19 files changed, 428 insertions, 214 deletions

diff --git a/Cargo.lock b/Cargo.lock
index cf4de8bbe..280d97432 100644
--- a/Cargo.lock
+++ b/Cargo.lock
@@ -216,12 +216,12 @@ dependencies = [
 
 [[package]]
 name = "crossbeam-channel"
-version = "0.4.2"
+version = "0.4.3"
 source = "registry+https://github.com/rust-lang/crates.io-index"
-checksum = "cced8691919c02aac3cb0a1bc2e9b73d89e832bf9a06fc579d4e71b68a2da061"
+checksum = "09ee0cc8804d5393478d743b035099520087a5186f3b93fa58cec08fa62407b6"
 dependencies = [
+ "cfg-if",
  "crossbeam-utils",
- "maybe-uninit",
 ]
 
 [[package]]
@@ -360,12 +360,6 @@ dependencies = [
 ]
 
 [[package]]
-name = "fs_extra"
-version = "1.1.0"
-source = "registry+https://github.com/rust-lang/crates.io-index"
-checksum = "5f2a4a2034423744d2cc7ca2068453168dcdb82c438419e639a26bd87839c674"
-
-[[package]]
 name = "fsevent"
 version = "2.0.2"
 source = "registry+https://github.com/rust-lang/crates.io-index"
@@ -542,38 +536,6 @@ source = "registry+https://github.com/rust-lang/crates.io-index"
 checksum = "dc6f3ad7b9d11a0c00842ff8de1b60ee58661048eb8049ed33c73594f359d7e6"
 
 [[package]]
-name = "jemalloc-ctl"
-version = "0.3.3"
-source = "registry+https://github.com/rust-lang/crates.io-index"
-checksum = "c502a5ff9dd2924f1ed32ba96e3b65735d837b4bfd978d3161b1702e66aca4b7"
-dependencies = [
- "jemalloc-sys",
- "libc",
- "paste",
-]
-
-[[package]]
-name = "jemalloc-sys"
-version = "0.3.2"
-source = "registry+https://github.com/rust-lang/crates.io-index"
-checksum = "0d3b9f3f5c9b31aa0f5ed3260385ac205db665baa41d49bb8338008ae94ede45"
-dependencies = [
- "cc",
- "fs_extra",
- "libc",
-]
-
-[[package]]
-name = "jemallocator"
-version = "0.3.2"
-source = "registry+https://github.com/rust-lang/crates.io-index"
-checksum = "43ae63fcfc45e99ab3d1b29a46782ad679e98436c3169d15a167a1108a724b69"
-dependencies = [
- "jemalloc-sys",
- "libc",
-]
-
-[[package]]
 name = "jod-thread"
 version = "0.1.2"
 source = "registry+https://github.com/rust-lang/crates.io-index"
@@ -875,25 +837,6 @@ dependencies = [
 ]
 
 [[package]]
-name = "paste"
-version = "0.1.18"
-source = "registry+https://github.com/rust-lang/crates.io-index"
-checksum = "45ca20c77d80be666aef2b45486da86238fabe33e38306bd3118fe4af33fa880"
-dependencies = [
- "paste-impl",
- "proc-macro-hack",
-]
-
-[[package]]
-name = "paste-impl"
-version = "0.1.18"
-source = "registry+https://github.com/rust-lang/crates.io-index"
-checksum = "d95a7db200b97ef370c8e6de0088252f7e0dfff7d047a28528e47456c0fc98b6"
-dependencies = [
- "proc-macro-hack",
-]
-
-[[package]]
 name = "paths"
 version = "0.1.0"
 
@@ -932,12 +875,6 @@ source = "registry+https://github.com/rust-lang/crates.io-index"
 checksum = "237a5ed80e274dbc66f86bd59c1e25edc039660be53194b5fe0a482e0f2612ea"
 
