1 files changed, 213 insertions, 0 deletions

diff --git a/crates/ide_assists/src/ast_transform.rs b/crates/ide_assists/src/ast_transform.rs
new file mode 100644
index 000000000..4a3ed7783
--- /dev/null
+++ b/crates/ide_assists/src/ast_transform.rs
@@ -0,0 +1,213 @@
+//! `AstTransformer`s are functions that replace nodes in an AST and can be easily combined.
+use hir::{HirDisplay, PathResolution, SemanticsScope};
+use ide_db::helpers::mod_path_to_ast;
+use rustc_hash::FxHashMap;
+use syntax::{
+    algo::SyntaxRewriter,
+    ast::{self, AstNode},
+    SyntaxNode,
+};
+
+pub fn apply<'a, N: AstNode>(transformer: &dyn AstTransform<'a>, node: N) -> N {
+    SyntaxRewriter::from_fn(|element| match element {
+        syntax::SyntaxElement::Node(n) => {
+            let replacement = transformer.get_substitution(&n, transformer)?;
+            Some(replacement.into())
+        }
+        _ => None,
+    })
+    .rewrite_ast(&node)
+}
+
+/// `AstTransform` helps with applying bulk transformations to syntax nodes.
+///
+/// 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;
+///   pub trait T<U> { fn foo(&self, _: U) -> A; }
+/// }
+///
+/// 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 {}
+///   }
+/// }
+/// ```
+///
+/// So, a single `AstTransform` describes such function from `SyntaxNode` to
+/// `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)))
+    }
+}
+
+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))
+    }
+}
+
+pub struct SubstituteTypeParams<'a> {
+    source_scope: &'a SemanticsScope<'a>,
+    substs: FxHashMap<hir::TypeParam, ast::Type>,
+}
+
+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_by_param: FxHashMap<_, _> = generic_def
+            .type_params(source_scope.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(source_scope.db)?;
+                    Some((
+                        k,
+                        ast::make::ty(
+                            &default
+                                .display_source_code(source_scope.db, source_scope.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)
+        }
+    }
+}
+
+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>,
+    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> 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() {
+            // 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())
+                    .map(|arg_list| apply(recur, arg_list));
+                if let Some(type_args) = type_args {
+                    let last_segment = path.segment().unwrap();
+                    path = path.with_segment(last_segment.with_generic_args(type_args))
+                }
+
+                Some(path.syntax().clone())
+            }
+            PathResolution::Local(_)
+            | PathResolution::TypeParam(_)
+            | PathResolution::SelfType(_)
+            | PathResolution::ConstParam(_) => None,
+            PathResolution::Macro(_) => None,
+            PathResolution::AssocItem(_) => None,
+        }
+    }
+}