3 files changed, 403 insertions, 0 deletions

diff --git a/crates/ra_assists/src/assists/add_new.rs b/crates/ra_assists/src/assists/add_new.rs
new file mode 100644
index 000000000..a8839cfba
--- /dev/null
+++ b/crates/ra_assists/src/assists/add_new.rs
@@ -0,0 +1,379 @@
+use format_buf::format;
+use hir::{db::HirDatabase, FromSource};
+use join_to_string::join;
+use ra_syntax::{
+    ast::{
+        self, AstNode, NameOwner, StructKind, TypeAscriptionOwner, TypeParamsOwner, VisibilityOwner,
+    },
+    TextUnit, T,
+};
+use std::fmt::Write;
+
+use crate::{Assist, AssistCtx, AssistId};
+
+// Assist: add_new
+//
+// Adds a new inherent impl for a type.
+//
+// ```
+// struct Ctx<T: Clone> {
+//      data: T,<|>
+// }
+// ```
+// ->
+// ```
+// struct Ctx<T: Clone> {
+//      data: T,
+// }
+//
+// impl<T: Clone> Ctx<T> {
+//     fn new(data: T) -> Self { Self { data } }
+// }
+//
+// ```
+pub(crate) fn add_new(ctx: AssistCtx<impl HirDatabase>) -> Option<Assist> {
+    let strukt = ctx.find_node_at_offset::<ast::StructDef>()?;
+
+    // We want to only apply this to non-union structs with named fields
+    let field_list = match (strukt.kind(), strukt.is_union()) {
+        (StructKind::Named(named), false) => named,
+        _ => return None,
+    };
+
+    // Return early if we've found an existing new fn
+    let impl_block = find_struct_impl(&ctx, &strukt)?;
+
+    ctx.add_assist(AssistId("add_new"), "add new fn", |edit| {
+        edit.target(strukt.syntax().text_range());
+
+        let mut buf = String::with_capacity(512);
+
+        if impl_block.is_some() {
+            buf.push('\n');
+        }
+
+        let vis = strukt.visibility().map(|v| format!("{} ", v.syntax()));
+        let vis = vis.as_ref().map(String::as_str).unwrap_or("");
+        write!(&mut buf, "    {}fn new(", vis).unwrap();
+
+        join(field_list.fields().map(|f| {
+            format!(
+                "{}: {}",
+                f.name().unwrap().syntax().text(),
+                f.ascribed_type().unwrap().syntax().text()
+            )
+        }))
+        .separator(", ")
+        .to_buf(&mut buf);
+
+        buf.push_str(") -> Self { Self {");
+
+        join(field_list.fields().map(|f| f.name().unwrap().syntax().text()))
+            .separator(", ")
+            .surround_with(" ", " ")
+            .to_buf(&mut buf);
+
+        buf.push_str("} }");
+
+        let (start_offset, end_offset) = if let Some(impl_block) = impl_block {
+            buf.push('\n');
+            let start = impl_block
+                .syntax()
+                .descendants_with_tokens()
+                .find(|t| t.kind() == T!['{'])
+                .unwrap()
+                .text_range()
+                .end();
+
+            (start, TextUnit::from_usize(1))
+        } else {
+            buf = generate_impl_text(&strukt, &buf);
+            let start = strukt.syntax().text_range().end();
+
+            (start, TextUnit::from_usize(3))
+        };
+
+        edit.set_cursor(start_offset + TextUnit::of_str(&buf) - end_offset);
+        edit.insert(start_offset, buf);
+    })
+}
+
+// Generates the surrounding `impl Type { <code> }` including type and lifetime
+// parameters
+fn generate_impl_text(strukt: &ast::StructDef, code: &str) -> String {
+    let type_params = strukt.type_param_list();
+    let mut buf = String::with_capacity(code.len());
+    buf.push_str("\n\nimpl");
+    if let Some(type_params) = &type_params {
+        format!(buf, "{}", type_params.syntax());
+    }
+    buf.push_str(" ");
+    buf.push_str(strukt.name().unwrap().text().as_str());
