let Some(ty) = &fun.ret_ty {
+            format_to!(fn_def, " -> {}", ty);
+        }
+    }
+    format_to!(fn_def, " {{");
+
+    match &fun.body {
+        FunctionBody::Expr(expr) => {
+            fn_def.push('\n');
+            let expr = expr.indent(indent);
+            format_to!(fn_def, "{}{}", indent + 1, expr.syntax());
+            fn_def.push('\n');
+        }
+        FunctionBody::Span { elements, leading_indent } => {
+            format_to!(fn_def, "{}", leading_indent);
+            for e in elements {
+                format_to!(fn_def, "{}", e);
+            }
+            if !fn_def.ends_with('\n') {
+                fn_def.push('\n');
+            }
+        }
+    }
+    format_to!(fn_def, "{}}}", indent);
+
+    fn_def
+}
+
+#[derive(Debug)]
+enum FunctionBody {
+    Expr(ast::Expr),
+    Span { elements: Vec<SyntaxElement>, leading_indent: String },
+}
+
+impl FunctionBody {
+    fn from_whole_node(node: SyntaxNode) -> Option<Self> {
+        match node.kind() {
+            PATH_EXPR => None,
+            BREAK_EXPR => ast::BreakExpr::cast(node).and_then(|e| e.expr()).map(Self::Expr),
+            RETURN_EXPR => ast::ReturnExpr::cast(node).and_then(|e| e.expr()).map(Self::Expr),
+            BLOCK_EXPR => ast::BlockExpr::cast(node)
+                .filter(|it| it.is_standalone())
+                .map(Into::into)
+                .map(Self::Expr),
+            _ => ast::Expr::cast(node).map(Self::Expr),
+        }
+    }
+
+    fn from_range(node: &SyntaxNode, range: TextRange) -> Option<FunctionBody> {
+        let mut first = node.token_at_offset(range.start()).left_biased()?;
+        let last = node.token_at_offset(range.end()).right_biased()?;
+
+        let mut leading_indent = String::new();
+
+        let leading_trivia = first
+            .siblings_with_tokens(Direction::Prev)
+            .skip(1)
+            .take_while(|e| e.kind() == SyntaxKind::WHITESPACE && e.as_token().is_some());
+
+        for e in leading_trivia {
+            let token = e.as_token().unwrap();
+            let text = token.text();
+            match text.rfind('\n') {
+                Some(pos) => {
+                    leading_indent = text[pos..].to_owned();
+                    break;
+                }
+                None => first = token.clone(),
+            }
+        }
+
+        let mut elements: Vec<_> = first
+            .siblings_with_tokens(Direction::Next)
+            .take_while(|e| e.as_token() != Some(&last))
+            .collect();
+
+        if !(last.kind() == SyntaxKind::WHITESPACE && last.text().lines().count() <= 2) {
+            elements.push(last.into());
+        }
+
+        Some(FunctionBody::Span { elements, leading_indent })
+    }
+
+    fn tail_expr(&self) -> Option<ast::Expr> {
+        match &self {
+            FunctionBody::Expr(expr) => Some(expr.clone()),
+            FunctionBody::Span { elements, .. } => {
+                elements.iter().rev().find_map(|e| e.as_node()).cloned().and_then(ast::Expr::cast)
+            }
+        }
+    }
+
+    fn scope_for_fn_insertion(&self) -> Option<SyntaxNode> {
+        match self {
+            FunctionBody::Expr(e) => scope_for_fn_insertion(e.syntax()),
+            FunctionBody::Span { elements, .. } => {
+                let node = elements.iter().find_map(|e| e.as_node())?;
+                scope_for_fn_insertion(&node)
+            }
+        }
+    }
+
+    fn descendants(&self) -> impl Iterator<Item = SyntaxNode> + '_ {
+        match self {
+            FunctionBody::Expr(expr) => Either::Right(expr.syntax().descendants()),
+            FunctionBody::Span { elements, .. } => Either::Left(
+                elements
+                    .iter()
+                    .filter_map(SyntaxElement::as_node)
+                    .flat_map(SyntaxNode::descendants),
+            ),
+        }
+    }
+
+    fn contains_node(&self, node: &SyntaxNode) -> bool {
+        fn is_node(body: &FunctionBody, n: &SyntaxNode) -> bool {
+            match body {
+                FunctionBody::Expr(expr) => n == expr.syntax(),
+                FunctionBody::Span { elements, .. } => {
+                    // FIXME: can it be quadratic?
