44 files changed, 1463 insertions, 1333 deletions

diff --git a/README.md b/README.md
index 2f6ed1e05..fabb8479d 100644
--- a/README.md
+++ b/README.md
@@ -2,13 +2,13 @@
   <img src="https://user-images.githubusercontent.com/1711539/72443316-5a79f280-37ae-11ea-858f-035209ece2dd.png" alt="rust-analyzer logo">
 </p>
 
-Rust Analyzer is an **experimental** modular compiler frontend for the Rust
+rust-analyzer is an **experimental** modular compiler frontend for the Rust
 language. It is a part of a larger rls-2.0 effort to create excellent IDE
 support for Rust. If you want to get involved, check the rls-2.0 working group:
 
 https://rust-lang.zulipchat.com/#narrow/stream/185405-t-compiler.2Fwg-rls-2.2E0
 
-Work on the Rust Analyzer is sponsored by
+Work on rust-analyzer is sponsored by
 
 [<img src="https://user-images.githubusercontent.com/1711539/58105231-cf306900-7bee-11e9-83d8-9f1102e59d29.png" alt="Ferrous Systems" width="300">](https://ferrous-systems.com/)
 - [Mozilla](https://www.mozilla.org/en-US/)
@@ -16,17 +16,17 @@ Work on the Rust Analyzer is sponsored by
 
 ## Language Server Quick Start
 
-Rust Analyzer is a work-in-progress, so you'll have to build it from source, and
-you might encounter critical bugs. That said, it is complete enough to provide a
-useful IDE experience and some people use it as a daily driver.
+rust-analyzer is a work-in-progress, so you might encounter critical bugs. That
+said, it is complete enough to provide a useful IDE experience and some people
+use it as a daily driver.
 
 To build rust-analyzer, you need:
 
-* latest stable rust for language server itself
+* latest stable Rust for the language server itself
 * latest stable npm and VS Code for VS Code extension
 
-To quickly install rust-analyzer with VS Code extension with standard setup
-(`code` and `cargo` in `$PATH`, etc), use this:
+To quickly install the rust-analyzer language server and VS Code extension with
+standard setup (`code` and `cargo` in `$PATH`, etc), use this:
 
 ```
 # clone the repo
@@ -44,8 +44,8 @@ cannot start, see [./docs/user](./docs/user).
 
 ## Documentation
 
-If you want to **contribute** to rust-analyzer or just curious about how things work
-under the hood, check the [./docs/dev](./docs/dev) folder.
+If you want to **contribute** to rust-analyzer or are just curious about how
+things work under the hood, check the [./docs/dev](./docs/dev) folder.
 
 If you want to **use** rust-analyzer's language server with your editor of
 choice, check [./docs/user](./docs/user) folder. It also contains some tips & tricks to help
diff --git a/crates/ra_assists/src/assist_ctx.rs b/crates/ra_assists/src/assist_ctx.rs
index 81f999090..5aab5fb8b 100644
--- a/crates/ra_assists/src/assist_ctx.rs
+++ b/crates/ra_assists/src/assist_ctx.rs
@@ -1,5 +1,4 @@
 //! This module defines `AssistCtx` -- the API surface that is exposed to assists.
-use either::Either;
 use hir::{InFile, SourceAnalyzer, SourceBinder};
 use ra_db::{FileRange, SourceDatabase};
 use ra_fmt::{leading_indent, reindent};
@@ -11,12 +10,36 @@ use ra_syntax::{
 };
 use ra_text_edit::TextEditBuilder;
 
-use crate::{AssistAction, AssistId, AssistLabel, ResolvedAssist};
+use crate::{AssistAction, AssistId, AssistLabel, GroupLabel, ResolvedAssist};
 
 #[derive(Clone, Debug)]
-pub(crate) enum Assist {
-    Unresolved { label: AssistLabel },
-    Resolved { assist: ResolvedAssist },
+pub(crate) struct Assist(pub(crate) Vec<AssistInfo>);
+
+#[derive(Clone, Debug)]
+pub(crate) struct AssistInfo {
+    pub(crate) label: AssistLabel,
+    pub(crate) group_label: Option<GroupLabel>,
+    pub(crate) action: Option<AssistAction>,
+}
+
+impl AssistInfo {
+    fn new(label: AssistLabel) -> AssistInfo {
+        AssistInfo { label, group_label: None, action: None }
+    }
+
+    fn resolved(self, action: AssistAction) -> AssistInfo {
+        AssistInfo { action: Some(action), ..self }
+    }
+
+    fn with_group(self, group_label: GroupLabel) -> AssistInfo {
+        AssistInfo { group_label: Some(group_label), ..self }
+    }
+
+    pub(crate) fn into_resolved(self) -> Option<ResolvedAssist> {
+        let label = self.label;
+        let group_label = self.group_label;
+        self.action.map(|action| ResolvedAssist { label, group_label, action })
+    }
 }
 
 pub(crate) type AssistHandler = fn(AssistCtx) -> Option<Assist>;
@@ -84,44 +107,21 @@ impl<'a> AssistCtx<'a> {
     ) -> Option<Assist> {
         let label = AssistLabel::new(label.into(), id);
 
-        let assist = if self.should_compute_edit {
+        let mut info = AssistInfo::new(label);
+        if self.should_compute_edit {
             let action = {
                 let mut edit = ActionBuilder::default();
                 f(&mut edit);
                 edit.build()
             };
-            Assist::Resolved { assist: ResolvedAssist { label, action_data: Either::Left(action) } }
-        } else {
-            Assist::Unresolved { label }
+            info = info.resolved(action)
         };
 
-        Some(assist)
+        Some(Assist(vec![info]))
     }
 
-    pub(crate) fn add_assist_group(
-        self,
-        id: AssistId,
-        label: impl Into<String>,
-        f: impl FnOnce() -> Vec<ActionBuilder>,
-    ) -> Option<Assist> {
-        let label = AssistLabel::new(label.into(), id);
-        let assist = if self.should_compute_edit {
-            let actions = f();
-            assert!(!actions.is_empty(), "Assist cannot have no");
-
-            Assist::Resolved {
-                assist: ResolvedAssist {
-                    label,
-                    action_data: Either::Right(
-                        actions.into_iter().map(ActionBuilder::build).collect(),
-                    ),
-                },
-            }
-        } else {
-            Assist::Unresolved { label }
-        };
-
-        Some(assist)
+    pub(crate) fn add_assist_group(self, group_name: impl Into<String>) -> AssistGroup<'a> {
+        AssistGroup { ctx: self, group_name: group_name.into(), assists: Vec::new() }
     }
 
     pub(crate) fn token_at_offset(&self) -> TokenAtOffset<SyntaxToken> {
@@ -155,20 +155,48 @@ impl<'a> AssistCtx<'a> {
     }
 }
 
+pub(crate) struct AssistGroup<'a> {
+    ctx: AssistCtx<'a>,
+    group_name: String,
+    assists: Vec<AssistInfo>,
+}
+
+impl<'a> AssistGroup<'a> {
+    pub(crate) fn add_assist(
+        &mut self,
+        id: AssistId,
+        label: impl Into<String>,
+        f: impl FnOnce(&mut ActionBuilder),
+    ) {
+        let label = AssistLabel::new(label.into(), id);
+
+        let mut info = AssistInfo::new(label).with_group(GroupLabel(self.group_name.clone()));
+        if self.ctx.should_compute_edit {
+            let action = {
+                let mut edit = ActionBuilder::default();
+                f(&mut edit);
+                edit.build()
+            };
+            info = info.resolved(action)
+        };
+
+        self.assists.push(info)
+    }
+
+    pub(crate) fn finish(self) -> Option<Assist> {
+        assert!(!self.assists.is_empty());
+        Some(Assist(self.assists))
+    }
+}
+
 #[derive(Default)]
 pub(crate) struct ActionBuilder {
     edit: TextEditBuilder,
     cursor_position: Option<TextUnit>,
     target: Option<TextRange>,
-    label: Option<String>,
 }
 
 impl ActionBuilder {
-    /// Adds a custom label to the action, if it needs to be different from the assist label
-    pub(crate) fn label(&mut self, label: impl Into<String>) {
-        self.label = Some(label.into())
-    }
-
     /// Replaces specified `range` of text with a given string.
     pub(crate) fn replace(&mut self, range: TextRange, replace_with: impl Into<String>) {
         self.edit.replace(range, replace_with.into())
@@ -227,7 +255,6 @@ impl ActionBuilder {
             edit: self.edit.finish(),
             cursor_position: self.cursor_position,
             target: self.target,
-            label: self.label,
         }
     }
 }
diff --git a/crates/ra_assists/src/doc_tests.rs b/crates/ra_assists/src/doc_tests.rs
index ae0e5605c..c0f9bc1fb 100644
--- a/crates/ra_assists/src/doc_tests.rs
+++ b/crates/ra_assists/src/doc_tests.rs
@@ -30,6 +30,6 @@ fn check(assist_id: &str, before: &str, after: &str) {
             )
         });
 
-    let actual = assist.get_first_action().edit.apply(&before);
+    let actual = assist.action.edit.apply(&before);
     assert_eq_text!(after, &actual);
 }
diff --git a/crates/ra_assists/src/doc_tests/generated.rs b/crates/ra_assists/src/doc_tests/generated.rs
index 0d95b957b..4ab09b167 100644
--- a/crates/ra_assists/src/doc_tests/generated.rs
+++ b/crates/ra_assists/src/doc_tests/generated.rs
@@ -1,4 +1,4 @@
-//! Generated file, do not edit by hand, see `crate/ra_tools/src/codegen`
+//! Generated file, do not edit by hand, see `xtask/src/codegen`
 
 use super::check;
 
@@ -161,21 +161,6 @@ impl Trait<u32> for () {
 }
 
 #[test]
-fn doctest_add_import() {
-    check(
-        "add_import",
-        r#####"
-fn process(map: std::collections::<|>HashMap<String, String>) {}
-"#####,
-        r#####"
-use std::collections::HashMap;
-
-fn process(map: HashMap<String, String>) {}
-"#####,
-    )
-}
-
-#[test]
 fn doctest_add_new() {
     check(
         "add_new",
@@ -592,6 +577,21 @@ fn handle(action: Action) {
 }
 
 #[test]
+fn doctest_replace_qualified_name_with_use() {
+    check(
+        "replace_qualified_name_with_use",
+        r#####"
+fn process(map: std::collections::<|>HashMap<String, String>) {}
+"#####,
+        r#####"
+use std::collections::HashMap;
+
+fn process(map: HashMap<String, String>) {}
+"#####,
+    )
+}
+
+#[test]
 fn doctest_split_import() {
     check(
         "split_import",
diff --git a/crates/ra_assists/src/handlers/add_new.rs b/crates/ra_assists/src/handlers/add_new.rs
index a08639311..2701eddb8 100644
--- a/crates/ra_assists/src/handlers/add_new.rs
+++ b/crates/ra_assists/src/handlers/add_new.rs
@@ -1,5 +1,5 @@
 use format_buf::format;
-use hir::InFile;
+use hir::{Adt, InFile};
 use join_to_string::join;
 use ra_syntax::{
     ast::{
@@ -135,16 +135,22 @@ fn find_struct_impl(ctx: &AssistCtx, strukt: &ast::StructDef) -> Option<Option<a
     })?;
     let mut sb = ctx.source_binder();
 
-    let struct_ty = {
+    let struct_def = {
         let src = InFile { file_id: ctx.frange.file_id.into(), value: strukt.clone() };
-        sb.to_def(src)?.ty(db)
+        sb.to_def(src)?
     };
 
     let block = module.descendants().filter_map(ast::ImplBlock::cast).find_map(|impl_blk| {
         let src = InFile { file_id: ctx.frange.file_id.into(), value: impl_blk.clone() };
         let blk = sb.to_def(src)?;
 
-        let same_ty = blk.target_ty(db) == struct_ty;
+        // FIXME: handle e.g. `struct S<T>; impl<U> S<U> {}`
+        // (we currently use the wrong type parameter)
+        // also we wouldn't want to use e.g. `impl S<u32>`
+        let same_ty = match blk.target_ty(db).as_adt() {
+            Some(def) => def == Adt::Struct(struct_def),
+            None => false,
+        };
         let not_trait_impl = blk.target_trait(db).is_none();
 
         if !(same_ty && not_trait_impl) {
diff --git a/crates/ra_assists/src/handlers/auto_import.rs b/crates/ra_assists/src/handlers/auto_import.rs
index 84b5474f9..e88c121eb 100644
--- a/crates/ra_assists/src/handlers/auto_import.rs
+++ b/crates/ra_assists/src/handlers/auto_import.rs
@@ -1,13 +1,9 @@
-use hir::ModPath;
 use ra_ide_db::imports_locator::ImportsLocator;
-use ra_syntax::{
-    ast::{self, AstNode},
-    SyntaxNode,
-};
+use ra_syntax::ast::{self, AstNode};
 
 use crate::{
-    assist_ctx::{ActionBuilder, Assist, AssistCtx},
-    auto_import_text_edit, AssistId,
+    assist_ctx::{Assist, AssistCtx},
+    insert_use_statement, AssistId,
 };
 use std::collections::BTreeSet;
 
@@ -67,24 +63,24 @@ pub(crate) fn auto_import(ctx: AssistCtx) -> Option<Assist> {
         return None;
     }
 
-    ctx.add_assist_group(AssistId("auto_import"), format!("Import {}", name_to_import), || {
-        proposed_imports
-            .into_iter()
-            .map(|import| import_to_action(import, &position, &path_to_import_syntax))
-            .collect()
-    })
-}
-
-fn import_to_action(import: ModPath, position: &SyntaxNode, anchor: &SyntaxNode) -> ActionBuilder {
-    let mut action_builder = ActionBuilder::default();
-    action_builder.label(format!("Import `{}`", &import));
-    auto_import_text_edit(position, anchor, &import, action_builder.text_edit_builder());
-    action_builder
+    let mut group = ctx.add_assist_group(format!("Import {}", name_to_import));
+    for import in proposed_imports {
+        group.add_assist(AssistId("auto_import"), format!("Import `{}`", &import), |edit| {
+            edit.target(path_to_import_syntax.text_range());
+            insert_use_statement(
+                &position,
+                path_to_import_syntax,
+                &import,
+                edit.text_edit_builder(),
+            );
+        });
+    }
+    group.finish()
 }
 
 #[cfg(test)]
 mod tests {
-    use crate::helpers::{check_assist, check_assist_not_applicable};
+    use crate::helpers::{check_assist, check_assist_not_applicable, check_assist_target};
 
     use super::*;
 
@@ -255,4 +251,19 @@ mod tests {
             ",
         );
     }
+
+    #[test]
+    fn auto_import_target() {
+        check_assist_target(
+            auto_import,
+            r"
+            struct AssistInfo {
+                group_label: Option<<|>GroupLabel>,
+            }
+
+            mod m { pub struct GroupLabel; }
+            ",
+            "GroupLabel",
+        )
+    }
 }
diff --git a/crates/ra_assists/src/handlers/add_import.rs b/crates/ra_assists/src/handlers/replace_qualified_name_with_use.rs
index f03dddac8..b70c88ec2 100644
--- a/crates/ra_assists/src/handlers/add_import.rs
+++ b/crates/ra_assists/src/handlers/replace_qualified_name_with_use.rs
@@ -12,10 +12,10 @@ use crate::{
     AssistId,
 };
 
-/// This function produces sequence of text edits into edit
-/// to import the target path in the most appropriate scope given
-/// the cursor position
-pub fn auto_import_text_edit(
+/// Creates and inserts a use statement for the given path to import.
