4 files changed, 862 insertions, 0 deletions

diff --git a/crates/ra_assists/src/doc_tests/generated.rs b/crates/ra_assists/src/doc_tests/generated.rs
index 0848ab6bc..64444ee3a 100644
--- a/crates/ra_assists/src/doc_tests/generated.rs
+++ b/crates/ra_assists/src/doc_tests/generated.rs
@@ -59,6 +59,33 @@ fn main() {
 }
 
 #[test]
+fn doctest_add_function() {
+    check(
+        "add_function",
+        r#####"
+struct Baz;
+fn baz() -> Baz { Baz }
+fn foo() {
+     bar<|>("", baz());
+}
+
+"#####,
+        r#####"
+struct Baz;
+fn baz() -> Baz { Baz }
+fn foo() {
+     bar("", baz());
+}
+
+fn bar(arg: &str, baz: Baz) {
+    unimplemented!()
+}
+
+"#####,
+    )
+}
+
+#[test]
 fn doctest_add_hash() {
     check(
         "add_hash",
diff --git a/crates/ra_assists/src/handlers/add_function.rs b/crates/ra_assists/src/handlers/add_function.rs
new file mode 100644
index 000000000..f6721d9df
--- /dev/null
+++ b/crates/ra_assists/src/handlers/add_function.rs
@@ -0,0 +1,807 @@
+use ra_syntax::{
+    ast::{self, AstNode},
+    SmolStr, SyntaxKind, SyntaxNode, TextUnit,
+};
+
+use crate::{Assist, AssistCtx, AssistId};
+use ast::{ArgListOwner, CallExpr, Expr};
+use hir::HirDisplay;
+use ra_fmt::leading_indent;
+use rustc_hash::{FxHashMap, FxHashSet};
+
+// Assist: add_function
+//
+// Adds a stub function with a signature matching the function under the cursor.
+//
+// ```
+// struct Baz;
+// fn baz() -> Baz { Baz }
+// fn foo() {
+//      bar<|>("", baz());
+// }
+//
+// ```
+// ->
+// ```
+// struct Baz;
+// fn baz() -> Baz { Baz }
+// fn foo() {
+//      bar("", baz());
+// }
+//
+// fn bar(arg: &str, baz: Baz) {
+//     unimplemented!()
+// }
+//
+// ```
+pub(crate) fn add_function(ctx: AssistCtx) -> Option<Assist> {
+    let path_expr: ast::PathExpr = ctx.find_node_at_offset()?;
+    let call = path_expr.syntax().parent().and_then(ast::CallExpr::cast)?;
+    let path = path_expr.path()?;
+
+    if path.qualifier().is_some() {
+        return None;
+    }
+
+    if ctx.sema.resolve_path(&path).is_some() {
+        // The function call already resolves, no need to add a function
+        return None;
+    }
+
+    let function_builder = FunctionBuilder::from_call(&ctx, &call)?;
+
+    ctx.add_assist(AssistId("add_function"), "Add function", |edit| {
+        edit.target(call.syntax().text_range());
+
+        let function_template = function_builder.render();
+        edit.set_cursor(function_template.cursor_offset);
+        edit.insert(function_template.insert_offset, function_template.fn_text);
+    })
+}
+
+struct FunctionTemplate {
+    insert_offset: TextUnit,
+    cursor_offset: TextUnit,
+    fn_text: String,
+}
+
+struct FunctionBuilder {
+    start_offset: TextUnit,
+    fn_name: String,
+    fn_generics: String,
+    fn_args: String,
+    indent: String,
+}
+
+impl FunctionBuilder {
+    fn from_call(ctx: &AssistCtx, call: &ast::CallExpr) -> Option<Self> {
+        let (start, indent) = next_space_for_fn(&call)?;
+        let fn_name = fn_name(&call)?;
+        let fn_generics = fn_generics(&call)?;
+        let fn_args = fn_args(ctx, &call)?;
+        let indent = if let Some(i) = &indent { i.to_string() } else { String::new() };
+        Some(Self { start_offset: start, fn_name, fn_generics, fn_args, indent })
+    }