+                    elements.iter().filter_map(SyntaxElement::as_node).any(|e| e == n)
+                }
+            }
+        }
+
+        node.ancestors().any(|a| is_node(self, &a))
+    }
+}
+
+fn scope_for_fn_insertion(node: &SyntaxNode) -> Option<SyntaxNode> {
+    let mut ancestors = node.ancestors().peekable();
+    let mut last_ancestor = 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
+}
+
+fn deduplicate_params(params: &mut Vec<Param>) {
+    let mut seen_params = FxHashSet::default();
+    params.retain(|p| seen_params.insert(p.name.clone()));
+}
+
+/// Returns a vector of local variables that are refferenced in `body`
+fn local_variables(body: &FunctionBody, ctx: &AssistContext) -> Vec<Local> {
+    body
+        .descendants()
+        .filter_map(ast::NameRef::cast)
+        .filter_map(|name_ref| NameRefClass::classify(&ctx.sema, &name_ref))
+        .map(|name_kind| name_kind.referenced(ctx.db()))
+        .filter_map(|definition| match definition {
+            Definition::Local(local) => Some(local),
+            _ => None,
+        })
+        .collect()
+}
+
+#[cfg(test)]
+mod tests {
+    use crate::tests::{check_assist, check_assist_not_applicable};
+
+    use super::*;
+
+    #[test]
+    fn no_args_from_binary_expr() {
+        check_assist(
+            extract_function,
+            r#"
+fn foo() {
+    foo($01 + 1$0);
+}"#,
+            r#"
+fn foo() {
+    foo(fun_name());
+}
+
+fn $0fun_name() -> i32 {
+    1 + 1
+}"#,
+        );
+    }
+    
+    #[test]
+    fn no_args_from_binary_expr_in_module() {
+        check_assist(
+            extract_function,
+            r#"
+mod bar {
+    fn foo() {
+        foo($01 + 1$0);
+    }
+}"#,
+            r#"
+mod bar {
+    fn foo() {
+        foo(fun_name());
+    }
+
+    fn $0fun_name() -> i32 {
+        1 + 1
+    }
+}"#,
+        );
+    }
+
+    #[test]
+    fn no_args_from_binary_expr_indented() {
+        check_assist(
+            extract_function,
+            r#"
+fn foo() {
+    $0{ 1 + 1 }$0;
+}"#,
+            r#"
+fn foo() {
+    fun_name();
+}
+
+fn $0fun_name() -> i32 {
+    { 1 + 1 }
+}"#,
+        );
+    }
+
+    #[test]
+    fn no_args_from_stmt_with_last_expr() {
+        check_assist(
+            extract_function,
+            r#"
+fn foo() -> i32 {
+    let k = 1;
+    $0let m = 1;
+    m + 1$0
+}"#,
+            r#"
+fn foo() -> i32 {
+    let k = 1;
+    fun_name()
+}
+
+fn $0fun_name() -> i32 {
+    let m = 1;
+    m + 1
+}"#,
+        );
+    }
+
+    #[test]
+    fn no_args_from_stmt_unit() {
+        check_assist(
+            extract_function,
+            r#"
+fn foo() {
+    let k = 3;
+    $0let m = 1;
+    let n = m + 1;$0
+    let g = 5;
+}"#,
+            r#"
+fn foo() {
+    let k = 3;
+    fun_name();
+    let g = 5;
+}
+
+fn $0fun_name() {
+    let m = 1;
+    let n = m + 1;
+}"#,
+        );
+    }
+
+    #[test]
+    fn no_args_from_loop_unit() {
+        check_assist(
+            extract_function,
+            r#"
+fn foo() {
+    $0loop {
+        let m = 1;
+    }$0
+}"#,
+            r#"
+fn foo() {
+    fun_name()
+}
+
+fn $0fun_name() -> ! {
+    loop {
+        let m = 1;