+/// The use statement is inserted in the scope most appropriate to the
+/// the cursor position given, additionally merged with the existing use imports.
+pub fn insert_use_statement(
     // Ideally the position of the cursor, used to
     position: &SyntaxNode,
     // The statement to use as anchor (last resort)
@@ -37,9 +37,9 @@ pub fn auto_import_text_edit(
     }
 }
 
-// Assist: add_import
+// Assist: replace_qualified_name_with_use
 //
-// Adds a use statement for a given fully-qualified path.
+// Adds a use statement for a given fully-qualified name.
 //
 // ```
 // fn process(map: std::collections::<|>HashMap<String, String>) {}
@@ -50,7 +50,7 @@ pub fn auto_import_text_edit(
 //
 // fn process(map: HashMap<String, String>) {}
 // ```
-pub(crate) fn add_import(ctx: AssistCtx) -> Option<Assist> {
+pub(crate) fn replace_qualified_name_with_use(ctx: AssistCtx) -> Option<Assist> {
     let path: ast::Path = ctx.find_node_at_offset()?;
     // We don't want to mess with use statements
     if path.syntax().ancestors().find_map(ast::UseItem::cast).is_some() {
@@ -72,9 +72,13 @@ pub(crate) fn add_import(ctx: AssistCtx) -> Option<Assist> {
         }
     };
 
-    ctx.add_assist(AssistId("add_import"), format!("Import {}", fmt_segments(&segments)), |edit| {
-        apply_auto_import(&position, &path, &segments, edit.text_edit_builder());
-    })
+    ctx.add_assist(
+        AssistId("replace_qualified_name_with_use"),
+        "Replace qualified path with use",
+        |edit| {
+            replace_with_use(&position, &path, &segments, edit.text_edit_builder());
+        },
+    )
 }
 
 fn collect_path_segments_raw(
@@ -107,12 +111,6 @@ fn collect_path_segments_raw(
     Some(segments.len() - oldlen)
 }
 
-fn fmt_segments(segments: &[SmolStr]) -> String {
-    let mut buf = String::new();
-    fmt_segments_raw(segments, &mut buf);
-    buf
-}
-
 fn fmt_segments_raw(segments: &[SmolStr], buf: &mut String) {
     let mut iter = segments.iter();
     if let Some(s) = iter.next() {
@@ -558,7 +556,7 @@ fn make_assist_add_nested_import(
     }
 }
 
-fn apply_auto_import(
+fn replace_with_use(
     container: &SyntaxNode,
     path: &ast::Path,
     target: &[SmolStr],
@@ -567,7 +565,7 @@ fn apply_auto_import(
     let action = best_action_for_target(container.clone(), path.syntax().clone(), target);
     make_assist(&action, target, edit);
     if let Some(last) = path.segment() {
-        // Here we are assuming the assist will provide a  correct use statement
+        // Here we are assuming the assist will provide a correct use statement
         // so we can delete the path qualifier
         edit.delete(TextRange::from_to(
             path.syntax().text_range().start(),
@@ -603,9 +601,9 @@ mod tests {
     use super::*;
 
     #[test]
-    fn test_auto_import_add_use_no_anchor() {
+    fn test_replace_add_use_no_anchor() {
         check_assist(
-            add_import,
+            replace_qualified_name_with_use,
             "
 std::fmt::Debug<|>
     ",
@@ -617,9 +615,9 @@ Debug<|>
         );
     }
     #[test]
-    fn test_auto_import_add_use_no_anchor_with_item_below() {
+    fn test_replace_add_use_no_anchor_with_item_below() {
         check_assist(
-            add_import,
+            replace_qualified_name_with_use,
             "
 std::fmt::Debug<|>
 
@@ -638,9 +636,9 @@ fn main() {
     }
 
     #[test]
-    fn test_auto_import_add_use_no_anchor_with_item_above() {
+    fn test_replace_add_use_no_anchor_with_item_above() {
         check_assist(
-            add_import,
+            replace_qualified_name_with_use,
             "
 fn main() {
 }
@@ -659,9 +657,9 @@ Debug<|>
     }
 
     #[test]
-    fn test_auto_import_add_use_no_anchor_2seg() {
+    fn test_replace_add_use_no_anchor_2seg() {
         check_assist(
-            add_import,
+            replace_qualified_name_with_use,
             "
 std::fmt<|>::Debug
     ",
@@ -674,9 +672,9 @@ fmt<|>::Debug
     }
 
     #[test]
-    fn test_auto_import_add_use() {
+    fn test_replace_add_use() {
         check_assist(
-            add_import,
+            replace_qualified_name_with_use,
             "
 use stdx;
 
@@ -694,9 +692,9 @@ impl Debug<|> for Foo {
     }
 
     #[test]
-    fn test_auto_import_file_use_other_anchor() {
+    fn test_replace_file_use_other_anchor() {
         check_assist(
-            add_import,
+            replace_qualified_name_with_use,
             "
 impl std::fmt::Debug<|> for Foo {
 }
@@ -711,9 +709,9 @@ impl Debug<|> for Foo {
     }
 
     #[test]
-    fn test_auto_import_add_use_other_anchor_indent() {
+    fn test_replace_add_use_other_anchor_indent() {
         check_assist(
-            add_import,
+            replace_qualified_name_with_use,
             "
     impl std::fmt::Debug<|> for Foo {
     }
@@ -728,9 +726,9 @@ impl Debug<|> for Foo {
     }
 
     #[test]
-    fn test_auto_import_split_different() {
+    fn test_replace_split_different() {
         check_assist(
-            add_import,
+            replace_qualified_name_with_use,
             "
 use std::fmt;
 
@@ -747,9 +745,9 @@ impl io<|> for Foo {
     }
 
     #[test]
-    fn test_auto_import_split_self_for_use() {
+    fn test_replace_split_self_for_use() {
         check_assist(
-            add_import,
+            replace_qualified_name_with_use,
             "
 use std::fmt;
 
@@ -766,9 +764,9 @@ impl Debug<|> for Foo {
     }
 
     #[test]
-    fn test_auto_import_split_self_for_target() {
+    fn test_replace_split_self_for_target() {
         check_assist(
-            add_import,
+            replace_qualified_name_with_use,
             "
 use std::fmt::Debug;
 
@@ -785,9 +783,9 @@ impl fmt<|> for Foo {
     }
 
     #[test]
-    fn test_auto_import_add_to_nested_self_nested() {
+    fn test_replace_add_to_nested_self_nested() {
         check_assist(
-            add_import,
+            replace_qualified_name_with_use,
             "
 use std::fmt::{Debug, nested::{Display}};
 
@@ -804,9 +802,9 @@ impl nested<|> for Foo {
     }
 
     #[test]
-    fn test_auto_import_add_to_nested_self_already_included() {
+    fn test_replace_add_to_nested_self_already_included() {
         check_assist(
-            add_import,
+            replace_qualified_name_with_use,
             "
 use std::fmt::{Debug, nested::{self, Display}};
 
@@ -823,9 +821,9 @@ impl nested<|> for Foo {
     }
 
     #[test]
-    fn test_auto_import_add_to_nested_nested() {
+    fn test_replace_add_to_nested_nested() {
         check_assist(
-            add_import,
+            replace_qualified_name_with_use,
             "
 use std::fmt::{Debug, nested::{Display}};
 
@@ -842,9 +840,9 @@ impl Debug<|> for Foo {
     }
 
     #[test]
-    fn test_auto_import_split_common_target_longer() {
+    fn test_replace_split_common_target_longer() {
         check_assist(
-            add_import,
+            replace_qualified_name_with_use,
             "
 use std::fmt::Debug;
 
@@ -861,9 +859,9 @@ impl Display<|> for Foo {
     }
 
     #[test]
-    fn test_auto_import_split_common_use_longer() {
+    fn test_replace_split_common_use_longer() {
         check_assist(
-            add_import,
+            replace_qualified_name_with_use,
             "
 use std::fmt::nested::Debug;
 
@@ -880,9 +878,9 @@ impl Display<|> for Foo {
     }
 
     #[test]
-    fn test_auto_import_use_nested_import() {
+    fn test_replace_use_nested_import() {
         check_assist(
-            add_import,
+            replace_qualified_name_with_use,
             "
 use crate::{
     ty::{Substs, Ty},
@@ -903,9 +901,9 @@ fn foo() { lower<|>::trait_env() }
     }
 
     #[test]
-    fn test_auto_import_alias() {
+    fn test_replace_alias() {
         check_assist(
-            add_import,
+            replace_qualified_name_with_use,
             "
 use std::fmt as foo;
 
@@ -922,9 +920,9 @@ impl Debug<|> for Foo {
     }
 
     #[test]
-    fn test_auto_import_not_applicable_one_segment() {
+    fn test_replace_not_applicable_one_segment() {
         check_assist_not_applicable(
-            add_import,
+            replace_qualified_name_with_use,
             "
 impl foo<|> for Foo {
 }
@@ -933,9 +931,9 @@ impl foo<|> for Foo {
     }
 
     #[test]
-    fn test_auto_import_not_applicable_in_use() {
+    fn test_replace_not_applicable_in_use() {
         check_assist_not_applicable(
-            add_import,
+            replace_qualified_name_with_use,
             "
 use std::fmt<|>;
 ",
@@ -943,9 +941,9 @@ use std::fmt<|>;
     }
 
     #[test]
-    fn test_auto_import_add_use_no_anchor_in_mod_mod() {
+    fn test_replace_add_use_no_anchor_in_mod_mod() {
         check_assist(
-            add_import,
+            replace_qualified_name_with_use,
             "
 mod foo {
     mod bar {
diff --git a/crates/ra_assists/src/lib.rs b/crates/ra_assists/src/lib.rs
index eca6dec4b..828a8e9e8 100644
--- a/crates/ra_assists/src/lib.rs
+++ b/crates/ra_assists/src/lib.rs
@@ -12,16 +12,13 @@ mod doc_tests;
 mod utils;
 pub mod ast_transform;
 
-use std::cmp::Ordering;
-
-use either::Either;
 use ra_db::FileRange;
 use ra_ide_db::RootDatabase;
 use ra_syntax::{TextRange, TextUnit};
 use ra_text_edit::TextEdit;
 
 pub(crate) use crate::assist_ctx::{Assist, AssistCtx, AssistHandler};
-pub use crate::handlers::add_import::auto_import_text_edit;
+pub use crate::handlers::replace_qualified_name_with_use::insert_use_statement;
 
 /// Unique identifier of the assist, should not be shown to the user
 /// directly.
@@ -35,6 +32,9 @@ pub struct AssistLabel {
     pub id: AssistId,
 }
 
+#[derive(Clone, Debug)]
+pub struct GroupLabel(pub String);
+
 impl AssistLabel {
     pub(crate) fn new(label: String, id: AssistId) -> AssistLabel {
         // FIXME: make fields private, so that this invariant can't be broken
@@ -45,7 +45,6 @@ impl AssistLabel {
 
 #[derive(Debug, Clone)]
 pub struct AssistAction {
-    pub label: Option<String>,
     pub edit: TextEdit,
     pub cursor_position: Option<TextUnit>,
     // FIXME: This belongs to `AssistLabel`
@@ -55,16 +54,8 @@ pub struct AssistAction {
 #[derive(Debug, Clone)]
 pub struct ResolvedAssist {
     pub label: AssistLabel,
-    pub action_data: Either<AssistAction, Vec<AssistAction>>,
-}
-
-impl ResolvedAssist {
-    pub fn get_first_action(&self) -> AssistAction {
-        match &self.action_data {
-            Either::Left(action) => action.clone(),
-            Either::Right(actions) => actions[0].clone(),
-        }
-    }
+    pub group_label: Option<GroupLabel>,
+    pub action: AssistAction,
 }
 
 /// Return all the assists applicable at the given position.
