6 files changed, 123 insertions, 208 deletions

diff --git a/crates/ide_assists/src/ast_transform.rs b/crates/ide_assists/src/ast_transform.rs
index e5ae718c9..5d9cc5551 100644
--- a/crates/ide_assists/src/ast_transform.rs
+++ b/crates/ide_assists/src/ast_transform.rs
@@ -1,31 +1,12 @@
 //! `AstTransformer`s are functions that replace nodes in an AST and can be easily combined.
-use hir::{HirDisplay, PathResolution, SemanticsScope};
+use hir::{HirDisplay, SemanticsScope};
 use ide_db::helpers::mod_path_to_ast;
 use rustc_hash::FxHashMap;
 use syntax::{
     ast::{self, AstNode},
-    ted, SyntaxNode,
+    ted,
 };
 
-pub fn apply<'a, N: AstNode>(transformer: &dyn AstTransform<'a>, node: &N) {
-    let mut skip_to = None;
-    for event in node.syntax().preorder() {
-        match event {
-            syntax::WalkEvent::Enter(node) if skip_to.is_none() => {
-                skip_to = transformer.get_substitution(&node, transformer).zip(Some(node));
-            }
-            syntax::WalkEvent::Enter(_) => (),
-            syntax::WalkEvent::Leave(node) => match &skip_to {
-                Some((replacement, skip_target)) if *skip_target == node => {
-                    ted::replace(node, replacement.clone_for_update());
-                    skip_to.take();
-                }
-                _ => (),
-            },
-        }
-    }
-}
-
 /// `AstTransform` helps with applying bulk transformations to syntax nodes.
 ///
 /// This is mostly useful for IDE code generation. If you paste some existing
@@ -35,8 +16,8 @@ pub fn apply<'a, N: AstNode>(transformer: &dyn AstTransform<'a>, node: &N) {
 ///
 /// ```
 /// mod x {
-///   pub struct A;
-///   pub trait T<U> { fn foo(&self, _: U) -> A; }
+///   pub struct A<V>;
+///   pub trait T<U> { fn foo(&self, _: U) -> A<U>; }
 /// }
 ///
 /// mod y {
@@ -45,7 +26,7 @@ pub fn apply<'a, N: AstNode>(transformer: &dyn AstTransform<'a>, node: &N) {
 ///   impl T<()> for () {
 ///      // If we invoke **Add Missing Members** here, we want to copy-paste `foo`.
 ///      // But we want a slightly-modified version of it:
-///      fn foo(&self, _: ()) -> x::A {}
+///      fn foo(&self, _: ()) -> x::A<()> {}
 ///   }
 /// }
 /// ```
@@ -54,49 +35,27 @@ pub fn apply<'a, N: AstNode>(transformer: &dyn AstTransform<'a>, node: &N) {
 /// `SyntaxNode`. Note that the API here is a bit too high-order and high-brow.
 /// We'd want to somehow express this concept simpler, but so far nobody got to
 /// simplifying this!
-pub trait AstTransform<'a> {
-    fn get_substitution(
-        &self,
-        node: &SyntaxNode,
-        recur: &dyn AstTransform<'a>,
-    ) -> Option<SyntaxNode>;
-
-    fn or<T: AstTransform<'a> + 'a>(self, other: T) -> Box<dyn AstTransform<'a> + 'a>
-    where
-        Self: Sized + 'a,
-    {
-        Box::new(Or(Box::new(self), Box::new(other)))
-    }
+pub(crate) struct AstTransform<'a> {
+    pub(crate) subst: (hir::Trait, ast::Impl),
+    pub(crate) target_scope: &'a SemanticsScope<'a>,
+    pub(crate) source_scope: &'a SemanticsScope<'a>,
 }
 