+    }
+}"#,
+        );
+    }
+
+    #[test]
+    fn no_args_from_loop_with_return() {
+        check_assist(
+            extract_function,
+            r#"
+fn foo() {
+    let v = $0loop {
+        let m = 1;
+        break m;
+    }$0;
+}"#,
+            r#"
+fn foo() {
+    let v = fun_name();
+}
+
+fn $0fun_name() -> i32 {
+    loop {
+        let m = 1;
+        break m;
+    }
+}"#,
+        );
+    }
+
+    #[test]
+    fn no_args_from_match() {
+        check_assist(
+            extract_function,
+            r#"
+fn foo() {
+    let v: i32 = $0match Some(1) {
+        Some(x) => x,
+        None => 0,
+    }$0;
+}"#,
+            r#"
+fn foo() {
+    let v: i32 = fun_name();
+}
+
+fn $0fun_name() -> i32 {
+    match Some(1) {
+        Some(x) => x,
+        None => 0,
+    }
+}"#,
+        );
+    }
+
+    #[test]
+    fn argument_form_expr() {
+        check_assist(
+            extract_function,
+            r"
+fn foo() -> u32 {
+    let n = 2;
+    $0n+2$0
+}",
+            r"
+fn foo() -> u32 {
+    let n = 2;
+    fun_name(n)
+}
+
+fn $0fun_name(n: u32) -> u32 {
+    n+2
+}",
+        )
+    }
+
+    #[test]
+    fn argument_used_twice_form_expr() {
+        check_assist(
+            extract_function,
+            r"
+fn foo() -> u32 {
+    let n = 2;
+    $0n+n$0
+}",
+            r"
+fn foo() -> u32 {
+    let n = 2;
+    fun_name(n)
+}
+
+fn $0fun_name(n: u32) -> u32 {
+    n+n
+}",
+        )
+    }
+
+    #[test]
+    fn two_arguments_form_expr() {
+        check_assist(
+            extract_function,
+            r"
+fn foo() -> u32 {
+    let n = 2;
+    let m = 3;
+    $0n+n*m$0
+}",
+            r"
+fn foo() -> u32 {
+    let n = 2;
+    let m = 3;
+    fun_name(n, m)
+}
+
+fn $0fun_name(n: u32, m: u32) -> u32 {
+    n+n*m
+}",
+        )
+    }
+
+    #[test]
+    fn argument_and_locals() {
+        check_assist(
+            extract_function,
+            r"
+fn foo() -> u32 {
+    let n = 2;
+    $0let m = 1;
+    n + m$0
+}",
+            r"
+fn foo() -> u32 {
+    let n = 2;
+    fun_name(n)
+}
+
+fn $0fun_name(n: u32) -> u32 {
+    let m = 1;
+    n + m
+}",
+        )
+    }
+
+    #[test]
+    fn in_comment_is_not_applicable() {
+        mark::check!(extract_function_in_comment_is_not_applicable);
+        check_assist_not_applicable(extract_function, r"fn main() { 1 + /* $0comment$0 */ 1; }");
+    }
+
+    #[test]
+    fn part_of_expr_stmt() {
+        check_assist(
+            extract_function,
+            "
+fn foo() {
+    $01$0 + 1;
+}",
+            "
+fn foo() {
+    fun_name() + 1;
+}
+
+fn $0fun_name() -> i32 {
+    1
+}",
+        );
+    }
+
+    #[test]
+    fn function_expr() {
+        check_assist(
+            extract_function,
+            r#"
+fn foo() {
+    $0bar(1 + 1)$0
+}"#,
+            r#"
+fn foo() {
+    fun_name();
+}
+
+fn $0fun_name() {
+    bar(1 + 1)
+}"#,
+        )
+    }
+
+    #[test]
+    fn extract_from_nested() {
+        check_assist(
+            extract_function,
+            r"
+fn main() {
+    let x = true;
+    let tuple = match x {
+        true => ($02 + 2$0, true)
+        _ => (0, false)
+    };