@@ -76,10 +67,8 @@ pub fn unresolved_assists(db: &RootDatabase, range: FileRange) -> Vec<AssistLabe
     handlers::all()
         .iter()
         .filter_map(|f| f(ctx.clone()))
-        .map(|a| match a {
-            Assist::Unresolved { label } => label,
-            Assist::Resolved { .. } => unreachable!(),
-        })
+        .flat_map(|it| it.0)
+        .map(|a| a.label)
         .collect()
 }
 
@@ -92,24 +81,13 @@ pub fn resolved_assists(db: &RootDatabase, range: FileRange) -> Vec<ResolvedAssi
     let mut a = handlers::all()
         .iter()
         .filter_map(|f| f(ctx.clone()))
-        .map(|a| match a {
-            Assist::Resolved { assist } => assist,
-            Assist::Unresolved { .. } => unreachable!(),
-        })
+        .flat_map(|it| it.0)
+        .map(|it| it.into_resolved().unwrap())
         .collect::<Vec<_>>();
-    sort_assists(&mut a);
+    a.sort_by_key(|it| it.action.target.map_or(TextUnit::from(!0u32), |it| it.len()));
     a
 }
 
-fn sort_assists(assists: &mut [ResolvedAssist]) {
-    assists.sort_by(|a, b| match (a.get_first_action().target, b.get_first_action().target) {
-        (Some(a), Some(b)) => a.len().cmp(&b.len()),
-        (Some(_), None) => Ordering::Less,
-        (None, Some(_)) => Ordering::Greater,
-        (None, None) => Ordering::Equal,
-    });
-}
-
 mod handlers {
     use crate::AssistHandler;
 
@@ -133,7 +111,7 @@ mod handlers {
     mod replace_if_let_with_match;
     mod split_import;
     mod remove_dbg;
-    pub(crate) mod add_import;
+    pub(crate) mod replace_qualified_name_with_use;
     mod add_missing_impl_members;
     mod move_guard;
     mod move_bounds;
@@ -158,7 +136,7 @@ mod handlers {
             replace_if_let_with_match::replace_if_let_with_match,
             split_import::split_import,
             remove_dbg::remove_dbg,
-            add_import::add_import,
+            replace_qualified_name_with_use::replace_qualified_name_with_use,
             add_missing_impl_members::add_missing_impl_members,
             add_missing_impl_members::add_missing_default_members,
             inline_local_variable::inline_local_variable,
@@ -184,7 +162,7 @@ mod helpers {
     use ra_syntax::TextRange;
     use test_utils::{add_cursor, assert_eq_text, extract_offset, extract_range};
 
-    use crate::{Assist, AssistCtx, AssistHandler};
+    use crate::{AssistCtx, AssistHandler};
 
     pub(crate) fn with_single_file(text: &str) -> (RootDatabase, FileId) {
         let (mut db, file_id) = RootDatabase::with_single_file(text);
@@ -202,10 +180,7 @@ mod helpers {
             FileRange { file_id, range: TextRange::offset_len(before_cursor_pos, 0.into()) };
         let assist =
             assist(AssistCtx::new(&db, frange, true)).expect("code action is not applicable");
-        let action = match assist {
-            Assist::Unresolved { .. } => unreachable!(),
-            Assist::Resolved { assist } => assist.get_first_action(),
-        };
+        let action = assist.0[0].action.clone().unwrap();
 
         let actual = action.edit.apply(&before);
         let actual_cursor_pos = match action.cursor_position {
@@ -225,10 +200,7 @@ mod helpers {
         let frange = FileRange { file_id, range };
         let assist =
             assist(AssistCtx::new(&db, frange, true)).expect("code action is not applicable");
-        let action = match assist {
-            Assist::Unresolved { .. } => unreachable!(),
-            Assist::Resolved { assist } => assist.get_first_action(),
-        };
+        let action = assist.0[0].action.clone().unwrap();
 
         let mut actual = action.edit.apply(&before);
         if let Some(pos) = action.cursor_position {
@@ -244,10 +216,7 @@ mod helpers {
             FileRange { file_id, range: TextRange::offset_len(before_cursor_pos, 0.into()) };
         let assist =
             assist(AssistCtx::new(&db, frange, true)).expect("code action is not applicable");
-        let action = match assist {
-            Assist::Unresolved { .. } => unreachable!(),
-            Assist::Resolved { assist } => assist.get_first_action(),
-        };
+        let action = assist.0[0].action.clone().unwrap();
 
         let range = action.target.expect("expected target on action");
         assert_eq_text!(&before[range.start().to_usize()..range.end().to_usize()], target);
@@ -259,10 +228,7 @@ mod helpers {
         let frange = FileRange { file_id, range };
         let assist =
             assist(AssistCtx::new(&db, frange, true)).expect("code action is not applicable");
-        let action = match assist {
-            Assist::Unresolved { .. } => unreachable!(),
-            Assist::Resolved { assist } => assist.get_first_action(),
-        };
+        let action = assist.0[0].action.clone().unwrap();
 
         let range = action.target.expect("expected target on action");
         assert_eq_text!(&before[range.start().to_usize()..range.end().to_usize()], target);
diff --git a/crates/ra_hir/src/code_model.rs b/crates/ra_hir/src/code_model.rs
index eaacf8c9e..4d9641728 100644
--- a/crates/ra_hir/src/code_model.rs
+++ b/crates/ra_hir/src/code_model.rs
@@ -10,9 +10,9 @@ use hir_def::{
     per_ns::PerNs,
     resolver::HasResolver,
     type_ref::{Mutability, TypeRef},
-    AdtId, ConstId, DefWithBodyId, EnumId, FunctionId, HasModule, ImplId, LocalEnumVariantId,
-    LocalModuleId, LocalStructFieldId, Lookup, ModuleId, StaticId, StructId, TraitId, TypeAliasId,
-    TypeParamId, UnionId,
+    AdtId, ConstId, DefWithBodyId, EnumId, FunctionId, GenericDefId, HasModule, ImplId,
+    LocalEnumVariantId, LocalModuleId, LocalStructFieldId, Lookup, ModuleId, StaticId, StructId,
+    TraitId, TypeAliasId, TypeParamId, UnionId,
 };
 use hir_expand::{
     diagnostics::DiagnosticSink,
@@ -21,7 +21,7 @@ use hir_expand::{
 };
 use hir_ty::{
     autoderef, display::HirFormatter, expr::ExprValidator, method_resolution, ApplicationTy,
-    Canonical, InEnvironment, TraitEnvironment, Ty, TyDefId, TypeCtor, TypeWalk,
+    Canonical, InEnvironment, Substs, TraitEnvironment, Ty, TyDefId, TypeCtor,
 };
 use ra_db::{CrateId, Edition, FileId};
 use ra_prof::profile;
@@ -270,7 +270,13 @@ impl StructField {
 
     pub fn ty(&self, db: &impl HirDatabase) -> Type {
         let var_id = self.parent.into();
-        let ty = db.field_types(var_id)[self.id].clone();
+        let generic_def_id: GenericDefId = match self.parent {
+            VariantDef::Struct(it) => it.id.into(),
+            VariantDef::Union(it) => it.id.into(),
+            VariantDef::EnumVariant(it) => it.parent.id.into(),
+        };
+        let substs = Substs::type_params(db, generic_def_id);
+        let ty = db.field_types(var_id)[self.id].clone().subst(&substs);
         Type::new(db, self.parent.module(db).id.krate.into(), var_id, ty)
     }
 
@@ -755,7 +761,7 @@ pub struct TypeParam {
 impl TypeParam {
     pub fn name(self, db: &impl HirDatabase) -> Name {
         let params = db.generic_params(self.id.parent);
-        params.types[self.id.local_id].name.clone()
+        params.types[self.id.local_id].name.clone().unwrap_or_else(Name::missing)
     }
 
     pub fn module(self, db: &impl HirDatabase) -> Module {
@@ -789,8 +795,9 @@ impl ImplBlock {
     pub fn target_ty(&self, db: &impl HirDatabase) -> Type {
         let impl_data = db.impl_data(self.id);
         let resolver = self.id.resolver(db);
+        let ctx = hir_ty::TyLoweringContext::new(db, &resolver);
         let environment = TraitEnvironment::lower(db, &resolver);
-        let ty = Ty::from_hir(db, &resolver, &impl_data.target_type);
+        let ty = Ty::from_hir(&ctx, &impl_data.target_type);
         Type {
             krate: self.id.lookup(db).container.module(db).krate,
             ty: InEnvironment { value: ty, environment },
@@ -851,9 +858,10 @@ impl Type {
     fn from_def(
         db: &impl HirDatabase,
         krate: CrateId,
-        def: impl HasResolver + Into<TyDefId>,
+        def: impl HasResolver + Into<TyDefId> + Into<GenericDefId>,
     ) -> Type {
-        let ty = db.ty(def.into());
+        let substs = Substs::type_params(db, def);
+        let ty = db.ty(def.into()).subst(&substs);
         Type::new(db, krate, def, ty)
     }
 
@@ -950,7 +958,7 @@ impl Type {
             match a_ty.ctor {
                 TypeCtor::Tuple { .. } => {
                     for ty in a_ty.parameters.iter() {
-                        let ty = ty.clone().subst(&a_ty.parameters);
+                        let ty = ty.clone();
                         res.push(self.derived(ty));
                     }
                 }
diff --git a/crates/ra_hir/src/lib.rs b/crates/ra_hir/src/lib.rs
index 86e422779..e1c7b7a20 100644
--- a/crates/ra_hir/src/lib.rs
+++ b/crates/ra_hir/src/lib.rs
@@ -58,7 +58,6 @@ pub use hir_def::{
     type_ref::Mutability,
 };
 pub use hir_expand::{
-    name::{name, Name},
-    HirFileId, InFile, MacroCallId, MacroCallLoc, MacroDefId, MacroFile, Origin,
+    name::Name, HirFileId, InFile, MacroCallId, MacroCallLoc, MacroDefId, MacroFile, Origin,
 };
 pub use hir_ty::{display::HirDisplay, CallableDef};
diff --git a/crates/ra_hir/src/source_analyzer.rs b/crates/ra_hir/src/source_analyzer.rs
index 4f8fc9602..bb9a35c5d 100644
--- a/crates/ra_hir/src/source_analyzer.rs
+++ b/crates/ra_hir/src/source_analyzer.rs
@@ -178,6 +178,10 @@ impl SourceAnalyzer {
         }
     }
 
+    fn trait_env(&self, db: &impl HirDatabase) -> Arc<TraitEnvironment> {
+        TraitEnvironment::lower(db, &self.resolver)
+    }
+
     pub fn type_of(&self, db: &impl HirDatabase, expr: &ast::Expr) -> Option<Type> {
         let expr_id = if let Some(expr) = self.expand_expr(db, InFile::new(self.file_id, expr)) {
             self.body_source_map.as_ref()?.node_expr(expr.as_ref())?
@@ -186,14 +190,14 @@ impl SourceAnalyzer {
         };
 
         let ty = self.infer.as_ref()?[expr_id].clone();
-        let environment = TraitEnvironment::lower(db, &self.resolver);
+        let environment = self.trait_env(db);
         Some(Type { krate: self.resolver.krate()?, ty: InEnvironment { value: ty, environment } })
     }
 
     pub fn type_of_pat(&self, db: &impl HirDatabase, pat: &ast::Pat) -> Option<Type> {
         let pat_id = self.pat_id(pat)?;
         let ty = self.infer.as_ref()?[pat_id].clone();
-        let environment = TraitEnvironment::lower(db, &self.resolver);
+        let environment = self.trait_env(db);
         Some(Type { krate: self.resolver.krate()?, ty: InEnvironment { value: ty, environment } })
     }
 
diff --git a/crates/ra_hir_def/src/generics.rs b/crates/ra_hir_def/src/generics.rs
index e9c28c730..f765e6edc 100644
--- a/crates/ra_hir_def/src/generics.rs
+++ b/crates/ra_hir_def/src/generics.rs
@@ -27,8 +27,16 @@ use crate::{
 /// Data about a generic parameter (to a function, struct, impl, ...).
 #[derive(Clone, PartialEq, Eq, Debug)]
 pub struct TypeParamData {
-    pub name: Name,
+    pub name: Option<Name>,
     pub default: Option<TypeRef>,
+    pub provenance: TypeParamProvenance,
+}
+
+#[derive(Copy, Clone, PartialEq, Eq, Debug)]
+pub enum TypeParamProvenance {
+    TypeParamList,
+    TraitSelf,
+    ArgumentImplTrait,
 }
 
 /// Data about the generic parameters of a function, struct, impl, etc.
@@ -45,10 +53,17 @@ pub struct GenericParams {
 /// associated type bindings like `Iterator<Item = u32>`.
 #[derive(Clone, PartialEq, Eq, Debug)]
 pub struct WherePredicate {
-    pub type_ref: TypeRef,
+    pub target: WherePredicateTarget,
     pub bound: TypeBound,
 }
 
+#[derive(Clone, PartialEq, Eq, Debug)]
+pub enum WherePredicateTarget {
+    TypeRef(TypeRef),
+    /// For desugared where predicates that can directly refer to a type param.