-struct Or<'a>(Box<dyn AstTransform<'a> + 'a>, Box<dyn AstTransform<'a> + 'a>);
-
-impl<'a> AstTransform<'a> for Or<'a> {
-    fn get_substitution(
-        &self,
-        node: &SyntaxNode,
-        recur: &dyn AstTransform<'a>,
-    ) -> Option<SyntaxNode> {
-        self.0.get_substitution(node, recur).or_else(|| self.1.get_substitution(node, recur))
+impl<'a> AstTransform<'a> {
+    pub(crate) fn apply(&self, item: ast::AssocItem) {
+        if let Some(ctx) = self.build_ctx() {
+            ctx.apply(item)
+        }
     }
-}
-
-pub struct SubstituteTypeParams<'a> {
-    source_scope: &'a SemanticsScope<'a>,
-    substs: FxHashMap<hir::TypeParam, ast::Type>,
-}
+    fn build_ctx(&self) -> Option<Ctx<'a>> {
+        let db = self.source_scope.db;
+        let target_module = self.target_scope.module()?;
+        let source_module = self.source_scope.module()?;
 
-impl<'a> SubstituteTypeParams<'a> {
-    pub fn for_trait_impl(
-        source_scope: &'a SemanticsScope<'a>,
-        // FIXME: there's implicit invariant that `trait_` and  `source_scope` match...
-        trait_: hir::Trait,
-        impl_def: ast::Impl,
-    ) -> SubstituteTypeParams<'a> {
-        let substs = get_syntactic_substs(impl_def).unwrap_or_default();
-        let generic_def: hir::GenericDef = trait_.into();
+        let substs = get_syntactic_substs(self.subst.1.clone()).unwrap_or_default();
+        let generic_def: hir::GenericDef = self.subst.0.into();
         let substs_by_param: FxHashMap<_, _> = generic_def
-            .type_params(source_scope.db)
+            .type_params(db)
             .into_iter()
             // this is a trait impl, so we need to skip the first type parameter -- this is a bit hacky
             .skip(1)
@@ -110,109 +69,96 @@ impl<'a> SubstituteTypeParams<'a> {
             .filter_map(|(k, v)| match v {
                 Some(v) => Some((k, v)),
                 None => {
-                    let default = k.default(source_scope.db)?;
+                    let default = k.default(db)?;
                     Some((
                         k,
-                        ast::make::ty(
-                            &default
-                                .display_source_code(source_scope.db, source_scope.module()?.into())
-                                .ok()?,
-                        ),
+                        ast::make::ty(&default.display_source_code(db, source_module.into()).ok()?),
                     ))
                 }
             })
             .collect();
-        return SubstituteTypeParams { source_scope, substs: substs_by_param };
-
-        // FIXME: It would probably be nicer if we could get this via HIR (i.e. get the
-        // trait ref, and then go from the types in the substs back to the syntax).
-        fn get_syntactic_substs(impl_def: ast::Impl) -> Option<Vec<ast::Type>> {
-            let target_trait = impl_def.trait_()?;
-            let path_type = match target_trait {
-                ast::Type::PathType(path) => path,
-                _ => return None,
-            };
-            let generic_arg_list = path_type.path()?.segment()?.generic_arg_list()?;
-
-            let mut result = Vec::new();
-            for generic_arg in generic_arg_list.generic_args() {
-                match generic_arg {
-                    ast::GenericArg::TypeArg(type_arg) => result.push(type_arg.ty()?),
-                    ast::GenericArg::AssocTypeArg(_)
-                    | ast::GenericArg::LifetimeArg(_)
-                    | ast::GenericArg::ConstArg(_) => (),
-                }
-            }
 
-            Some(result)
-        }
+        let res = Ctx { substs: substs_by_param, target_module, source_scope: self.source_scope };
+        Some(res)
     }
 }
 
-impl<'a> AstTransform<'a> for SubstituteTypeParams<'a> {
-    fn get_substitution(
-        &self,
-        node: &SyntaxNode,
-        _recur: &dyn AstTransform<'a>,
-    ) -> Option<SyntaxNode> {
-        let type_ref = ast::Type::cast(node.clone())?;
-        let path = match &type_ref {
-            ast::Type::PathType(path_type) => path_type.path()?,
-            _ => return None,
-        };
-        let resolution = self.source_scope.speculative_resolve(&path)?;
-        match resolution {
-            hir::PathResolution::TypeParam(tp) => Some(self.substs.get(&tp)?.syntax().clone()),
-            _ => None,
-        }
-    }
-}
-
-pub struct QualifyPaths<'a> {
-    target_scope: &'a SemanticsScope<'a>,
+struct Ctx<'a> {
+    substs: FxHashMap<hir::TypeParam, ast::Type>,
+    target_module: hir::Module,
     source_scope: &'a SemanticsScope<'a>,
 }
 