+}",
+            r"
+fn main() {
+    let x = true;
+    let tuple = match x {
+        true => (fun_name(), true)
+        _ => (0, false)
+    };
+}
+
+fn $0fun_name() -> i32 {
+    2 + 2
+}",
+        );
+    }
+
+    #[test]
+    fn param_from_closure() {
+        check_assist(
+            extract_function,
+            r"
+fn main() {
+    let lambda = |x: u32| $0x * 2$0;
+}",
+            r"
+fn main() {
+    let lambda = |x: u32| fun_name(x);
+}
+
+fn $0fun_name(x: u32) -> u32 {
+    x * 2
+}",
+        );
+    }
+
+    #[test]
+    fn extract_return_stmt() {
+        check_assist(
+            extract_function,
+            r"
+fn foo() -> u32 {
+    $0return 2 + 2$0;
+}",
+            r"
+fn foo() -> u32 {
+    return fun_name();
+}
+
+fn $0fun_name() -> u32 {
+    2 + 2
+}",
+        );
+    }
+
+    #[test]
+    fn does_not_add_extra_whitespace() {
+        check_assist(
+            extract_function,
+            r"
+fn foo() -> u32 {
+
+
+    $0return 2 + 2$0;
+}",
+            r"
+fn foo() -> u32 {
+
+
+    return fun_name();
+}
+
+fn $0fun_name() -> u32 {
+    2 + 2
+}",
+        );
+    }
+
+    #[test]
+    fn break_stmt() {
+        check_assist(
+            extract_function,
+            r"
+fn main() {
+    let result = loop {
+        $0break 2 + 2$0;
+    };
+}",
+            r"
+fn main() {
+    let result = loop {
+        break fun_name();
+    };
+}
+
+fn $0fun_name() -> i32 {
+    2 + 2
+}",
+        );
+    }
+
+    #[test]
+    fn extract_cast() {
+        check_assist(
+            extract_function,
+            r"
+fn main() {
+    let v = $00f32 as u32$0;
+}",
+            r"
+fn main() {
+    let v = fun_name();
+}
+
+fn $0fun_name() -> u32 {
+    0f32 as u32
+}",
+        );
+    }
+
+    #[test]
+    fn return_not_applicable() {
+        check_assist_not_applicable(extract_function, r"fn foo() { $0return$0; } ");
+    }
+}

crates/assists/src/lib.rs

@@ -117,6 +117,7 @@ mod handlers {
     mod convert_integer_literal;
     mod early_return;
     mod expand_glob_import;
+    mod extract_function;
     mod extract_struct_from_enum_variant;
     mod extract_variable;
     mod fill_match_arms;
@@ -174,6 +175,7 @@ mod handlers {
             early_return::convert_to_guarded_return,
             expand_glob_import::expand_glob_import,
             move_module_to_file::move_module_to_file,
+            extract_function::extract_function,
             extract_struct_from_enum_variant::extract_struct_from_enum_variant,
             extract_variable::extract_variable,
             fill_match_arms::fill_match_arms,

crates/assists/src/tests/generated.rs

@@ -256,6 +256,33 @@ fn qux(bar: Bar, baz: Baz) {}
     )
 }
 
+#[test]
+fn doctest_extract_function() {
+    check_doc_test(
+        "extract_function",
+        r#####"
+fn main() {
+    let n = 1;
+    $0let m = n + 2;
+    let k = m + n;$0
+    let g = 3;
+}
+"#####,
+        r#####"
+fn main() {
+    let n = 1;
+    fun_name(n);
+    let g = 3;
+}
+
+fn $0fun_name(n: i32) {
+    let m = n + 2;
+    let k = m + n;
+}
+"#####,
+    )
+}
+
 #[test]
 fn doctest_extract_struct_from_enum_variant() {
     check_doc_test(