+    TypeParam(LocalTypeParamId),
+}
+
 type SourceMap = ArenaMap<LocalTypeParamId, Either<ast::TraitDef, ast::TypeParam>>;
 
 impl GenericParams {
@@ -68,6 +83,11 @@ impl GenericParams {
             GenericDefId::FunctionId(it) => {
                 let src = it.lookup(db).source(db);
                 generics.fill(&mut sm, &src.value);
+                // lower `impl Trait` in arguments
+                let data = db.function_data(it);
+                for param in &data.params {
+                    generics.fill_implicit_impl_trait_args(param);
+                }
                 src.file_id
             }
             GenericDefId::AdtId(AdtId::StructId(it)) => {
@@ -89,8 +109,11 @@ impl GenericParams {
                 let src = it.lookup(db).source(db);
 
                 // traits get the Self type as an implicit first type parameter
-                let self_param_id =
-                    generics.types.alloc(TypeParamData { name: name![Self], default: None });
+                let self_param_id = generics.types.alloc(TypeParamData {
+                    name: Some(name![Self]),
+                    default: None,
+                    provenance: TypeParamProvenance::TraitSelf,
+                });
                 sm.insert(self_param_id, Either::Left(src.value.clone()));
                 // add super traits as bounds on Self
                 // i.e., trait Foo: Bar is equivalent to trait Foo where Self: Bar
@@ -142,7 +165,11 @@ impl GenericParams {
             let name = type_param.name().map_or_else(Name::missing, |it| it.as_name());
             // FIXME: Use `Path::from_src`
             let default = type_param.default_type().map(TypeRef::from_ast);
-            let param = TypeParamData { name: name.clone(), default };
+            let param = TypeParamData {
+                name: Some(name.clone()),
+                default,
+                provenance: TypeParamProvenance::TypeParamList,
+            };
             let param_id = self.types.alloc(param);
             sm.insert(param_id, Either::Right(type_param.clone()));
 
@@ -170,11 +197,43 @@ impl GenericParams {
             return;
         }
         let bound = TypeBound::from_ast(bound);
-        self.where_predicates.push(WherePredicate { type_ref, bound });
+        self.where_predicates
+            .push(WherePredicate { target: WherePredicateTarget::TypeRef(type_ref), bound });
+    }
+
+    fn fill_implicit_impl_trait_args(&mut self, type_ref: &TypeRef) {
+        type_ref.walk(&mut |type_ref| {
+            if let TypeRef::ImplTrait(bounds) = type_ref {
+                let param = TypeParamData {
+                    name: None,
+                    default: None,
+                    provenance: TypeParamProvenance::ArgumentImplTrait,
+                };
+                let param_id = self.types.alloc(param);
+                for bound in bounds {
+                    self.where_predicates.push(WherePredicate {
+                        target: WherePredicateTarget::TypeParam(param_id),
+                        bound: bound.clone(),
+                    });
+                }
+            }
+        });
     }
 
     pub fn find_by_name(&self, name: &Name) -> Option<LocalTypeParamId> {
-        self.types.iter().find_map(|(id, p)| if &p.name == name { Some(id) } else { None })
+        self.types
+            .iter()
+            .find_map(|(id, p)| if p.name.as_ref() == Some(name) { Some(id) } else { None })
+    }
+
+    pub fn find_trait_self_param(&self) -> Option<LocalTypeParamId> {
+        self.types.iter().find_map(|(id, p)| {
+            if p.provenance == TypeParamProvenance::TraitSelf {
+                Some(id)
+            } else {
+                None
+            }
+        })
     }
 }
 
diff --git a/crates/ra_hir_def/src/resolver.rs b/crates/ra_hir_def/src/resolver.rs
index f7bac5801..05cf4646a 100644
--- a/crates/ra_hir_def/src/resolver.rs
+++ b/crates/ra_hir_def/src/resolver.rs
@@ -490,10 +490,12 @@ impl Scope {
             }
             Scope::GenericParams { params, def } => {
                 for (local_id, param) in params.types.iter() {
-                    f(
-                        param.name.clone(),
-                        ScopeDef::GenericParam(TypeParamId { local_id, parent: *def }),
-                    )
+                    if let Some(name) = &param.name {
+                        f(
+                            name.clone(),
+                            ScopeDef::GenericParam(TypeParamId { local_id, parent: *def }),
+                        )
+                    }
                 }
             }
             Scope::ImplBlockScope(i) => {
diff --git a/crates/ra_hir_def/src/type_ref.rs b/crates/ra_hir_def/src/type_ref.rs
index 5f10e9a88..102fdb13d 100644
--- a/crates/ra_hir_def/src/type_ref.rs
+++ b/crates/ra_hir_def/src/type_ref.rs
@@ -124,6 +124,48 @@ impl TypeRef {
     pub(crate) fn unit() -> TypeRef {
         TypeRef::Tuple(Vec::new())
     }
+
+    pub fn walk(&self, f: &mut impl FnMut(&TypeRef)) {
+        go(self, f);
+
+        fn go(type_ref: &TypeRef, f: &mut impl FnMut(&TypeRef)) {
+            f(type_ref);
+            match type_ref {
+                TypeRef::Fn(types) | TypeRef::Tuple(types) => types.iter().for_each(|t| go(t, f)),
+                TypeRef::RawPtr(type_ref, _)
+                | TypeRef::Reference(type_ref, _)
+                | TypeRef::Array(type_ref)
+                | TypeRef::Slice(type_ref) => go(&type_ref, f),
+                TypeRef::ImplTrait(bounds) | TypeRef::DynTrait(bounds) => {
+                    for bound in bounds {
+                        match bound {
+                            TypeBound::Path(path) => go_path(path, f),
+                            TypeBound::Error => (),
+                        }
+                    }
+                }
+                TypeRef::Path(path) => go_path(path, f),
+                TypeRef::Never | TypeRef::Placeholder | TypeRef::Error => {}
+            };
+        }
+
+        fn go_path(path: &Path, f: &mut impl FnMut(&TypeRef)) {
+            if let Some(type_ref) = path.type_anchor() {
+                go(type_ref, f);
+            }
+            for segment in path.segments().iter() {
+                if let Some(args_and_bindings) = segment.args_and_bindings {
+                    for arg in &args_and_bindings.args {
+                        let crate::path::GenericArg::Type(type_ref) = arg;
+                        go(type_ref, f);
+                    }
+                    for (_, type_ref) in &args_and_bindings.bindings {
+                        go(type_ref, f);
+                    }
+                }
+            }
+        }
+    }
 }
 
 pub(crate) fn type_bounds_from_ast(type_bounds_opt: Option<ast::TypeBoundList>) -> Vec<TypeBound> {
diff --git a/crates/ra_hir_expand/src/name.rs b/crates/ra_hir_expand/src/name.rs
index 133805bdb..b2e10f445 100644
--- a/crates/ra_hir_expand/src/name.rs
+++ b/crates/ra_hir_expand/src/name.rs
@@ -187,13 +187,6 @@ pub mod known {
         PartialOrd,
         Eq,
         PartialEq,
-        // FIXME delete those after `ImportResolver` is removed.
-        hash,
-        fmt,
-        io,
-        Display,
-        Iterator,
-        Hasher,
     );
 
     // self/Self cannot be used as an identifier
diff --git a/crates/ra_hir_ty/src/db.rs b/crates/ra_hir_ty/src/db.rs
index eb521c7a0..e9bfcfa17 100644
--- a/crates/ra_hir_ty/src/db.rs
+++ b/crates/ra_hir_ty/src/db.rs
@@ -3,17 +3,18 @@
 use std::sync::Arc;
 
 use hir_def::{
-    db::DefDatabase, DefWithBodyId, GenericDefId, ImplId, LocalStructFieldId, TraitId, VariantId,
+    db::DefDatabase, DefWithBodyId, GenericDefId, ImplId, LocalStructFieldId, TraitId, TypeParamId,
+    VariantId,
 };
 use ra_arena::map::ArenaMap;
-use ra_db::{salsa, CrateId};
+use ra_db::{impl_intern_key, salsa, CrateId};
 use ra_prof::profile;
 
 use crate::{
     method_resolution::CrateImplBlocks,
     traits::{chalk, AssocTyValue, Impl},
-    CallableDef, FnSig, GenericPredicate, InferenceResult, Substs, TraitRef, Ty, TyDefId, TypeCtor,
-    ValueTyDefId,
+    Binders, CallableDef, GenericPredicate, InferenceResult, PolyFnSig, Substs, TraitRef, Ty,
+    TyDefId, TypeCtor, ValueTyDefId,
 };
 
 #[salsa::query_group(HirDatabaseStorage)]
@@ -27,34 +28,33 @@ pub trait HirDatabase: DefDatabase {
 
     #[salsa::invoke(crate::lower::ty_query)]
     #[salsa::cycle(crate::lower::ty_recover)]
-    fn ty(&self, def: TyDefId) -> Ty;
+    fn ty(&self, def: TyDefId) -> Binders<Ty>;
 
     #[salsa::invoke(crate::lower::value_ty_query)]
-    fn value_ty(&self, def: ValueTyDefId) -> Ty;
+    fn value_ty(&self, def: ValueTyDefId) -> Binders<Ty>;
 
     #[salsa::invoke(crate::lower::impl_self_ty_query)]
     #[salsa::cycle(crate::lower::impl_self_ty_recover)]
-    fn impl_self_ty(&self, def: ImplId) -> Ty;
+    fn impl_self_ty(&self, def: ImplId) -> Binders<Ty>;
 
     #[salsa::invoke(crate::lower::impl_trait_query)]
-    fn impl_trait(&self, def: ImplId) -> Option<TraitRef>;
+    fn impl_trait(&self, def: ImplId) -> Option<Binders<TraitRef>>;
 
     #[salsa::invoke(crate::lower::field_types_query)]
-    fn field_types(&self, var: VariantId) -> Arc<ArenaMap<LocalStructFieldId, Ty>>;
+    fn field_types(&self, var: VariantId) -> Arc<ArenaMap<LocalStructFieldId, Binders<Ty>>>;
 
     #[salsa::invoke(crate::callable_item_sig)]
-    fn callable_item_signature(&self, def: CallableDef) -> FnSig;
+    fn callable_item_signature(&self, def: CallableDef) -> PolyFnSig;
 
     #[salsa::invoke(crate::lower::generic_predicates_for_param_query)]
     #[salsa::cycle(crate::lower::generic_predicates_for_param_recover)]
     fn generic_predicates_for_param(
         &self,
-        def: GenericDefId,
-        param_idx: u32,
-    ) -> Arc<[GenericPredicate]>;
+        param_id: TypeParamId,
+    ) -> Arc<[Binders<GenericPredicate>]>;
 
     #[salsa::invoke(crate::lower::generic_predicates_query)]
-    fn generic_predicates(&self, def: GenericDefId) -> Arc<[GenericPredicate]>;
+    fn generic_predicates(&self, def: GenericDefId) -> Arc<[Binders<GenericPredicate>]>;
 
     #[salsa::invoke(crate::lower::generic_defaults_query)]
     fn generic_defaults(&self, def: GenericDefId) -> Substs;
@@ -77,6 +77,8 @@ pub trait HirDatabase: DefDatabase {
     #[salsa::interned]
     fn intern_type_ctor(&self, type_ctor: TypeCtor) -> crate::TypeCtorId;
     #[salsa::interned]
+    fn intern_type_param_id(&self, param_id: TypeParamId) -> GlobalTypeParamId;
+    #[salsa::interned]
     fn intern_chalk_impl(&self, impl_: Impl) -> crate::traits::GlobalImplId;
     #[salsa::interned]
     fn intern_assoc_ty_value(&self, assoc_ty_value: AssocTyValue) -> crate::traits::AssocTyValueId;
@@ -117,3 +119,7 @@ fn infer(db: &impl HirDatabase, def: DefWithBodyId) -> Arc<InferenceResult> {
 fn hir_database_is_object_safe() {
     fn _assert_object_safe(_: &dyn HirDatabase) {}
 }
+
+#[derive(Debug, Clone, Copy, PartialEq, Eq, Hash)]
+pub struct GlobalTypeParamId(salsa::InternId);
+impl_intern_key!(GlobalTypeParamId);
diff --git a/crates/ra_hir_ty/src/infer.rs b/crates/ra_hir_ty/src/infer.rs
index e2eda3134..a9d958c8b 100644
--- a/crates/ra_hir_ty/src/infer.rs
+++ b/crates/ra_hir_ty/src/infer.rs
@@ -34,7 +34,6 @@ use hir_expand::{diagnostics::DiagnosticSink, name::name};
 use ra_arena::map::ArenaMap;
 use ra_prof::profile;
 use ra_syntax::SmolStr;
-use test_utils::tested_by;
 
 use super::{
     primitive::{FloatTy, IntTy},
@@ -42,7 +41,9 @@ use super::{
     ApplicationTy, GenericPredicate, InEnvironment, ProjectionTy, Substs, TraitEnvironment,
     TraitRef, Ty, TypeCtor, TypeWalk, Uncertain,
 };
-use crate::{db::HirDatabase, infer::diagnostics::InferenceDiagnostic};
+use crate::{
+    db::HirDatabase, infer::diagnostics::InferenceDiagnostic, lower::ImplTraitLoweringMode,
+};
 
 pub(crate) use unify::unify;
 
@@ -271,38 +272,21 @@ impl<'a, D: HirDatabase> InferenceContext<'a, D> {
         self.result.diagnostics.push(diagnostic);
     }
 
-    fn make_ty(&mut self, type_ref: &TypeRef) -> Ty {
-        let ty = Ty::from_hir(
-            self.db,
-            // FIXME use right resolver for block
-            &self.resolver,
-            type_ref,
-        );
+    fn make_ty_with_mode(
+        &mut self,
+        type_ref: &TypeRef,
+        impl_trait_mode: ImplTraitLoweringMode,
+    ) -> Ty {
+        // FIXME use right resolver for block
+        let ctx = crate::lower::TyLoweringContext::new(self.db, &self.resolver)
+            .with_impl_trait_mode(impl_trait_mode);
+        let ty = Ty::from_hir(&ctx, type_ref);
         let ty = self.insert_type_vars(ty);
         self.normalize_associated_types_in(ty)
     }
 
-    /// Replaces `impl Trait` in `ty` by type variables and obligations for
-    /// those variables. This is done for function arguments when calling a
-    /// function, and for return types when inside the function body, i.e. in
-    /// the cases where the `impl Trait` is 'transparent'. In other cases, `impl
-    /// Trait` is represented by `Ty::Opaque`.
-    fn insert_vars_for_impl_trait(&mut self, ty: Ty) -> Ty {
-        ty.fold(&mut |ty| match ty {
-            Ty::Opaque(preds) => {
-                tested_by!(insert_vars_for_impl_trait);
-                let var = self.table.new_type_var();
-                let var_subst = Substs::builder(1).push(var.clone()).build();
-                self.obligations.extend(
-                    preds
-                        .iter()
-                        .map(|pred| pred.clone().subst_bound_vars(&var_subst))
-                        .filter_map(Obligation::from_predicate),
-                );
-                var
-            }
-            _ => ty,
-        })
+    fn make_ty(&mut self, type_ref: &TypeRef) -> Ty {
+        self.make_ty_with_mode(type_ref, ImplTraitLoweringMode::Disallowed)
     }
 
     /// Replaces Ty::Unknown by a new type var, so we can maybe still infer it.