-impl<'a> QualifyPaths<'a> {
-    pub fn new(target_scope: &'a SemanticsScope<'a>, source_scope: &'a SemanticsScope<'a>) -> Self {
-        Self { target_scope, source_scope }
+impl<'a> Ctx<'a> {
+    fn apply(&self, item: ast::AssocItem) {
+        for event in item.syntax().preorder() {
+            let node = match event {
+                syntax::WalkEvent::Enter(_) => continue,
+                syntax::WalkEvent::Leave(it) => it,
+            };
+            if let Some(path) = ast::Path::cast(node.clone()) {
+                self.transform_path(path);
+            }
+        }
     }
-}
-
-impl<'a> AstTransform<'a> for QualifyPaths<'a> {
-    fn get_substitution(
-        &self,
-        node: &SyntaxNode,
-        recur: &dyn AstTransform<'a>,
-    ) -> Option<SyntaxNode> {
-        // FIXME handle value ns?
-        let from = self.target_scope.module()?;
-        let p = ast::Path::cast(node.clone())?;
-        if p.segment().and_then(|s| s.param_list()).is_some() {
+    fn transform_path(&self, path: ast::Path) -> Option<()> {
+        if path.qualifier().is_some() {
+            return None;
+        }
+        if path.segment().and_then(|s| s.param_list()).is_some() {
             // don't try to qualify `Fn(Foo) -> Bar` paths, they are in prelude anyway
             return None;
         }
-        let resolution = self.source_scope.speculative_resolve(&p)?;
-        match resolution {
-            PathResolution::Def(def) => {
-                let found_path = from.find_use_path(self.source_scope.db.upcast(), def)?;
-                let mut path = mod_path_to_ast(&found_path);
 
-                let type_args = p.segment().and_then(|s| s.generic_arg_list());
-                if let Some(type_args) = type_args {
-                    apply(recur, &type_args);
-                    let last_segment = path.segment().unwrap();
-                    path = path.with_segment(last_segment.with_generic_args(type_args))
-                }
+        let resolution = self.source_scope.speculative_resolve(&path)?;
 
-                Some(path.syntax().clone())
+        match resolution {
+            hir::PathResolution::TypeParam(tp) => {
+                if let Some(subst) = self.substs.get(&tp) {
+                    ted::replace(path.syntax(), subst.clone_subtree().clone_for_update().syntax())
+                }
+            }
+            hir::PathResolution::Def(def) => {
+                let found_path =
+                    self.target_module.find_use_path(self.source_scope.db.upcast(), def)?;
+                let res = mod_path_to_ast(&found_path).clone_for_update();
+                if let Some(args) = path.segment().and_then(|it| it.generic_arg_list()) {
+                    if let Some(segment) = res.segment() {
+                        let old = segment.get_or_create_generic_arg_list();
+                        ted::replace(old.syntax(), args.clone_subtree().syntax().clone_for_update())
+                    }
+                }
+                ted::replace(path.syntax(), res.syntax())
             }
-            PathResolution::Local(_)
-            | PathResolution::TypeParam(_)
-            | PathResolution::SelfType(_)
-            | PathResolution::ConstParam(_) => None,
-            PathResolution::Macro(_) => None,
-            PathResolution::AssocItem(_) => None,
+            hir::PathResolution::Local(_)
+            | hir::PathResolution::ConstParam(_)
+            | hir::PathResolution::SelfType(_)
+            | hir::PathResolution::Macro(_)
+            | hir::PathResolution::AssocItem(_) => (),
         }
+        Some(())
     }
 }
+
+// FIXME: It would probably be nicer if we could get this via HIR (i.e. get the
+// trait ref, and then go from the types in the substs back to the syntax).
+fn get_syntactic_substs(impl_def: ast::Impl) -> Option<Vec<ast::Type>> {
+    let target_trait = impl_def.trait_()?;
+    let path_type = match target_trait {
+        ast::Type::PathType(path) => path,
+        _ => return None,
+    };
+    let generic_arg_list = path_type.path()?.segment()?.generic_arg_list()?;
+
+    let mut result = Vec::new();
+    for generic_arg in generic_arg_list.generic_args() {
+        match generic_arg {
+            ast::GenericArg::TypeArg(type_arg) => result.push(type_arg.ty()?),
+            ast::GenericArg::AssocTypeArg(_)
+            | ast::GenericArg::LifetimeArg(_)
+            | ast::GenericArg::ConstArg(_) => (),
+        }
+    }
+
+    Some(result)
+}
diff --git a/crates/ide_assists/src/utils.rs b/crates/ide_assists/src/utils.rs
index fc7caee04..2e79a3aed 100644
--- a/crates/ide_assists/src/utils.rs
+++ b/crates/ide_assists/src/utils.rs
@@ -24,7 +24,7 @@ use syntax::{
 
