2 files changed, 161 insertions, 0 deletions

diff --git a/crates/ide_db/src/lib.rs b/crates/ide_db/src/lib.rs
index 7bbd08d6f..bde8767dd 100644
--- a/crates/ide_db/src/lib.rs
+++ b/crates/ide_db/src/lib.rs
@@ -14,6 +14,7 @@ pub mod ty_filter;
 pub mod traits;
 pub mod call_info;
 pub mod helpers;
+pub mod path_transform;
 
 pub mod search;
 pub mod rename;
diff --git a/crates/ide_db/src/path_transform.rs b/crates/ide_db/src/path_transform.rs
new file mode 100644
index 000000000..f3d7aa920
--- /dev/null
+++ b/crates/ide_db/src/path_transform.rs
@@ -0,0 +1,160 @@
+//! See [`PathTransform`].
+
+use crate::helpers::mod_path_to_ast;
+use hir::{HirDisplay, SemanticsScope};
+use rustc_hash::FxHashMap;
+use syntax::{
+    ast::{self, AstNode},
+    ted,
+};
+
+/// `PathTransform` substitutes path in SyntaxNodes in bulk.
+///
+/// This is mostly useful for IDE code generation. If you paste some existing
+/// code into a new context (for example, to add method overrides to an `impl`
+/// block), you generally want to appropriately qualify the names, and sometimes
+/// you might want to substitute generic parameters as well:
+///
+/// ```
+/// mod x {
+///   pub struct A<V>;
+///   pub trait T<U> { fn foo(&self, _: U) -> A<U>; }
+/// }
+///
+/// mod y {
+///   use x::T;
+///
+///   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<()> {}
+///   }
+/// }
+/// ```
+pub struct PathTransform<'a> {
+    pub subst: (hir::Trait, ast::Impl),
+    pub target_scope: &'a SemanticsScope<'a>,
+    pub source_scope: &'a SemanticsScope<'a>,
+}
+
+impl<'a> PathTransform<'a> {
+    pub fn apply(&self, item: ast::AssocItem) {
+        if let Some(ctx) = self.build_ctx() {
+            ctx.apply(item)
+        }
+    }
+    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()?;
+
+        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(db)
+            .into_iter()
+            // this is a trait impl, so we need to skip the first type parameter -- this is a bit hacky
+            .skip(1)
+            // The actual list of trait type parameters may be longer than the one
+            // used in the `impl` block due to trailing default type parameters.
+            // For that case we extend the `substs` with an empty iterator so we
+            // can still hit those trailing values and check if they actually have
+            // a default type. If they do, go for that type from `hir` to `ast` so
+            // the resulting change can be applied correctly.
+            .zip(substs.into_iter().map(Some).chain(std::iter::repeat(None)))
+            .filter_map(|(k, v)| match v {
+                Some(v) => Some((k, v)),
+                None => {
+                    let default = k.default(db)?;
+                    Some((
+                        k,
+                        ast::make::ty(&default.display_source_code(db, source_module.into()).ok()?),
+                    ))
+                }
+            })
+            .collect();
+
+        let res = Ctx { substs: substs_by_param, target_module, source_scope: self.source_scope };
+        Some(res)
+    }
+}
+
+struct Ctx<'a> {
+    substs: FxHashMap<hir::TypeParam, ast::Type>,
+    target_module: hir::Module,
+    source_scope: &'a SemanticsScope<'a>,
+}
+
+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);
+            }
+        }
+    }
+    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(&path)?;
+
+        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())
+            }
+            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)
+}