+    if let Some(type_params) = type_params {
+        let lifetime_params = type_params
+            .lifetime_params()
+            .filter_map(|it| it.lifetime_token())
+            .map(|it| it.text().clone());
+        let type_params =
+            type_params.type_params().filter_map(|it| it.name()).map(|it| it.text().clone());
+        join(lifetime_params.chain(type_params)).surround_with("<", ">").to_buf(&mut buf);
+    }
+
+    format!(&mut buf, " {{\n{}\n}}\n", code);
+
+    buf
+}
+
+// Uses a syntax-driven approach to find any impl blocks for the struct that
+// exist within the module/file
+//
+// Returns `None` if we've found an existing `new` fn
+//
+// FIXME: change the new fn checking to a more semantic approach when that's more
+// viable (e.g. we process proc macros, etc)
+fn find_struct_impl(
+    ctx: &AssistCtx<impl HirDatabase>,
+    strukt: &ast::StructDef,
+) -> Option<Option<ast::ImplBlock>> {
+    let db = ctx.db;
+    let module = strukt.syntax().ancestors().find(|node| {
+        ast::Module::can_cast(node.kind()) || ast::SourceFile::can_cast(node.kind())
+    })?;
+
+    let struct_ty = {
+        let src = hir::Source { file_id: ctx.frange.file_id.into(), ast: strukt.clone() };
+        hir::Struct::from_source(db, src).unwrap().ty(db)
+    };
+
+    let mut found_new_fn = false;
+
+    let block = module.descendants().filter_map(ast::ImplBlock::cast).find(|impl_blk| {
+        if found_new_fn {
+            return false;
+        }
+
+        let src = hir::Source { file_id: ctx.frange.file_id.into(), ast: impl_blk.clone() };
+        let blk = hir::ImplBlock::from_source(db, src).unwrap();
+
+        let same_ty = blk.target_ty(db) == struct_ty;
+        let not_trait_impl = blk.target_trait(db).is_none();
+
+        found_new_fn = has_new_fn(impl_blk);
+
+        same_ty && not_trait_impl
+    });
+
+    if found_new_fn {
+        None
+    } else {
+        Some(block)
+    }
+}
+
+fn has_new_fn(imp: &ast::ImplBlock) -> bool {
+    if let Some(il) = imp.item_list() {
+        for item in il.impl_items() {
+            if let ast::ImplItem::FnDef(f) = item {
+                if f.name().unwrap().text().eq_ignore_ascii_case("new") {
+                    return true;
+                }
+            }
+        }
+    }
+
+    false
+}
+
+#[cfg(test)]
+mod tests {
+    use super::*;
+    use crate::helpers::{check_assist, check_assist_not_applicable, check_assist_target};
+
+    #[test]
+    #[rustfmt::skip]
+    fn test_add_new() {
+        // Check output of generation
+        check_assist(
+            add_new,
+"struct Foo {<|>}",
+"struct Foo {}
+
+impl Foo {
+    fn new() -> Self { Self {  } }<|>
+}
+",
+        );
+        check_assist(
+            add_new,
+"struct Foo<T: Clone> {<|>}",
+"struct Foo<T: Clone> {}
+
+impl<T: Clone> Foo<T> {
+    fn new() -> Self { Self {  } }<|>
+}
+",
+        );
+        check_assist(
+            add_new,
+"struct Foo<'a, T: Foo<'a>> {<|>}",
+"struct Foo<'a, T: Foo<'a>> {}
+
+impl<'a, T: Foo<'a>> Foo<'a, T> {
+    fn new() -> Self { Self {  } }<|>
+}
+",
+        );
+        check_assist(
+            add_new,
+"struct Foo { baz: String <|>}",
+"struct Foo { baz: String }
+
+impl Foo {
+    fn new(baz: String) -> Self { Self { baz } }<|>
+}
+",
+        );
+        check_assist(
+            add_new,
+"struct Foo { baz: String, qux: Vec<i32> <|>}",
+"struct Foo { baz: String, qux: Vec<i32> }
+
+impl Foo {
+    fn new(baz: String, qux: Vec<i32>) -> Self { Self { baz, qux } }<|>
+}
+",
+        );
+