@@ -446,19 +430,20 @@ impl<'a, D: HirDatabase> InferenceContext<'a, D> {
             None => return (Ty::Unknown, None),
         };
         let resolver = &self.resolver;
+        let ctx = crate::lower::TyLoweringContext::new(self.db, &self.resolver);
         // FIXME: this should resolve assoc items as well, see this example:
         // https://play.rust-lang.org/?gist=087992e9e22495446c01c0d4e2d69521
         match resolver.resolve_path_in_type_ns_fully(self.db, path.mod_path()) {
             Some(TypeNs::AdtId(AdtId::StructId(strukt))) => {
-                let substs = Ty::substs_from_path(self.db, resolver, path, strukt.into());
+                let substs = Ty::substs_from_path(&ctx, path, strukt.into());
                 let ty = self.db.ty(strukt.into());
-                let ty = self.insert_type_vars(ty.apply_substs(substs));
+                let ty = self.insert_type_vars(ty.subst(&substs));
                 (ty, Some(strukt.into()))
             }
             Some(TypeNs::EnumVariantId(var)) => {
-                let substs = Ty::substs_from_path(self.db, resolver, path, var.into());
+                let substs = Ty::substs_from_path(&ctx, path, var.into());
                 let ty = self.db.ty(var.parent.into());
-                let ty = self.insert_type_vars(ty.apply_substs(substs));
+                let ty = self.insert_type_vars(ty.subst(&substs));
                 (ty, Some(var.into()))
             }
             Some(_) | None => (Ty::Unknown, None),
@@ -471,13 +456,18 @@ impl<'a, D: HirDatabase> InferenceContext<'a, D> {
 
     fn collect_fn(&mut self, data: &FunctionData) {
         let body = Arc::clone(&self.body); // avoid borrow checker problem
-        for (type_ref, pat) in data.params.iter().zip(body.params.iter()) {
-            let ty = self.make_ty(type_ref);
+        let ctx = crate::lower::TyLoweringContext::new(self.db, &self.resolver)
+            .with_impl_trait_mode(ImplTraitLoweringMode::Param);
+        let param_tys =
+            data.params.iter().map(|type_ref| Ty::from_hir(&ctx, type_ref)).collect::<Vec<_>>();
+        for (ty, pat) in param_tys.into_iter().zip(body.params.iter()) {
+            let ty = self.insert_type_vars(ty);
+            let ty = self.normalize_associated_types_in(ty);
 
             self.infer_pat(*pat, &ty, BindingMode::default());
         }
-        let return_ty = self.make_ty(&data.ret_type);
-        self.return_ty = self.insert_vars_for_impl_trait(return_ty);
+        let return_ty = self.make_ty_with_mode(&data.ret_type, ImplTraitLoweringMode::Disallowed); // FIXME implement RPIT
+        self.return_ty = return_ty;
     }
 
     fn infer_body(&mut self) {
diff --git a/crates/ra_hir_ty/src/infer/coerce.rs b/crates/ra_hir_ty/src/infer/coerce.rs
index 83c0c2c3f..f68a1439f 100644
--- a/crates/ra_hir_ty/src/infer/coerce.rs
+++ b/crates/ra_hir_ty/src/infer/coerce.rs
@@ -57,8 +57,8 @@ impl<'a, D: HirDatabase> InferenceContext<'a, D> {
                 let trait_ref = db.impl_trait(impl_id)?;
 
                 // `CoerseUnsized` has one generic parameter for the target type.
-                let cur_from_ty = trait_ref.substs.0.get(0)?;
-                let cur_to_ty = trait_ref.substs.0.get(1)?;
+                let cur_from_ty = trait_ref.value.substs.0.get(0)?;
+                let cur_to_ty = trait_ref.value.substs.0.get(1)?;
 
                 match (&cur_from_ty, cur_to_ty) {
                     (ty_app!(ctor1, st1), ty_app!(ctor2, st2)) => {
@@ -66,9 +66,7 @@ impl<'a, D: HirDatabase> InferenceContext<'a, D> {
                         // This works for smart-pointer-like coercion, which covers all impls from std.
                         st1.iter().zip(st2.iter()).enumerate().find_map(|(i, (ty1, ty2))| {
                             match (ty1, ty2) {
-                                (Ty::Param { idx: p1, .. }, Ty::Param { idx: p2, .. })
-                                    if p1 != p2 =>
-                                {
+                                (Ty::Bound(idx1), Ty::Bound(idx2)) if idx1 != idx2 => {
                                     Some(((*ctor1, *ctor2), i))
                                 }
                                 _ => None,
@@ -256,8 +254,8 @@ impl<'a, D: HirDatabase> InferenceContext<'a, D> {
                 let unsize_generic_index = {
                     let mut index = None;
                     let mut multiple_param = false;
-                    field_tys[last_field_id].walk(&mut |ty| match ty {
-                        &Ty::Param { idx, .. } => {
+                    field_tys[last_field_id].value.walk(&mut |ty| match ty {
+                        &Ty::Bound(idx) => {
                             if index.is_none() {
                                 index = Some(idx);
                             } else if Some(idx) != index {
@@ -276,10 +274,8 @@ impl<'a, D: HirDatabase> InferenceContext<'a, D> {
                 // Check other fields do not involve it.
                 let mut multiple_used = false;
                 fields.for_each(|(field_id, _data)| {
-                    field_tys[field_id].walk(&mut |ty| match ty {
-                        &Ty::Param { idx, .. } if idx == unsize_generic_index => {
-                            multiple_used = true
-                        }
+                    field_tys[field_id].value.walk(&mut |ty| match ty {
+                        &Ty::Bound(idx) if idx == unsize_generic_index => multiple_used = true,
                         _ => {}
                     })
                 });
diff --git a/crates/ra_hir_ty/src/infer/expr.rs b/crates/ra_hir_ty/src/infer/expr.rs
index 31259a01d..3c9c02d03 100644
--- a/crates/ra_hir_ty/src/infer/expr.rs
+++ b/crates/ra_hir_ty/src/infer/expr.rs
@@ -10,7 +10,7 @@ use hir_def::{
     resolver::resolver_for_expr,
     AdtId, AssocContainerId, Lookup, StructFieldId,
 };
-use hir_expand::name::{name, Name};
+use hir_expand::name::Name;
 use ra_syntax::ast::RangeOp;
 
 use crate::{
@@ -19,8 +19,8 @@ use crate::{
     method_resolution, op,
     traits::InEnvironment,
     utils::{generics, variant_data, Generics},
-    ApplicationTy, CallableDef, InferTy, IntTy, Mutability, Obligation, Substs, TraitRef, Ty,
-    TypeCtor, TypeWalk, Uncertain,
+    ApplicationTy, Binders, CallableDef, InferTy, IntTy, Mutability, Obligation, Substs, TraitRef,
+    Ty, TypeCtor, Uncertain,
 };
 
 use super::{BindingMode, Expectation, InferenceContext, InferenceDiagnostic, TypeMismatch};
@@ -236,8 +236,7 @@ impl<'a, D: HirDatabase> InferenceContext<'a, D> {
                         self.result.record_field_resolutions.insert(field.expr, field_def);
                     }
                     let field_ty = field_def
-                        .map_or(Ty::Unknown, |it| field_types[it.local_id].clone())
-                        .subst(&substs);
+                        .map_or(Ty::Unknown, |it| field_types[it.local_id].clone().subst(&substs));
                     self.infer_expr_coerce(field.expr, &Expectation::has_type(field_ty));
                 }
                 if let Some(expr) = spread {
@@ -588,10 +587,10 @@ impl<'a, D: HirDatabase> InferenceContext<'a, D> {
                 self.write_method_resolution(tgt_expr, func);
                 (ty, self.db.value_ty(func.into()), Some(generics(self.db, func.into())))
             }
-            None => (receiver_ty, Ty::Unknown, None),
+            None => (receiver_ty, Binders::new(0, Ty::Unknown), None),
         };
         let substs = self.substs_for_method_call(def_generics, generic_args, &derefed_receiver_ty);
-        let method_ty = method_ty.apply_substs(substs);
+        let method_ty = method_ty.subst(&substs);
         let method_ty = self.insert_type_vars(method_ty);
         self.register_obligations_for_call(&method_ty);
         let (expected_receiver_ty, param_tys, ret_ty) = match method_ty.callable_sig(self.db) {
@@ -635,7 +634,6 @@ impl<'a, D: HirDatabase> InferenceContext<'a, D> {
                     continue;
                 }
 
-                let param_ty = self.insert_vars_for_impl_trait(param_ty);
                 let param_ty = self.normalize_associated_types_in(param_ty);
                 self.infer_expr_coerce(arg, &Expectation::has_type(param_ty.clone()));
             }
@@ -648,13 +646,15 @@ impl<'a, D: HirDatabase> InferenceContext<'a, D> {
         generic_args: Option<&GenericArgs>,
         receiver_ty: &Ty,
     ) -> Substs {
-        let (total_len, _parent_len, child_len) =
-            def_generics.as_ref().map_or((0, 0, 0), |g| g.len_split());
+        let (parent_params, self_params, type_params, impl_trait_params) =
+            def_generics.as_ref().map_or((0, 0, 0, 0), |g| g.provenance_split());
+        assert_eq!(self_params, 0); // method shouldn't have another Self param
+        let total_len = parent_params + type_params + impl_trait_params;
         let mut substs = Vec::with_capacity(total_len);
         // Parent arguments are unknown, except for the receiver type
         if let Some(parent_generics) = def_generics.as_ref().map(|p| p.iter_parent()) {
             for (_id, param) in parent_generics {
-                if param.name == name![Self] {
+                if param.provenance == hir_def::generics::TypeParamProvenance::TraitSelf {
                     substs.push(receiver_ty.clone());
                 } else {
                     substs.push(Ty::Unknown);
@@ -664,7 +664,7 @@ impl<'a, D: HirDatabase> InferenceContext<'a, D> {
         // handle provided type arguments
         if let Some(generic_args) = generic_args {
             // if args are provided, it should be all of them, but we can't rely on that
-            for arg in generic_args.args.iter().take(child_len) {
+            for arg in generic_args.args.iter().take(type_params) {
                 match arg {
                     GenericArg::Type(type_ref) => {
                         let ty = self.make_ty(type_ref);
diff --git a/crates/ra_hir_ty/src/infer/pat.rs b/crates/ra_hir_ty/src/infer/pat.rs
index a14662884..e7283f24c 100644
--- a/crates/ra_hir_ty/src/infer/pat.rs
+++ b/crates/ra_hir_ty/src/infer/pat.rs
@@ -12,7 +12,7 @@ use hir_expand::name::Name;
 use test_utils::tested_by;
 
 use super::{BindingMode, InferenceContext};
-use crate::{db::HirDatabase, utils::variant_data, Substs, Ty, TypeCtor, TypeWalk};
+use crate::{db::HirDatabase, utils::variant_data, Substs, Ty, TypeCtor};
 
 impl<'a, D: HirDatabase> InferenceContext<'a, D> {
     fn infer_tuple_struct_pat(
@@ -34,8 +34,7 @@ impl<'a, D: HirDatabase> InferenceContext<'a, D> {
             let expected_ty = var_data
                 .as_ref()
                 .and_then(|d| d.field(&Name::new_tuple_field(i)))
-                .map_or(Ty::Unknown, |field| field_tys[field].clone())
-                .subst(&substs);
+                .map_or(Ty::Unknown, |field| field_tys[field].clone().subst(&substs));
             let expected_ty = self.normalize_associated_types_in(expected_ty);
             self.infer_pat(subpat, &expected_ty, default_bm);
         }
@@ -65,7 +64,7 @@ impl<'a, D: HirDatabase> InferenceContext<'a, D> {
         for subpat in subpats {
             let matching_field = var_data.as_ref().and_then(|it| it.field(&subpat.name));
             let expected_ty =
-                matching_field.map_or(Ty::Unknown, |field| field_tys[field].clone()).subst(&substs);
+                matching_field.map_or(Ty::Unknown, |field| field_tys[field].clone().subst(&substs));
             let expected_ty = self.normalize_associated_types_in(expected_ty);
             self.infer_pat(subpat.pat, &expected_ty, default_bm);
         }
diff --git a/crates/ra_hir_ty/src/infer/path.rs b/crates/ra_hir_ty/src/infer/path.rs
index 2c1d4831d..686ce7a21 100644
--- a/crates/ra_hir_ty/src/infer/path.rs
+++ b/crates/ra_hir_ty/src/infer/path.rs
@@ -9,9 +9,9 @@ use hir_def::{
 };
 use hir_expand::name::Name;
 
-use crate::{db::HirDatabase, method_resolution, Substs, Ty, TypeWalk, ValueTyDefId};
+use crate::{db::HirDatabase, method_resolution, Substs, Ty, ValueTyDefId};
 
-use super::{ExprOrPatId, InferenceContext, TraitEnvironment, TraitRef};
+use super::{ExprOrPatId, InferenceContext, TraitRef};
 
 impl<'a, D: HirDatabase> InferenceContext<'a, D> {
     pub(super) fn infer_path(
@@ -39,7 +39,8 @@ impl<'a, D: HirDatabase> InferenceContext<'a, D> {
             }
             let ty = self.make_ty(type_ref);
             let remaining_segments_for_ty = path.segments().take(path.segments().len() - 1);
-            let ty = Ty::from_type_relative_path(self.db, resolver, ty, remaining_segments_for_ty);
+            let ctx = crate::lower::TyLoweringContext::new(self.db, &resolver);
+            let ty = Ty::from_type_relative_path(&ctx, ty, remaining_segments_for_ty);
             self.resolve_ty_assoc_item(
                 ty,
                 &path.segments().last().expect("path had at least one segment").name,
@@ -69,12 +70,16 @@ impl<'a, D: HirDatabase> InferenceContext<'a, D> {
             ValueNs::EnumVariantId(it) => it.into(),
         };
 
-        let mut ty = self.db.value_ty(typable);
-        if let Some(self_subst) = self_subst {
-            ty = ty.subst(&self_subst);
-        }
-        let substs = Ty::substs_from_path(self.db, &self.resolver, path, typable);
-        let ty = ty.subst(&substs);
+        let ty = self.db.value_ty(typable);
+        // self_subst is just for the parent
+        let parent_substs = self_subst.unwrap_or_else(Substs::empty);
+        let ctx = crate::lower::TyLoweringContext::new(self.db, &self.resolver);
+        let substs = Ty::substs_from_path(&ctx, path, typable);
+        let full_substs = Substs::builder(substs.len())
+            .use_parent_substs(&parent_substs)
+            .fill(substs.0[parent_substs.len()..].iter().cloned())
+            .build();
+        let ty = ty.subst(&full_substs);
         Some(ty)
     }
 
@@ -98,13 +103,9 @@ impl<'a, D: HirDatabase> InferenceContext<'a, D> {
             (TypeNs::TraitId(trait_), true) => {
                 let segment =
                     remaining_segments.last().expect("there should be at least one segment here");
-                let trait_ref = TraitRef::from_resolved_path(
-                    self.db,
-                    &self.resolver,
-                    trait_.into(),
-                    resolved_segment,