+    fn render(&self) -> FunctionTemplate {
+        let mut fn_buf = String::with_capacity(128);
+        fn_buf.push_str("\n\n");
+        fn_buf.push_str(&self.indent);
+        fn_buf.push_str("fn ");
+        fn_buf.push_str(&self.fn_name);
+        fn_buf.push_str(&self.fn_generics);
+        fn_buf.push_str(&self.fn_args);
+        fn_buf.push_str(" {\n");
+        fn_buf.push_str(&self.indent);
+        fn_buf.push_str("    ");
+
+        // We take the offset here to put the cursor in front of the `unimplemented!()` body
+        let offset = TextUnit::of_str(&fn_buf);
+
+        fn_buf.push_str("unimplemented!()\n");
+        fn_buf.push_str(&self.indent);
+        fn_buf.push_str("}");
+
+        let cursor_pos = self.start_offset + offset;
+        FunctionTemplate {
+            fn_text: fn_buf,
+            cursor_offset: cursor_pos,
+            insert_offset: self.start_offset,
+        }
+    }
+}
+
+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(ctx: &AssistCtx, call: &CallExpr) -> Option<String> {
+    let mut arg_names = Vec::new();
+    let mut arg_types = Vec::new();
+    for arg in call.arg_list()?.args() {
+        let arg_name = match fn_arg_name(&arg) {
+            Some(name) => name,
+            None => String::from("arg"),
+        };
+        arg_names.push(arg_name);
+        arg_types.push(match fn_arg_type(ctx, &arg) {
+            Some(ty) => ty,
+            None => String::from("()"),
+        });
+    }
+    deduplicate_arg_names(&mut arg_names);
+    Some(format!(
+        "({})",
+        arg_names
+            .into_iter()
+            .zip(arg_types)
+            .map(|(name, ty)| format!("{}: {}", name, ty))
+            .collect::<Vec<_>>()
+            .join(", ")
+    ))
+}
+
+/// Makes duplicate argument names unique by appending incrementing numbers.
+///
+/// ```
+/// let mut names: Vec<String> =
+///     vec!["foo".into(), "foo".into(), "bar".into(), "baz".into(), "bar".into()];
+/// deduplicate_arg_names(&mut names);
+/// let expected: Vec<String> =
+///     vec!["foo_1".into(), "foo_2".into(), "bar_1".into(), "baz".into(), "bar_2".into()];
+/// assert_eq!(names, expected);
+/// ```
+fn deduplicate_arg_names(arg_names: &mut Vec<String>) {
+    let arg_name_counts = arg_names.iter().fold(FxHashMap::default(), |mut m, name| {
+        *m.entry(name).or_insert(0) += 1;
+        m
+    });
+    let duplicate_arg_names: FxHashSet<String> = arg_name_counts
+        .into_iter()
+        .filter(|(_, count)| *count >= 2)
+        .map(|(name, _)| name.clone())
+        .collect();
+
+    let mut counter_per_name = FxHashMap::default();
+    for arg_name in arg_names.iter_mut() {
+        if duplicate_arg_names.contains(arg_name) {
+            let counter = counter_per_name.entry(arg_name.clone()).or_insert(1);
+            arg_name.push('_');
+            arg_name.push_str(&counter.to_string());
+            *counter += 1;
+        }
+    }
+}
+
+fn fn_arg_name(fn_arg: &Expr) -> Option<String> {
+    match fn_arg {
+        Expr::CastExpr(cast_expr) => fn_arg_name(&cast_expr.expr()?),
+        _ => Some(
+            fn_arg
+                .syntax()
+                .descendants()
+                .filter(|d| ast::NameRef::can_cast(d.kind()))
+                .last()?