 use crate::{
     assist_context::{AssistBuilder, AssistContext},
-    ast_transform::{self, AstTransform, QualifyPaths, SubstituteTypeParams},
+    ast_transform::AstTransform,
 };
 
 pub(crate) fn unwrap_trivial_block(block: ast::BlockExpr) -> ast::Expr {
@@ -132,14 +132,18 @@ pub fn add_trait_assoc_items_to_impl(
     target_scope: hir::SemanticsScope,
 ) -> (ast::Impl, ast::AssocItem) {
     let source_scope = sema.scope_for_def(trait_);
-    let ast_transform = QualifyPaths::new(&target_scope, &source_scope)
-        .or(SubstituteTypeParams::for_trait_impl(&source_scope, trait_, impl_.clone()));
-
-    let items = items
-        .into_iter()
-        .map(|it| it.clone_for_update())
-        .inspect(|it| ast_transform::apply(&*ast_transform, it))
-        .map(|it| edit::remove_attrs_and_docs(&it).clone_subtree().clone_for_update());
+
+    let transform = AstTransform {
+        subst: (trait_, impl_.clone()),
+        source_scope: &source_scope,
+        target_scope: &target_scope,
+    };
+
+    let items = items.into_iter().map(|assoc_item| {
+        let assoc_item = assoc_item.clone_for_update();
+        transform.apply(assoc_item.clone());
+        edit::remove_attrs_and_docs(&assoc_item).clone_subtree().clone_for_update()
+    });
 
     let res = impl_.clone_for_update();
 
diff --git a/crates/syntax/src/ast/edit.rs b/crates/syntax/src/ast/edit.rs
index 61952377f..19107ee38 100644
--- a/crates/syntax/src/ast/edit.rs
+++ b/crates/syntax/src/ast/edit.rs
@@ -6,14 +6,12 @@ use std::{
     ops::{self, RangeInclusive},
 };
 
-use arrayvec::ArrayVec;
-
 use crate::{
     algo,
     ast::{self, make, AstNode},
-    ted, AstToken, InsertPosition, NodeOrToken, SyntaxElement, SyntaxKind,
+    ted, AstToken, NodeOrToken, SyntaxElement, SyntaxKind,
     SyntaxKind::{ATTR, COMMENT, WHITESPACE},
-    SyntaxNode, SyntaxToken, T,
+    SyntaxNode, SyntaxToken,
 };
 