-                    None,
-                );
+                let ctx = crate::lower::TyLoweringContext::new(self.db, &self.resolver);
+                let trait_ref =
+                    TraitRef::from_resolved_path(&ctx, trait_.into(), resolved_segment, None);
                 self.resolve_trait_assoc_item(trait_ref, segment, id)
             }
             (def, _) => {
@@ -114,9 +115,9 @@ impl<'a, D: HirDatabase> InferenceContext<'a, D> {
                 // as Iterator>::Item::default`)
                 let remaining_segments_for_ty =
                     remaining_segments.take(remaining_segments.len() - 1);
+                let ctx = crate::lower::TyLoweringContext::new(self.db, &self.resolver);
                 let ty = Ty::from_partly_resolved_hir_path(
-                    self.db,
-                    &self.resolver,
+                    &ctx,
                     def,
                     resolved_segment,
                     remaining_segments_for_ty,
@@ -173,13 +174,9 @@ impl<'a, D: HirDatabase> InferenceContext<'a, D> {
             AssocItemId::ConstId(c) => ValueNs::ConstId(c),
             AssocItemId::TypeAliasId(_) => unreachable!(),
         };
-        let substs = Substs::build_for_def(self.db, item)
-            .use_parent_substs(&trait_ref.substs)
-            .fill_with_params()
-            .build();
 
         self.write_assoc_resolution(id, item);
-        Some((def, Some(substs)))
+        Some((def, Some(trait_ref.substs)))
     }
 
     fn resolve_ty_assoc_item(
@@ -193,14 +190,13 @@ impl<'a, D: HirDatabase> InferenceContext<'a, D> {
         }
 
         let canonical_ty = self.canonicalizer().canonicalize_ty(ty.clone());
-        let env = TraitEnvironment::lower(self.db, &self.resolver);
         let krate = self.resolver.krate()?;
         let traits_in_scope = self.resolver.traits_in_scope(self.db);
 
         method_resolution::iterate_method_candidates(
             &canonical_ty.value,
             self.db,
-            env,
+            self.trait_env.clone(),
             krate,
             &traits_in_scope,
             Some(name),
@@ -219,12 +215,8 @@ impl<'a, D: HirDatabase> InferenceContext<'a, D> {
                             .fill(iter::repeat_with(|| self.table.new_type_var()))
                             .build();
                         let impl_self_ty = self.db.impl_self_ty(impl_id).subst(&impl_substs);
-                        let substs = Substs::build_for_def(self.db, item)
-                            .use_parent_substs(&impl_substs)
-                            .fill_with_params()
-                            .build();
                         self.unify(&impl_self_ty, &ty);
-                        Some(substs)
+                        Some(impl_substs)
                     }
                     AssocContainerId::TraitId(trait_) => {
                         // we're picking this method
@@ -232,15 +224,11 @@ impl<'a, D: HirDatabase> InferenceContext<'a, D> {
                             .push(ty.clone())
                             .fill(std::iter::repeat_with(|| self.table.new_type_var()))
                             .build();
-                        let substs = Substs::build_for_def(self.db, item)
-                            .use_parent_substs(&trait_substs)
-                            .fill_with_params()
-                            .build();
                         self.obligations.push(super::Obligation::Trait(TraitRef {
                             trait_,
-                            substs: trait_substs,
+                            substs: trait_substs.clone(),
                         }));
-                        Some(substs)
+                        Some(trait_substs)
                     }
                     AssocContainerId::ContainerId(_) => None,
                 };
diff --git a/crates/ra_hir_ty/src/lib.rs b/crates/ra_hir_ty/src/lib.rs
index 08d501ccd..c5fe18c85 100644
--- a/crates/ra_hir_ty/src/lib.rs
+++ b/crates/ra_hir_ty/src/lib.rs
@@ -44,8 +44,8 @@ use std::sync::Arc;
 use std::{fmt, iter, mem};
 
 use hir_def::{
-    expr::ExprId, type_ref::Mutability, AdtId, AssocContainerId, DefWithBodyId, GenericDefId,
-    HasModule, Lookup, TraitId, TypeAliasId,
+    expr::ExprId, generics::TypeParamProvenance, type_ref::Mutability, AdtId, AssocContainerId,
+    DefWithBodyId, GenericDefId, HasModule, Lookup, TraitId, TypeAliasId, TypeParamId,
 };
 use hir_expand::name::Name;
 use ra_db::{impl_intern_key, salsa, CrateId};
@@ -60,7 +60,9 @@ use display::{HirDisplay, HirFormatter};
 pub use autoderef::autoderef;
 pub use infer::{do_infer_query, InferTy, InferenceResult};
 pub use lower::CallableDef;
-pub use lower::{callable_item_sig, TyDefId, ValueTyDefId};
+pub use lower::{
+    callable_item_sig, ImplTraitLoweringMode, TyDefId, TyLoweringContext, ValueTyDefId,
+};
 pub use traits::{InEnvironment, Obligation, ProjectionPredicate, TraitEnvironment};
 
 /// A type constructor or type name: this might be something like the primitive
@@ -285,22 +287,20 @@ pub enum Ty {
     /// trait and all its parameters are fully known.
     Projection(ProjectionTy),
 
-    /// A type parameter; for example, `T` in `fn f<T>(x: T) {}
-    Param {
-        /// The index of the parameter (starting with parameters from the
-        /// surrounding impl, then the current function).
-        idx: u32,
-        /// The name of the parameter, for displaying.
-        // FIXME get rid of this
-        name: Name,
-    },
-
-    /// A bound type variable. Used during trait resolution to represent Chalk
-    /// variables, and in `Dyn` and `Opaque` bounds to represent the `Self` type.
+    /// A placeholder for a type parameter; for example, `T` in `fn f<T>(x: T)
+    /// {}` when we're type-checking the body of that function. In this
+    /// situation, we know this stands for *some* type, but don't know the exact
+    /// type.
+    Param(TypeParamId),
+
+    /// A bound type variable. This is used in various places: when representing
+    /// some polymorphic type like the type of function `fn f<T>`, the type
+    /// parameters get turned into variables; during trait resolution, inference
+    /// variables get turned into bound variables and back; and in `Dyn` the
+    /// `Self` type is represented with a bound variable as well.
     Bound(u32),
 
-    /// A type variable used during type checking. Not to be confused with a
-    /// type parameter.
+    /// A type variable used during type checking.
     Infer(InferTy),
 
     /// A trait object (`dyn Trait` or bare `Trait` in pre-2018 Rust).
@@ -364,15 +364,19 @@ impl Substs {
     }
 
     /// Return Substs that replace each parameter by itself (i.e. `Ty::Param`).
-    pub(crate) fn identity(generic_params: &Generics) -> Substs {
-        Substs(
-            generic_params.iter().map(|(idx, p)| Ty::Param { idx, name: p.name.clone() }).collect(),
-        )
+    pub(crate) fn type_params_for_generics(generic_params: &Generics) -> Substs {
+        Substs(generic_params.iter().map(|(id, _)| Ty::Param(id)).collect())
+    }
+
+    /// Return Substs that replace each parameter by itself (i.e. `Ty::Param`).
+    pub fn type_params(db: &impl HirDatabase, def: impl Into<GenericDefId>) -> Substs {
+        let params = generics(db, def.into());
+        Substs::type_params_for_generics(&params)
     }
 
     /// Return Substs that replace each parameter by a bound variable.
     pub(crate) fn bound_vars(generic_params: &Generics) -> Substs {
-        Substs(generic_params.iter().map(|(idx, _p)| Ty::Bound(idx)).collect())
+        Substs(generic_params.iter().enumerate().map(|(idx, _)| Ty::Bound(idx as u32)).collect())
     }
 
     pub fn build_for_def(db: &impl HirDatabase, def: impl Into<GenericDefId>) -> SubstsBuilder {
@@ -420,11 +424,6 @@ impl SubstsBuilder {
         self.fill((starting_from..).map(Ty::Bound))
     }
 
-    pub fn fill_with_params(self) -> Self {
-        let start = self.vec.len() as u32;
-        self.fill((start..).map(|idx| Ty::Param { idx, name: Name::missing() }))
-    }
-
     pub fn fill_with_unknown(self) -> Self {
         self.fill(iter::repeat(Ty::Unknown))
     }
@@ -451,6 +450,32 @@ impl Deref for Substs {
     }
 }
 
+#[derive(Copy, Clone, PartialEq, Eq, Debug)]
+pub struct Binders<T> {
+    pub num_binders: usize,
+    pub value: T,
+}
+
+impl<T> Binders<T> {
+    pub fn new(num_binders: usize, value: T) -> Self {
+        Self { num_binders, value }
+    }
+}
+
+impl<T: TypeWalk> Binders<T> {
+    /// Substitutes all variables.
+    pub fn subst(self, subst: &Substs) -> T {
+        assert_eq!(subst.len(), self.num_binders);
+        self.value.subst_bound_vars(subst)
+    }
+
+    /// Substitutes just a prefix of the variables (shifting the rest).
+    pub fn subst_prefix(self, subst: &Substs) -> Binders<T> {
+        assert!(subst.len() < self.num_binders);
+        Binders::new(self.num_binders - subst.len(), self.value.subst_bound_vars(subst))
+    }
+}
+
 /// A trait with type parameters. This includes the `Self`, so this represents a concrete type implementing the trait.
 /// Name to be bikeshedded: TraitBound? TraitImplements?
 #[derive(Clone, PartialEq, Eq, Debug, Hash)]
@@ -551,6 +576,9 @@ pub struct FnSig {
     params_and_return: Arc<[Ty]>,
 }
 
+/// A polymorphic function signature.
+pub type PolyFnSig = Binders<FnSig>;
+
 impl FnSig {
     pub fn from_params_and_return(mut params: Vec<Ty>, ret: Ty) -> FnSig {
         params.push(ret);
@@ -730,22 +758,7 @@ pub trait TypeWalk {
         self
     }
 
-    /// Replaces type parameters in this type using the given `Substs`. (So e.g.
-    /// if `self` is `&[T]`, where type parameter T has index 0, and the
-    /// `Substs` contain `u32` at index 0, we'll have `&[u32]` afterwards.)
-    fn subst(self, substs: &Substs) -> Self
-    where
-        Self: Sized,
-    {
-        self.fold(&mut |ty| match ty {
-            Ty::Param { idx, name } => {
-                substs.get(idx as usize).cloned().unwrap_or(Ty::Param { idx, name })
-            }
-            ty => ty,
-        })
-    }
-
-    /// Substitutes `Ty::Bound` vars (as opposed to type parameters).
+    /// Substitutes `Ty::Bound` vars with the given substitution.
     fn subst_bound_vars(mut self, substs: &Substs) -> Self
     where
         Self: Sized,
@@ -755,6 +768,9 @@ pub trait TypeWalk {
                 &mut Ty::Bound(idx) => {
                     if idx as usize >= binders && (idx as usize - binders) < substs.len() {
                         *ty = substs.0[idx as usize - binders].clone();
+                    } else if idx as usize >= binders + substs.len() {
+                        // shift free binders
+                        *ty = Ty::Bound(idx - substs.len() as u32);
                     }
                 }
                 _ => {}
@@ -880,7 +896,7 @@ impl HirDisplay for ApplicationTy {
                 write!(f, ") -> {}", sig.ret().display(f.db))?;
             }
             TypeCtor::FnDef(def) => {
-                let sig = f.db.callable_item_signature(def);
+                let sig = f.db.callable_item_signature(def).subst(&self.parameters);
                 let name = match def {
                     CallableDef::FunctionId(ff) => f.db.function_data(ff).name.clone(),
                     CallableDef::StructId(s) => f.db.struct_data(s).name.clone(),
@@ -896,9 +912,16 @@ impl HirDisplay for ApplicationTy {
                     }
                 }
                 if self.parameters.len() > 0 {
-                    write!(f, "<")?;
-                    f.write_joined(&*self.parameters.0, ", ")?;
-                    write!(f, ">")?;
+                    let generics = generics(f.db, def.into());
+                    let (parent_params, self_param, type_params, _impl_trait_params) =
+                        generics.provenance_split();
+                    let total_len = parent_params + self_param + type_params;
+                    // We print all params except implicit impl Trait params. Still a bit weird; should we leave out parent and self?
+                    if total_len > 0 {
+                        write!(f, "<")?;
+                        f.write_joined(&self.parameters.0[..total_len], ", ")?;
+                        write!(f, ">")?;
+                    }
                 }
                 write!(f, "(")?;
                 f.write_joined(sig.params(), ", ")?;
@@ -1009,7 +1032,24 @@ impl HirDisplay for Ty {
         match self {
             Ty::Apply(a_ty) => a_ty.hir_fmt(f)?,
             Ty::Projection(p_ty) => p_ty.hir_fmt(f)?,
-            Ty::Param { name, .. } => write!(f, "{}", name)?,
+            Ty::Param(id) => {
+                let generics = generics(f.db, id.parent);
+                let param_data = &generics.params.types[id.local_id];
+                match param_data.provenance {
+                    TypeParamProvenance::TypeParamList | TypeParamProvenance::TraitSelf => {
+                        write!(f, "{}", param_data.name.clone().unwrap_or_else(Name::missing))?
+                    }
+                    TypeParamProvenance::ArgumentImplTrait => {
+                        write!(f, "impl ")?;
+                        let bounds = f.db.generic_predicates_for_param(*id);
+                        let substs = Substs::type_params_for_generics(&generics);
+                        write_bounds_like_dyn_trait(
+                            &bounds.iter().map(|b| b.clone().subst(&substs)).collect::<Vec<_>>(),
+                            f,
+                        )?;
+                    }
+                }
+            }
             Ty::Bound(idx) => write!(f, "?{}", idx)?,
             Ty::Dyn(predicates) | Ty::Opaque(predicates) => {
                 match self {
@@ -1017,66 +1057,7 @@ impl HirDisplay for Ty {
                     Ty::Opaque(_) => write!(f, "impl ")?,
                     _ => unreachable!(),
                 };
-                // Note: This code is written to produce nice results (i.e.
-                // corresponding to surface Rust) for types that can occur in
-                // actual Rust. It will have weird results if the predicates
-                // aren't as expected (i.e. self types = $0, projection
-                // predicates for a certain trait come after the Implemented
-                // predicate for that trait).