+                .to_string(),
+        ),
+    }
+}
+
+fn fn_arg_type(ctx: &AssistCtx, fn_arg: &Expr) -> Option<String> {
+    let ty = ctx.sema.type_of_expr(fn_arg)?;
+    if ty.is_unknown() {
+        return None;
+    }
+    Some(ty.display(ctx.sema.db).to_string())
+}
+
+/// 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 add_function_with_no_args() {
+        check_assist(
+            add_function,
+            r"
+fn foo() {
+    bar<|>();
+}
+",
+            r"
+fn foo() {
+    bar();
+}
+
+fn bar() {
+    <|>unimplemented!()
+}
+",
+        )
+    }
+
+    #[test]
+    fn add_function_from_method() {
+        // This ensures that the function is correctly generated
+        // in the next outer mod or file
+        check_assist(
+            add_function,
+            r"
+impl Foo {
+    fn foo() {
+        bar<|>();
+    }
+}
+",
+            r"
+impl Foo {
+    fn foo() {
+        bar();
+    }
+}
+
+fn bar() {
+    <|>unimplemented!()
+}
+",
+        )
+    }
+
+    #[test]
+    fn add_function_directly_after_current_block() {
+        // The new fn should not be created at the end of the file or module
+        check_assist(
+            add_function,
+            r"
+fn foo1() {
+    bar<|>();
+}
+
+fn foo2() {}
+",
+            r"
+fn foo1() {
+    bar();
+}
+
+fn bar() {
+    <|>unimplemented!()
+}
+
+fn foo2() {}
+",
+        )
+    }
+
+    #[test]
+    fn add_function_with_no_args_in_same_module() {
+        check_assist(
+            add_function,
+            r"
+mod baz {
+    fn foo() {
+        bar<|>();
+    }
+}
+",
+            r"
+mod baz {
+    fn foo() {
+        bar();
+    }
+
+    fn bar() {
+        <|>unimplemented!()
+    }
+}
+",
+        )
+    }
+
+    #[test]
+    fn add_function_with_function_call_arg() {
+        check_assist(
+            add_function,
+            r"
+struct Baz;
+fn baz() -> Baz { unimplemented!() }
+fn foo() {
+    bar<|>(baz());
+}
+",
+            r"
+struct Baz;
+fn baz() -> Baz { unimplemented!() }
+fn foo() {
+    bar(baz());
+}
+
+fn bar(baz: Baz) {
+    <|>unimplemented!()
+}
+",
+        );
+    }
+
+    #[test]
+    fn add_function_with_method_call_arg() {
+        check_assist(
+            add_function,
+            r"
+struct Baz;
+impl Baz {
+    fn foo(&self) -> Baz {
+        ba<|>r(self.baz())
+    }
+    fn baz(&self) -> Baz {
+        Baz
+    }
+}
+",
+            r"
+struct Baz;
+impl Baz {
+    fn foo(&self) -> Baz {
+        bar(self.baz())
+    }
+    fn baz(&self) -> Baz {
+        Baz
+    }
+}
+
+fn bar(baz: Baz) {
+    <|>unimplemented!()
+}
+",
+        )
+    }
+
+    #[test]
+    fn add_function_with_string_literal_arg() {
+        check_assist(
+            add_function,
+            r#"
+fn foo() {
+    <|>bar("bar")
+}
+"#,
+            r#"
+fn foo() {
+    bar("bar")
+}
+
+fn bar(arg: &str) {
+    <|>unimplemented!()
+}
+"#,
+        )
+    }
+
+    #[test]
+    fn add_function_with_char_literal_arg() {
+        check_assist(
+            add_function,
+            r#"
+fn foo() {
+    <|>bar('x')
+}
+"#,
+            r#"
+fn foo() {
+    bar('x')
+}
+
+fn bar(arg: char) {
+    <|>unimplemented!()
+}
+"#,
+        )
+    }
+
+    #[test]
+    fn add_function_with_int_literal_arg() {
+        check_assist(
+            add_function,
+            r"
+fn foo() {
+    <|>bar(42)
+}
+",
+            r"
+fn foo() {
+    bar(42)
+}
+
+fn bar(arg: i32) {
+    <|>unimplemented!()
+}
+",
+        )
+    }
+
+    #[test]
+    fn add_function_with_cast_int_literal_arg() {
+        check_assist(
+            add_function,
+            r"
+fn foo() {
+    <|>bar(42 as u8)
+}
+",
+            r"
+fn foo() {
+    bar(42 as u8)
+}
+
+fn bar(arg: u8) {
+    <|>unimplemented!()
+}
+",
+        )
+    }
+
+    #[test]
+    fn name_of_cast_variable_is_used() {
+        // Ensures that the name of the cast type isn't used
+        // in the generated function signature.