+        // Check that visibility modifiers don't get brought in for fields
+        check_assist(
+            add_new,
+"struct Foo { pub baz: String, pub qux: Vec<i32> <|>}",
+"struct Foo { pub baz: String, pub qux: Vec<i32> }
+
+impl Foo {
+    fn new(baz: String, qux: Vec<i32>) -> Self { Self { baz, qux } }<|>
+}
+",
+        );
+
+        // Check that it reuses existing impls
+        check_assist(
+            add_new,
+"struct Foo {<|>}
+
+impl Foo {}
+",
+"struct Foo {}
+
+impl Foo {
+    fn new() -> Self { Self {  } }<|>
+}
+",
+        );
+        check_assist(
+            add_new,
+"struct Foo {<|>}
+
+impl Foo {
+    fn qux(&self) {}
+}
+",
+"struct Foo {}
+
+impl Foo {
+    fn new() -> Self { Self {  } }<|>
+
+    fn qux(&self) {}
+}
+",
+        );
+
+        check_assist(
+            add_new,
+"struct Foo {<|>}
+
+impl Foo {
+    fn qux(&self) {}
+    fn baz() -> i32 {
+        5
+    }
+}
+",
+"struct Foo {}
+
+impl Foo {
+    fn new() -> Self { Self {  } }<|>
+
+    fn qux(&self) {}
+    fn baz() -> i32 {
+        5
+    }
+}
+",
+        );
+
+        // Check visibility of new fn based on struct
+        check_assist(
+            add_new,
+"pub struct Foo {<|>}",
+"pub struct Foo {}
+
+impl Foo {
+    pub fn new() -> Self { Self {  } }<|>
+}
+",
+        );
+        check_assist(
+            add_new,
+"pub(crate) struct Foo {<|>}",
+"pub(crate) struct Foo {}
+
+impl Foo {
+    pub(crate) fn new() -> Self { Self {  } }<|>
+}
+",
+        );
+    }
+
+    #[test]
+    fn add_new_not_applicable_if_fn_exists() {
+        check_assist_not_applicable(
+            add_new,
+            "
+struct Foo {<|>}
+
+impl Foo {
+    fn new() -> Self {
+        Self
+    }    
+}",
+        );
+
+        check_assist_not_applicable(
+            add_new,
+            "
+struct Foo {<|>}
+
+impl Foo {
+    fn New() -> Self {
+        Self
+    }    
+}",
+        );
+    }
+
+    #[test]
+    fn add_new_target() {
+        check_assist_target(
+            add_new,
+            "
+struct SomeThingIrrelevant;
+/// Has a lifetime parameter
+struct Foo<'a, T: Foo<'a>> {<|>}
+struct EvenMoreIrrelevant;
+",
+            "/// Has a lifetime parameter
+struct Foo<'a, T: Foo<'a>> {}",
+        );
+    }
+}
diff --git a/crates/ra_assists/src/doc_tests/generated.rs b/crates/ra_assists/src/doc_tests/generated.rs
index 1bee76f59..176761efb 100644
--- a/crates/ra_assists/src/doc_tests/generated.rs
+++ b/crates/ra_assists/src/doc_tests/generated.rs
@@ -157,6 +157,28 @@ fn process(map: HashMap<String, String>) {}
 }
 
 #[test]
+fn doctest_add_new() {
+    check(
+        "add_new",
+        r#####"
+struct Ctx<T: Clone> {
+     data: T,<|>
+}
+"#####,
+        r#####"
+struct Ctx<T: Clone> {
+     data: T,
+}
+
+impl<T: Clone> Ctx<T> {
+    fn new(data: T) -> Self { Self { data } }
+}
+
+"#####,
+    )
+}
+
+#[test]
 fn doctest_apply_demorgan() {
     check(
         "apply_demorgan",
diff --git a/crates/ra_assists/src/lib.rs b/crates/ra_assists/src/lib.rs
index 39c1c283f..f2f0dacbf 100644
--- a/crates/ra_assists/src/lib.rs
+++ b/crates/ra_assists/src/lib.rs
@@ -95,6 +95,7 @@ mod assists {
     mod add_derive;
     mod add_explicit_type;
     mod add_impl;
+    mod add_new;
     mod apply_demorgan;
     mod flip_comma;
     mod flip_binexpr;
@@ -119,6 +120,7 @@ mod assists {
             add_derive::add_derive,
             add_explicit_type::add_explicit_type,
             add_impl::add_impl,
+            add_new::add_new,
             apply_demorgan::apply_demorgan,
             change_visibility::change_visibility,
             fill_match_arms::fill_match_arms,