-                let mut first = true;
-                let mut angle_open = false;
-                for p in predicates.iter() {
-                    match p {
-                        GenericPredicate::Implemented(trait_ref) => {
-                            if angle_open {
-                                write!(f, ">")?;
-                            }
-                            if !first {
-                                write!(f, " + ")?;
-                            }
-                            // We assume that the self type is $0 (i.e. the
-                            // existential) here, which is the only thing that's
-                            // possible in actual Rust, and hence don't print it
-                            write!(f, "{}", f.db.trait_data(trait_ref.trait_).name.clone())?;
-                            if trait_ref.substs.len() > 1 {
-                                write!(f, "<")?;
-                                f.write_joined(&trait_ref.substs[1..], ", ")?;
-                                // there might be assoc type bindings, so we leave the angle brackets open
-                                angle_open = true;
-                            }
-                        }
-                        GenericPredicate::Projection(projection_pred) => {
-                            // in types in actual Rust, these will always come
-                            // after the corresponding Implemented predicate
-                            if angle_open {
-                                write!(f, ", ")?;
-                            } else {
-                                write!(f, "<")?;
-                                angle_open = true;
-                            }
-                            let name =
-                                f.db.type_alias_data(projection_pred.projection_ty.associated_ty)
-                                    .name
-                                    .clone();
-                            write!(f, "{} = ", name)?;
-                            projection_pred.ty.hir_fmt(f)?;
-                        }
-                        GenericPredicate::Error => {
-                            if angle_open {
-                                // impl Trait<X, {error}>
-                                write!(f, ", ")?;
-                            } else if !first {
-                                // impl Trait + {error}
-                                write!(f, " + ")?;
-                            }
-                            p.hir_fmt(f)?;
-                        }
-                    }
-                    first = false;
-                }
-                if angle_open {
-                    write!(f, ">")?;
-                }
+                write_bounds_like_dyn_trait(&predicates, f)?;
             }
             Ty::Unknown => write!(f, "{{unknown}}")?,
             Ty::Infer(..) => write!(f, "_")?,
@@ -1085,6 +1066,71 @@ impl HirDisplay for Ty {
     }
 }
 
+fn write_bounds_like_dyn_trait(
+    predicates: &[GenericPredicate],
+    f: &mut HirFormatter<impl HirDatabase>,
+) -> fmt::Result {
+    // Note: This code is written to produce nice results (i.e.
+    // corresponding to surface Rust) for types that can occur in
+    // actual Rust. It will have weird results if the predicates
+    // aren't as expected (i.e. self types = $0, projection
+    // predicates for a certain trait come after the Implemented
+    // predicate for that trait).
+    let mut first = true;
+    let mut angle_open = false;
+    for p in predicates.iter() {
+        match p {
+            GenericPredicate::Implemented(trait_ref) => {
+                if angle_open {
+                    write!(f, ">")?;
+                }
+                if !first {
+                    write!(f, " + ")?;
+                }
+                // We assume that the self type is $0 (i.e. the
+                // existential) here, which is the only thing that's
+                // possible in actual Rust, and hence don't print it
+                write!(f, "{}", f.db.trait_data(trait_ref.trait_).name.clone())?;
+                if trait_ref.substs.len() > 1 {
+                    write!(f, "<")?;
+                    f.write_joined(&trait_ref.substs[1..], ", ")?;
+                    // there might be assoc type bindings, so we leave the angle brackets open
+                    angle_open = true;
+                }
+            }
+            GenericPredicate::Projection(projection_pred) => {
+                // in types in actual Rust, these will always come
+                // after the corresponding Implemented predicate
+                if angle_open {
+                    write!(f, ", ")?;
+                } else {
+                    write!(f, "<")?;
+                    angle_open = true;
+                }
+                let name =
+                    f.db.type_alias_data(projection_pred.projection_ty.associated_ty).name.clone();
+                write!(f, "{} = ", name)?;
+                projection_pred.ty.hir_fmt(f)?;
+            }
+            GenericPredicate::Error => {
+                if angle_open {
+                    // impl Trait<X, {error}>
+                    write!(f, ", ")?;
+                } else if !first {
+                    // impl Trait + {error}
+                    write!(f, " + ")?;
+                }
+                p.hir_fmt(f)?;
+            }
+        }
+        first = false;
+    }
+    if angle_open {
+        write!(f, ">")?;
+    }
+    Ok(())
+}
+
 impl TraitRef {
     fn hir_fmt_ext(&self, f: &mut HirFormatter<impl HirDatabase>, use_as: bool) -> fmt::Result {
         if f.should_truncate() {
diff --git a/crates/ra_hir_ty/src/lower.rs b/crates/ra_hir_ty/src/lower.rs
index 2c2ecee9c..c68c5852b 100644
--- a/crates/ra_hir_ty/src/lower.rs
+++ b/crates/ra_hir_ty/src/lower.rs
@@ -10,12 +10,13 @@ use std::sync::Arc;
 
 use hir_def::{
     builtin_type::BuiltinType,
-    generics::WherePredicate,
+    generics::{TypeParamProvenance, WherePredicate, WherePredicateTarget},
     path::{GenericArg, Path, PathSegment, PathSegments},
     resolver::{HasResolver, Resolver, TypeNs},
     type_ref::{TypeBound, TypeRef},
     AdtId, ConstId, EnumId, EnumVariantId, FunctionId, GenericDefId, HasModule, ImplId,
-    LocalStructFieldId, Lookup, StaticId, StructId, TraitId, TypeAliasId, UnionId, VariantId,
+    LocalStructFieldId, Lookup, StaticId, StructId, TraitId, TypeAliasId, TypeParamId, UnionId,
+    VariantId,
 };
 use ra_arena::map::ArenaMap;
 use ra_db::CrateId;
@@ -27,63 +28,158 @@ use crate::{
         all_super_traits, associated_type_by_name_including_super_traits, generics, make_mut_slice,
         variant_data,
     },
-    FnSig, GenericPredicate, ProjectionPredicate, ProjectionTy, Substs, TraitEnvironment, TraitRef,
-    Ty, TypeCtor, TypeWalk,
+    Binders, FnSig, GenericPredicate, PolyFnSig, ProjectionPredicate, ProjectionTy, Substs,
+    TraitEnvironment, TraitRef, Ty, TypeCtor,
 };
 
+#[derive(Debug)]
+pub struct TyLoweringContext<'a, DB: HirDatabase> {
+    pub db: &'a DB,
+    pub resolver: &'a Resolver,
+    /// Note: Conceptually, it's thinkable that we could be in a location where
+    /// some type params should be represented as placeholders, and others
+    /// should be converted to variables. I think in practice, this isn't
+    /// possible currently, so this should be fine for now.
+    pub type_param_mode: TypeParamLoweringMode,
+    pub impl_trait_mode: ImplTraitLoweringMode,
+    pub impl_trait_counter: std::cell::Cell<u16>,
+}
+
+impl<'a, DB: HirDatabase> TyLoweringContext<'a, DB> {
+    pub fn new(db: &'a DB, resolver: &'a Resolver) -> Self {
+        let impl_trait_counter = std::cell::Cell::new(0);
+        let impl_trait_mode = ImplTraitLoweringMode::Disallowed;
+        let type_param_mode = TypeParamLoweringMode::Placeholder;
+        Self { db, resolver, impl_trait_mode, impl_trait_counter, type_param_mode }
+    }
+
+    pub fn with_impl_trait_mode(self, impl_trait_mode: ImplTraitLoweringMode) -> Self {
+        Self { impl_trait_mode, ..self }
+    }
+
+    pub fn with_type_param_mode(self, type_param_mode: TypeParamLoweringMode) -> Self {
+        Self { type_param_mode, ..self }
+    }
+}
+
+#[derive(Copy, Clone, Debug, PartialEq, Eq)]
+pub enum ImplTraitLoweringMode {
+    /// `impl Trait` gets lowered into an opaque type that doesn't unify with
+    /// anything except itself. This is used in places where values flow 'out',
+    /// i.e. for arguments of the function we're currently checking, and return
+    /// types of functions we're calling.
+    Opaque,
+    /// `impl Trait` gets lowered into a type variable. Used for argument
+    /// position impl Trait when inside the respective function, since it allows
+    /// us to support that without Chalk.
+    Param,
+    /// `impl Trait` gets lowered into a variable that can unify with some
+    /// type. This is used in places where values flow 'in', i.e. for arguments
+    /// of functions we're calling, and the return type of the function we're
+    /// currently checking.
+    Variable,
+    /// `impl Trait` is disallowed and will be an error.
+    Disallowed,
+}
+
+#[derive(Copy, Clone, Debug, PartialEq, Eq)]
+pub enum TypeParamLoweringMode {
+    Placeholder,
+    Variable,
+}
+
 impl Ty {
-    pub fn from_hir(db: &impl HirDatabase, resolver: &Resolver, type_ref: &TypeRef) -> Self {
+    pub fn from_hir(ctx: &TyLoweringContext<'_, impl HirDatabase>, type_ref: &TypeRef) -> Self {
         match type_ref {
             TypeRef::Never => Ty::simple(TypeCtor::Never),
             TypeRef::Tuple(inner) => {
-                let inner_tys: Arc<[Ty]> =
-                    inner.iter().map(|tr| Ty::from_hir(db, resolver, tr)).collect();
+                let inner_tys: Arc<[Ty]> = inner.iter().map(|tr| Ty::from_hir(ctx, tr)).collect();
                 Ty::apply(
                     TypeCtor::Tuple { cardinality: inner_tys.len() as u16 },
                     Substs(inner_tys),
                 )
             }
-            TypeRef::Path(path) => Ty::from_hir_path(db, resolver, path),
+            TypeRef::Path(path) => Ty::from_hir_path(ctx, path),
             TypeRef::RawPtr(inner, mutability) => {
-                let inner_ty = Ty::from_hir(db, resolver, inner);
+                let inner_ty = Ty::from_hir(ctx, inner);
                 Ty::apply_one(TypeCtor::RawPtr(*mutability), inner_ty)
             }
             TypeRef::Array(inner) => {
-                let inner_ty = Ty::from_hir(db, resolver, inner);
+                let inner_ty = Ty::from_hir(ctx, inner);
                 Ty::apply_one(TypeCtor::Array, inner_ty)
             }
             TypeRef::Slice(inner) => {
-                let inner_ty = Ty::from_hir(db, resolver, inner);
+                let inner_ty = Ty::from_hir(ctx, inner);
                 Ty::apply_one(TypeCtor::Slice, inner_ty)
             }
             TypeRef::Reference(inner, mutability) => {
-                let inner_ty = Ty::from_hir(db, resolver, inner);
+                let inner_ty = Ty::from_hir(ctx, inner);
                 Ty::apply_one(TypeCtor::Ref(*mutability), inner_ty)
             }
             TypeRef::Placeholder => Ty::Unknown,
             TypeRef::Fn(params) => {
-                let sig = Substs(params.iter().map(|tr| Ty::from_hir(db, resolver, tr)).collect());
+                let sig = Substs(params.iter().map(|tr| Ty::from_hir(ctx, tr)).collect());
                 Ty::apply(TypeCtor::FnPtr { num_args: sig.len() as u16 - 1 }, sig)
             }
             TypeRef::DynTrait(bounds) => {
                 let self_ty = Ty::Bound(0);
                 let predicates = bounds
                     .iter()
-                    .flat_map(|b| {
-                        GenericPredicate::from_type_bound(db, resolver, b, self_ty.clone())
-                    })
+                    .flat_map(|b| GenericPredicate::from_type_bound(ctx, b, self_ty.clone()))
                     .collect();
                 Ty::Dyn(predicates)
             }
             TypeRef::ImplTrait(bounds) => {
-                let self_ty = Ty::Bound(0);
-                let predicates = bounds
-                    .iter()
-                    .flat_map(|b| {
-                        GenericPredicate::from_type_bound(db, resolver, b, self_ty.clone())
-                    })
-                    .collect();
-                Ty::Opaque(predicates)
+                match ctx.impl_trait_mode {
+                    ImplTraitLoweringMode::Opaque => {
+                        let self_ty = Ty::Bound(0);
+                        let predicates = bounds
+                            .iter()
+                            .flat_map(|b| {
+                                GenericPredicate::from_type_bound(ctx, b, self_ty.clone())
+                            })
+                            .collect();
+                        Ty::Opaque(predicates)
+                    }
+                    ImplTraitLoweringMode::Param => {
+                        let idx = ctx.impl_trait_counter.get();
+                        ctx.impl_trait_counter.set(idx + 1);
+                        if let Some(def) = ctx.resolver.generic_def() {
+                            let generics = generics(ctx.db, def);
+                            let param = generics
+                                .iter()
+                                .filter(|(_, data)| {
+                                    data.provenance == TypeParamProvenance::ArgumentImplTrait
+                                })
+                                .nth(idx as usize)
+                                .map_or(Ty::Unknown, |(id, _)| Ty::Param(id));
+                            param
+                        } else {
+                            Ty::Unknown
+                        }
+                    }
+                    ImplTraitLoweringMode::Variable => {
+                        let idx = ctx.impl_trait_counter.get();
+                        ctx.impl_trait_counter.set(idx + 1);
+                        let (parent_params, self_params, list_params, _impl_trait_params) =
+                            if let Some(def) = ctx.resolver.generic_def() {
+                                let generics = generics(ctx.db, def);
+                                generics.provenance_split()
+                            } else {
+                                (0, 0, 0, 0)
+                            };
+                        Ty::Bound(
+                            idx as u32
+                                + parent_params as u32
+                                + self_params as u32
+                                + list_params as u32,
+                        )
+                    }
+                    ImplTraitLoweringMode::Disallowed => {
+                        // FIXME: report error
+                        Ty::Unknown
+                    }
+                }
             }
             TypeRef::Error => Ty::Unknown,
         }
@@ -93,10 +189,9 @@ impl Ty {
     /// lower the self types of the predicates since that could lead to cycles.