 impl ast::BinExpr {
@@ -25,46 +23,6 @@ impl ast::BinExpr {
     }
 }
 
-impl ast::Path {
-    #[must_use]
-    pub fn with_segment(&self, segment: ast::PathSegment) -> ast::Path {
-        if let Some(old) = self.segment() {
-            return self.replace_children(
-                single_node(old.syntax().clone()),
-                iter::once(segment.syntax().clone().into()),
-            );
-        }
-        self.clone()
-    }
-}
-
-impl ast::PathSegment {
-    #[must_use]
-    pub fn with_generic_args(&self, type_args: ast::GenericArgList) -> ast::PathSegment {
-        self._with_generic_args(type_args, false)
-    }
-
-    #[must_use]
-    pub fn with_turbo_fish(&self, type_args: ast::GenericArgList) -> ast::PathSegment {
-        self._with_generic_args(type_args, true)
-    }
-
-    fn _with_generic_args(&self, type_args: ast::GenericArgList, turbo: bool) -> ast::PathSegment {
-        if let Some(old) = self.generic_arg_list() {
-            return self.replace_children(
-                single_node(old.syntax().clone()),
-                iter::once(type_args.syntax().clone().into()),
-            );
-        }
-        let mut to_insert: ArrayVec<SyntaxElement, 2> = ArrayVec::new();
-        if turbo {
-            to_insert.push(make::token(T![::]).into());
-        }
-        to_insert.push(type_args.syntax().clone().into());
-        self.insert_children(InsertPosition::Last, to_insert)
-    }
-}
-
 impl ast::UseTree {
     /// Splits off the given prefix, making it the path component of the use tree, appending the rest of the path to all UseTreeList items.
     #[must_use]
@@ -234,16 +192,6 @@ fn prev_tokens(token: SyntaxToken) -> impl Iterator<Item = SyntaxToken> {
 
 pub trait AstNodeEdit: AstNode + Clone + Sized {
     #[must_use]
-    fn insert_children(
-        &self,
-        position: InsertPosition<SyntaxElement>,
-        to_insert: impl IntoIterator<Item = SyntaxElement>,
-    ) -> Self {
-        let new_syntax = algo::insert_children(self.syntax(), position, to_insert);
-        Self::cast(new_syntax).unwrap()
-    }
-
-    #[must_use]
     fn replace_children(
         &self,
         to_replace: RangeInclusive<SyntaxElement>,
diff --git a/crates/syntax/src/ast/edit_in_place.rs b/crates/syntax/src/ast/edit_in_place.rs
index 2676ed8c9..ca8103668 100644
--- a/crates/syntax/src/ast/edit_in_place.rs
+++ b/crates/syntax/src/ast/edit_in_place.rs
@@ -239,6 +239,16 @@ impl ast::TypeBoundList {
     }
 }
 
+impl ast::PathSegment {
+    pub fn get_or_create_generic_arg_list(&self) -> ast::GenericArgList {
+        if self.generic_arg_list().is_none() {
+            let arg_list = make::generic_arg_list().clone_for_update();
+            ted::append_child(self.syntax(), arg_list.syntax())
+        }
+        self.generic_arg_list().unwrap()
+    }
+}
+
 impl ast::UseTree {
     pub fn remove(&self) {
         for &dir in [Direction::Next, Direction::Prev].iter() {
diff --git a/crates/syntax/src/ast/make.rs b/crates/syntax/src/ast/make.rs
index d13926ded..0cf170626 100644
--- a/crates/syntax/src/ast/make.rs
+++ b/crates/syntax/src/ast/make.rs
@@ -106,6 +106,10 @@ pub fn impl_trait(trait_: ast::Path, ty: ast::Path) -> ast::Impl {
     ast_from_text(&format!("impl {} for {} {{}}", trait_, ty))
 }
 
+pub(crate) fn generic_arg_list() -> ast::GenericArgList {
+    ast_from_text("const S: T<> = ();")
+}
+
 pub fn path_segment(name_ref: ast::NameRef) -> ast::PathSegment {
     ast_from_text(&format!("use {};", name_ref))
 }
diff --git a/crates/syntax/src/ted.rs b/crates/syntax/src/ted.rs
index a50c0dbca..ae970f44f 100644
--- a/crates/syntax/src/ted.rs
+++ b/crates/syntax/src/ted.rs
@@ -184,6 +184,9 @@ fn ws_between(left: &SyntaxElement, right: &SyntaxElement) -> Option<SyntaxToken
     if left.kind() == T![&] && right.kind() == SyntaxKind::LIFETIME {
         return None;
     }
+    if right.kind() == SyntaxKind::GENERIC_ARG_LIST {
+        return None;
+    }
 
     if right.kind() == SyntaxKind::USE {
         let mut indent = IndentLevel::from_element(left);