+        check_assist(
+            add_function,
+            r"
+fn foo() {
+    let x = 42;
+    bar<|>(x as u8)
+}
+",
+            r"
+fn foo() {
+    let x = 42;
+    bar(x as u8)
+}
+
+fn bar(x: u8) {
+    <|>unimplemented!()
+}
+",
+        )
+    }
+
+    #[test]
+    fn add_function_with_variable_arg() {
+        check_assist(
+            add_function,
+            r"
+fn foo() {
+    let worble = ();
+    <|>bar(worble)
+}
+",
+            r"
+fn foo() {
+    let worble = ();
+    bar(worble)
+}
+
+fn bar(worble: ()) {
+    <|>unimplemented!()
+}
+",
+        )
+    }
+
+    #[test]
+    fn add_function_with_impl_trait_arg() {
+        check_assist(
+            add_function,
+            r"
+trait Foo {}
+fn foo() -> impl Foo {
+    unimplemented!()
+}
+fn baz() {
+    <|>bar(foo())
+}
+",
+            r"
+trait Foo {}
+fn foo() -> impl Foo {
+    unimplemented!()
+}
+fn baz() {
+    bar(foo())
+}
+
+fn bar(foo: impl Foo) {
+    <|>unimplemented!()
+}
+",
+        )
+    }
+
+    #[test]
+    #[ignore]
+    // FIXME print paths properly to make this test pass
+    fn add_function_with_qualified_path_arg() {
+        check_assist(
+            add_function,
+            r"
+mod Baz {
+    pub struct Bof;
+    pub fn baz() -> Bof { Bof }
+}
+mod Foo {
+    fn foo() {
+        <|>bar(super::Baz::baz())
+    }
+}
+",
+            r"
+mod Baz {
+    pub struct Bof;
+    pub fn baz() -> Bof { Bof }
+}
+mod Foo {
+    fn foo() {
+        bar(super::Baz::baz())
+    }
+
+    fn bar(baz: super::Baz::Bof) {
+        <|>unimplemented!()
+    }
+}
+",
+        )
+    }
+
+    #[test]
+    #[ignore]
+    // FIXME fix printing the generics of a `Ty` to make this test pass
+    fn add_function_with_generic_arg() {
+        check_assist(
+            add_function,
+            r"
+fn foo<T>(t: T) {
+    <|>bar(t)
+}
+",
+            r"
+fn foo<T>(t: T) {
+    bar(t)
+}
+
+fn bar<T>(t: T) {
+    <|>unimplemented!()
+}
+",
+        )
+    }
+
+    #[test]
+    #[ignore]
+    // FIXME Fix function type printing to make this test pass
+    fn add_function_with_fn_arg() {
+        check_assist(
+            add_function,
+            r"
+struct Baz;
+impl Baz {
+    fn new() -> Self { Baz }
+}
+fn foo() {
+    <|>bar(Baz::new);
+}
+",
+            r"
+struct Baz;
+impl Baz {
+    fn new() -> Self { Baz }
+}
+fn foo() {
+    bar(Baz::new);
+}
+
+fn bar(arg: fn() -> Baz) {
+    <|>unimplemented!()
+}
+",
+        )
+    }
+
+    #[test]
+    #[ignore]
+    // FIXME Fix closure type printing to make this test pass
+    fn add_function_with_closure_arg() {
+        check_assist(
+            add_function,
+            r"
+fn foo() {
+    let closure = |x: i64| x - 1;
+    <|>bar(closure)
+}
+",
+            r"
+fn foo() {
+    let closure = |x: i64| x - 1;
+    bar(closure)
+}
+
+fn bar(closure: impl Fn(i64) -> i64) {
+    <|>unimplemented!()
+}
+",
+        )
+    }
+
+    #[test]
+    fn unresolveable_types_default_to_unit() {
+        check_assist(
+            add_function,
+            r"
+fn foo() {
+    <|>bar(baz)
+}
+",
+            r"
+fn foo() {
+    bar(baz)
+}
+
+fn bar(baz: ()) {
+    <|>unimplemented!()
+}
+",
+        )
+    }
+
+    #[test]
+    fn arg_names_dont_overlap() {
+        check_assist(
+            add_function,
+            r"
+struct Baz;
+fn baz() -> Baz { Baz }