     /// So we just check here if the `type_ref` resolves to a generic param, and which.
     fn from_hir_only_param(
-        db: &impl HirDatabase,
-        resolver: &Resolver,
+        ctx: &TyLoweringContext<'_, impl HirDatabase>,
         type_ref: &TypeRef,
-    ) -> Option<u32> {
+    ) -> Option<TypeParamId> {
         let path = match type_ref {
             TypeRef::Path(path) => path,
             _ => return None,
@@ -107,29 +202,26 @@ impl Ty {
         if path.segments().len() > 1 {
             return None;
         }
-        let resolution = match resolver.resolve_path_in_type_ns(db, path.mod_path()) {
+        let resolution = match ctx.resolver.resolve_path_in_type_ns(ctx.db, path.mod_path()) {
             Some((it, None)) => it,
             _ => return None,
         };
         if let TypeNs::GenericParam(param_id) = resolution {
-            let generics = generics(db, resolver.generic_def().expect("generics in scope"));
-            let idx = generics.param_idx(param_id);
-            Some(idx)
+            Some(param_id)
         } else {
             None
         }
     }
 
     pub(crate) fn from_type_relative_path(
-        db: &impl HirDatabase,
-        resolver: &Resolver,
+        ctx: &TyLoweringContext<'_, impl HirDatabase>,
         ty: Ty,
         remaining_segments: PathSegments<'_>,
     ) -> Ty {
         if remaining_segments.len() == 1 {
             // resolve unselected assoc types
             let segment = remaining_segments.first().unwrap();
-            Ty::select_associated_type(db, resolver, ty, segment)
+            Ty::select_associated_type(ctx, ty, segment)
         } else if remaining_segments.len() > 1 {
             // FIXME report error (ambiguous associated type)
             Ty::Unknown
@@ -139,20 +231,18 @@ impl Ty {
     }
 
     pub(crate) fn from_partly_resolved_hir_path(
-        db: &impl HirDatabase,
-        resolver: &Resolver,
+        ctx: &TyLoweringContext<'_, impl HirDatabase>,
         resolution: TypeNs,
         resolved_segment: PathSegment<'_>,
         remaining_segments: PathSegments<'_>,
     ) -> Ty {
         let ty = match resolution {
             TypeNs::TraitId(trait_) => {
-                let trait_ref =
-                    TraitRef::from_resolved_path(db, resolver, trait_, resolved_segment, None);
+                let trait_ref = TraitRef::from_resolved_path(ctx, trait_, resolved_segment, None);
                 return if remaining_segments.len() == 1 {
                     let segment = remaining_segments.first().unwrap();
                     let associated_ty = associated_type_by_name_including_super_traits(
-                        db,
+                        ctx.db,
                         trait_ref.trait_,
                         &segment.name,
                     );
@@ -177,37 +267,55 @@ impl Ty {
                 };
             }
             TypeNs::GenericParam(param_id) => {
-                let generics = generics(db, resolver.generic_def().expect("generics in scope"));
-                let idx = generics.param_idx(param_id);
-                // FIXME: maybe return name in resolution?
-                let name = generics.param_name(param_id);
-                Ty::Param { idx, name }
+                let generics =
+                    generics(ctx.db, ctx.resolver.generic_def().expect("generics in scope"));
+                match ctx.type_param_mode {
+                    TypeParamLoweringMode::Placeholder => Ty::Param(param_id),
+                    TypeParamLoweringMode::Variable => {
+                        let idx = generics.param_idx(param_id).expect("matching generics");
+                        Ty::Bound(idx)
+                    }
+                }
             }
-            TypeNs::SelfType(impl_id) => db.impl_self_ty(impl_id).clone(),
-            TypeNs::AdtSelfType(adt) => db.ty(adt.into()),
-
-            TypeNs::AdtId(it) => Ty::from_hir_path_inner(db, resolver, resolved_segment, it.into()),
-            TypeNs::BuiltinType(it) => {
-                Ty::from_hir_path_inner(db, resolver, resolved_segment, it.into())
+            TypeNs::SelfType(impl_id) => {
+                let generics = generics(ctx.db, impl_id.into());
+                let substs = match ctx.type_param_mode {
+                    TypeParamLoweringMode::Placeholder => {
+                        Substs::type_params_for_generics(&generics)
+                    }
+                    TypeParamLoweringMode::Variable => Substs::bound_vars(&generics),
+                };
+                ctx.db.impl_self_ty(impl_id).subst(&substs)
             }
-            TypeNs::TypeAliasId(it) => {
-                Ty::from_hir_path_inner(db, resolver, resolved_segment, it.into())
+            TypeNs::AdtSelfType(adt) => {
+                let generics = generics(ctx.db, adt.into());
+                let substs = match ctx.type_param_mode {
+                    TypeParamLoweringMode::Placeholder => {
+                        Substs::type_params_for_generics(&generics)
+                    }
+                    TypeParamLoweringMode::Variable => Substs::bound_vars(&generics),
+                };
+                ctx.db.ty(adt.into()).subst(&substs)
             }
+
+            TypeNs::AdtId(it) => Ty::from_hir_path_inner(ctx, resolved_segment, it.into()),
+            TypeNs::BuiltinType(it) => Ty::from_hir_path_inner(ctx, resolved_segment, it.into()),
+            TypeNs::TypeAliasId(it) => Ty::from_hir_path_inner(ctx, resolved_segment, it.into()),
             // FIXME: report error
             TypeNs::EnumVariantId(_) => return Ty::Unknown,
         };
 
-        Ty::from_type_relative_path(db, resolver, ty, remaining_segments)
+        Ty::from_type_relative_path(ctx, ty, remaining_segments)
     }
 
-    pub(crate) fn from_hir_path(db: &impl HirDatabase, resolver: &Resolver, path: &Path) -> Ty {
+    pub(crate) fn from_hir_path(ctx: &TyLoweringContext<'_, impl HirDatabase>, path: &Path) -> Ty {
         // Resolve the path (in type namespace)
         if let Some(type_ref) = path.type_anchor() {
-            let ty = Ty::from_hir(db, resolver, &type_ref);
-            return Ty::from_type_relative_path(db, resolver, ty, path.segments());
+            let ty = Ty::from_hir(ctx, &type_ref);
+            return Ty::from_type_relative_path(ctx, ty, path.segments());
         }
         let (resolution, remaining_index) =
-            match resolver.resolve_path_in_type_ns(db, path.mod_path()) {
+            match ctx.resolver.resolve_path_in_type_ns(ctx.db, path.mod_path()) {
                 Some(it) => it,
                 None => return Ty::Unknown,
             };
@@ -218,39 +326,44 @@ impl Ty {
             ),
             Some(i) => (path.segments().get(i - 1).unwrap(), path.segments().skip(i)),
         };
-        Ty::from_partly_resolved_hir_path(
-            db,
-            resolver,
-            resolution,
-            resolved_segment,
-            remaining_segments,
-        )
+        Ty::from_partly_resolved_hir_path(ctx, resolution, resolved_segment, remaining_segments)
     }
 
     fn select_associated_type(
-        db: &impl HirDatabase,
-        resolver: &Resolver,
+        ctx: &TyLoweringContext<'_, impl HirDatabase>,
         self_ty: Ty,
         segment: PathSegment<'_>,
     ) -> Ty {
-        let param_idx = match self_ty {
-            Ty::Param { idx, .. } => idx,
-            _ => return Ty::Unknown, // Error: Ambiguous associated type
-        };
-        let def = match resolver.generic_def() {
+        let def = match ctx.resolver.generic_def() {
             Some(def) => def,
             None => return Ty::Unknown, // this can't actually happen
         };
-        let predicates = db.generic_predicates_for_param(def.into(), param_idx);
-        let traits_from_env = predicates.iter().filter_map(|pred| match pred {
-            GenericPredicate::Implemented(tr) if tr.self_ty() == &self_ty => Some(tr.trait_),
+        let param_id = match self_ty {
+            Ty::Param(id) if ctx.type_param_mode == TypeParamLoweringMode::Placeholder => id,
+            Ty::Bound(idx) if ctx.type_param_mode == TypeParamLoweringMode::Variable => {
+                let generics = generics(ctx.db, def);
+                let param_id = if let Some((id, _)) = generics.iter().nth(idx as usize) {
+                    id
+                } else {
+                    return Ty::Unknown;
+                };
+                param_id
+            }
+            _ => return Ty::Unknown, // Error: Ambiguous associated type
+        };
+        let predicates = ctx.db.generic_predicates_for_param(param_id);
+        let traits_from_env = predicates.iter().filter_map(|pred| match &pred.value {
+            GenericPredicate::Implemented(tr) => Some(tr.trait_),
             _ => None,
         });
-        let traits = traits_from_env.flat_map(|t| all_super_traits(db, t));
+        let traits = traits_from_env.flat_map(|t| all_super_traits(ctx.db, t));
         for t in traits {
-            if let Some(associated_ty) = db.trait_data(t).associated_type_by_name(&segment.name) {
-                let substs =
-                    Substs::build_for_def(db, t).push(self_ty.clone()).fill_with_unknown().build();
+            if let Some(associated_ty) = ctx.db.trait_data(t).associated_type_by_name(&segment.name)
+            {
+                let substs = Substs::build_for_def(ctx.db, t)
+                    .push(self_ty.clone())
+                    .fill_with_unknown()
+                    .build();
                 // FIXME handle type parameters on the segment
                 return Ty::Projection(ProjectionTy { associated_ty, parameters: substs });
             }
@@ -259,8 +372,7 @@ impl Ty {
     }
 
     fn from_hir_path_inner(
-        db: &impl HirDatabase,
-        resolver: &Resolver,
+        ctx: &TyLoweringContext<'_, impl HirDatabase>,
         segment: PathSegment<'_>,
         typable: TyDefId,
     ) -> Ty {
@@ -269,15 +381,14 @@ impl Ty {
             TyDefId::AdtId(it) => Some(it.into()),
             TyDefId::TypeAliasId(it) => Some(it.into()),
         };
-        let substs = substs_from_path_segment(db, resolver, segment, generic_def, false);
-        db.ty(typable).subst(&substs)
+        let substs = substs_from_path_segment(ctx, segment, generic_def, false);
+        ctx.db.ty(typable).subst(&substs)
     }
 
     /// Collect generic arguments from a path into a `Substs`. See also
     /// `create_substs_for_ast_path` and `def_to_ty` in rustc.
     pub(super) fn substs_from_path(
-        db: &impl HirDatabase,
-        resolver: &Resolver,
+        ctx: &TyLoweringContext<'_, impl HirDatabase>,
         path: &Path,
         // Note that we don't call `db.value_type(resolved)` here,
         // `ValueTyDefId` is just a convenient way to pass generics and
@@ -305,52 +416,49 @@ impl Ty {
                 (segment, Some(var.parent.into()))
             }
         };
-        substs_from_path_segment(db, resolver, segment, generic_def, false)
+        substs_from_path_segment(ctx, segment, generic_def, false)
     }
 }
 
 pub(super) fn substs_from_path_segment(
-    db: &impl HirDatabase,
-    resolver: &Resolver,
+    ctx: &TyLoweringContext<'_, impl HirDatabase>,
     segment: PathSegment<'_>,
     def_generic: Option<GenericDefId>,
-    add_self_param: bool,
+    _add_self_param: bool,
 ) -> Substs {
     let mut substs = Vec::new();
-    let def_generics = def_generic.map(|def| generics(db, def.into()));
-
-    let (total_len, parent_len, child_len) = def_generics.map_or((0, 0, 0), |g| g.len_split());
-    substs.extend(iter::repeat(Ty::Unknown).take(parent_len));
-    if add_self_param {
-        // FIXME this add_self_param argument is kind of a hack: Traits have the
-        // Self type as an implicit first type parameter, but it can't be
-        // actually provided in the type arguments
-        // (well, actually sometimes it can, in the form of type-relative paths: `<Foo as Default>::default()`)
-        substs.push(Ty::Unknown);
-    }
+    let def_generics = def_generic.map(|def| generics(ctx.db, def.into()));
+
+    let (parent_params, self_params, type_params, impl_trait_params) =
+        def_generics.map_or((0, 0, 0, 0), |g| g.provenance_split());
+    substs.extend(iter::repeat(Ty::Unknown).take(parent_params));
     if let Some(generic_args) = &segment.args_and_bindings {
+        if !generic_args.has_self_type {
+            substs.extend(iter::repeat(Ty::Unknown).take(self_params));
+        }
+        let expected_num =
+            if generic_args.has_self_type { self_params + type_params } else { type_params };
+        let skip = if generic_args.has_self_type && self_params == 0 { 1 } else { 0 };
         // if args are provided, it should be all of them, but we can't rely on that
-        let self_param_correction = if add_self_param { 1 } else { 0 };
-        let child_len = child_len - self_param_correction;
-        for arg in generic_args.args.iter().take(child_len) {
+        for arg in generic_args.args.iter().skip(skip).take(expected_num) {
             match arg {
                 GenericArg::Type(type_ref) => {
-                    let ty = Ty::from_hir(db, resolver, type_ref);
+                    let ty = Ty::from_hir(ctx, type_ref);
                     substs.push(ty);
                 }
             }
         }
     }
+    let total_len = parent_params + self_params + type_params + impl_trait_params;
     // add placeholders for args that were not provided
-    let supplied_params = substs.len();
-    for _ in supplied_params..total_len {
+    for _ in substs.len()..total_len {
         substs.push(Ty::Unknown);
     }
     assert_eq!(substs.len(), total_len);
 
     // handle defaults
     if let Some(def_generic) = def_generic {
-        let default_substs = db.generic_defaults(def_generic.into());
+        let default_substs = ctx.db.generic_defaults(def_generic.into());
         assert_eq!(substs.len(), default_substs.len());
 
         for (i, default_ty) in default_substs.iter().enumerate() {
@@ -365,27 +473,25 @@ pub(super) fn substs_from_path_segment(
 
 impl TraitRef {
     fn from_path(
-        db: &impl HirDatabase,
-        resolver: &Resolver,
+        ctx: &TyLoweringContext<'_, impl HirDatabase>,
         path: &Path,
         explicit_self_ty: Option<Ty>,
     ) -> Option<Self> {
-        let resolved = match resolver.resolve_path_in_type_ns_fully(db, path.mod_path())? {
+        let resolved = match ctx.resolver.resolve_path_in_type_ns_fully(ctx.db, path.mod_path())? {
             TypeNs::TraitId(tr) => tr,
             _ => return None,
         };
         let segment = path.segments().last().expect("path should have at least one segment");
-        Some(TraitRef::from_resolved_path(db, resolver, resolved.into(), segment, explicit_self_ty))
+        Some(TraitRef::from_resolved_path(ctx, resolved.into(), segment, explicit_self_ty))
     }
 
     pub(crate) fn from_resolved_path(
-        db: &impl HirDatabase,
-        resolver: &Resolver,
+        ctx: &TyLoweringContext<'_, impl HirDatabase>,
         resolved: TraitId,
         segment: PathSegment<'_>,
         explicit_self_ty: Option<Ty>,
     ) -> Self {