+fn foo() {
+    <|>bar(baz(), baz())
+}
+",
+            r"
+struct Baz;
+fn baz() -> Baz { Baz }
+fn foo() {
+    bar(baz(), baz())
+}
+
+fn bar(baz_1: Baz, baz_2: Baz) {
+    <|>unimplemented!()
+}
+",
+        )
+    }
+
+    #[test]
+    fn arg_name_counters_start_at_1_per_name() {
+        check_assist(
+            add_function,
+            r#"
+struct Baz;
+fn baz() -> Baz { Baz }
+fn foo() {
+    <|>bar(baz(), baz(), "foo", "bar")
+}
+"#,
+            r#"
+struct Baz;
+fn baz() -> Baz { Baz }
+fn foo() {
+    bar(baz(), baz(), "foo", "bar")
+}
+
+fn bar(baz_1: Baz, baz_2: Baz, arg_1: &str, arg_2: &str) {
+    <|>unimplemented!()
+}
+"#,
+        )
+    }
+
+    #[test]
+    fn add_function_not_applicable_if_function_already_exists() {
+        check_assist_not_applicable(
+            add_function,
+            r"
+fn foo() {
+    bar<|>();
+}
+
+fn bar() {}
+",
+        )
+    }
+
+    #[test]
+    fn add_function_not_applicable_if_unresolved_variable_in_call_is_selected() {
+        check_assist_not_applicable(
+            // bar is resolved, but baz isn't.
+            // The assist is only active if the cursor is on an unresolved path,
+            // but the assist should only be offered if the path is a function call.
+            add_function,
+            r"
+fn foo() {
+    bar(b<|>az);
+}
+
+fn bar(baz: ()) {}
+",
+        )
+    }
+
+    #[test]
+    fn add_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(
+            add_function,
+            r"
+fn foo() {
+    other_crate::bar<|>();
+}
+        ",
+        )
+    }
+
+    #[test]
+    #[ignore]
+    fn create_method_with_no_args() {
+        check_assist(
+            add_function,
+            r"
+struct Foo;
+impl Foo {
+    fn foo(&self) {
+        self.bar()<|>;
+    }
+}
+        ",
+            r"
+struct Foo;
+impl Foo {
+    fn foo(&self) {
+        self.bar();
+    }
+    fn bar(&self) {
+        unimplemented!();
+    }
+}
+        ",
+        )
+    }
+}
diff --git a/crates/ra_assists/src/lib.rs b/crates/ra_assists/src/lib.rs
index fa1f3dd26..66854cb5a 100644
--- a/crates/ra_assists/src/lib.rs
+++ b/crates/ra_assists/src/lib.rs
@@ -96,6 +96,7 @@ mod handlers {
     mod add_custom_impl;
     mod add_derive;
     mod add_explicit_type;
+    mod add_function;
     mod add_impl;
     mod add_missing_impl_members;
     mod add_new;
@@ -128,6 +129,7 @@ mod handlers {
             add_custom_impl::add_custom_impl,
             add_derive::add_derive,
             add_explicit_type::add_explicit_type,
+            add_function::add_function,
             add_impl::add_impl,
             add_missing_impl_members::add_missing_default_members,
             add_missing_impl_members::add_missing_impl_members,
diff --git a/docs/user/assists.md b/docs/user/assists.md
index 94b5ef85d..754131f6f 100644
--- a/docs/user/assists.md
+++ b/docs/user/assists.md
@@ -56,6 +56,32 @@ fn main() {
 }
 ```
 
+## `add_function`
+
+Adds a stub function with a signature matching the function under the cursor.
+
+```rust
+// BEFORE
+struct Baz;
+fn baz() -> Baz { Baz }
+fn foo() {
+     bar┃("", baz());
+}
+
+
+// AFTER
+struct Baz;
+fn baz() -> Baz { Baz }
+fn foo() {
+     bar("", baz());
+}
+
+fn bar(arg: &str, baz: Baz) {
+    unimplemented!()
+}
+
+```
+
 ## `add_hash`
 
 Adds a hash to a raw string literal.