 [[package]]
-name = "proc-macro-hack"
-version = "0.5.16"
-source = "registry+https://github.com/rust-lang/crates.io-index"
-checksum = "7e0456befd48169b9f13ef0f0ad46d492cf9d2dbb918bcf38e01eed4ce3ec5e4"
-
-[[package]]
 name = "proc-macro2"
 version = "1.0.19"
 source = "registry+https://github.com/rust-lang/crates.io-index"
@@ -1198,9 +1135,8 @@ name = "ra_prof"
 version = "0.1.0"
 dependencies = [
  "backtrace",
- "jemalloc-ctl",
- "jemallocator",
- "mimalloc",
+ "cfg-if",
+ "libc",
  "once_cell",
  "ra_arena",
 ]
@@ -1425,6 +1361,7 @@ dependencies = [
  "log",
  "lsp-server",
  "lsp-types",
+ "mimalloc",
  "parking_lot",
  "pico-args",
  "ra_cfg",
diff --git a/crates/ra_assists/src/handlers/extract_variable.rs b/crates/ra_assists/src/handlers/extract_variable.rs
index 481baf1a4..098adf078 100644
--- a/crates/ra_assists/src/handlers/extract_variable.rs
+++ b/crates/ra_assists/src/handlers/extract_variable.rs
@@ -2,7 +2,6 @@ use ra_syntax::{
     ast::{self, AstNode},
     SyntaxKind::{
         BLOCK_EXPR, BREAK_EXPR, COMMENT, LAMBDA_EXPR, LOOP_EXPR, MATCH_ARM, PATH_EXPR, RETURN_EXPR,
-        WHITESPACE,
     },
     SyntaxNode,
 };
@@ -36,22 +35,20 @@ pub(crate) fn extract_variable(acc: &mut Assists, ctx: &AssistContext) -> Option
         mark::hit!(extract_var_in_comment_is_not_applicable);
         return None;
     }
-    let expr = node.ancestors().find_map(valid_target_expr)?;
-    let (anchor_stmt, wrap_in_block) = anchor_stmt(expr.clone())?;
-    let indent = anchor_stmt.prev_sibling_or_token()?.as_token()?.clone();
-    if indent.kind() != WHITESPACE {
-        return None;
-    }
-    let target = expr.syntax().text_range();
+    let to_extract = node.ancestors().find_map(valid_target_expr)?;
+    let anchor = Anchor::from(&to_extract)?;
+    let indent = anchor.syntax().prev_sibling_or_token()?.as_token()?.clone();
+    let target = to_extract.syntax().text_range();
     acc.add(
         AssistId("extract_variable", AssistKind::RefactorExtract),
         "Extract into variable",
         target,
         move |edit| {
-            let field_shorthand = match expr.syntax().parent().and_then(ast::RecordField::cast) {
-                Some(field) => field.name_ref(),
-                None => None,
-            };
+            let field_shorthand =
+                match to_extract.syntax().parent().and_then(ast::RecordField::cast) {
+                    Some(field) => field.name_ref(),
+                    None => None,
+                };
 
             let mut buf = String::new();
 
@@ -60,26 +57,20 @@ pub(crate) fn extract_variable(acc: &mut Assists, ctx: &AssistContext) -> Option
                 None => "var_name".to_string(),
             };
             let expr_range = match &field_shorthand {
-                Some(it) => it.syntax().text_range().cover(expr.syntax().text_range()),
-                None => expr.syntax().text_range(),
+                Some(it) => it.syntax().text_range().cover(to_extract.syntax().text_range()),
+                None => to_extract.syntax().text_range(),
             };
 
-            if wrap_in_block {
+            if let Anchor::WrapInBlock(_) = anchor {
                 format_to!(buf, "{{ let {} = ", var_name);
             } else {
                 format_to!(buf, "let {} = ", var_name);
             };
-            format_to!(buf, "{}", expr.syntax());
+            format_to!(buf, "{}", to_extract.syntax());
 
-            let full_stmt = ast::ExprStmt::cast(anchor_stmt.clone());
-            let is_full_stmt = if let Some(expr_stmt) = &full_stmt {
-                Some(expr.syntax().clone()) == expr_stmt.expr().map(|e| e.syntax().clone())
-            } else {
-                false
-            };
-            if is_full_stmt {
+            if let Anchor::Replace(stmt) = anchor {
                 mark::hit!(test_extract_var_expr_stmt);
-                if full_stmt.unwrap().semicolon_token().is_none() {
+                if stmt.semicolon_token().is_none() {
                     buf.push_str(";");
                 }
                 match ctx.config.snippet_cap {
@@ -107,7 +98,7 @@ pub(crate) fn extract_variable(acc: &mut Assists, ctx: &AssistContext) -> Option
             }
 
             edit.replace(expr_range, var_name.clone());
-            let offset = anchor_stmt.text_range().start();
+            let offset = anchor.syntax().text_range().start();
             match ctx.config.snippet_cap {
                 Some(cap) => {
                     let snip =
@@ -117,8 +108,8 @@ pub(crate) fn extract_variable(acc: &mut Assists, ctx: &AssistContext) -> Option
                 None => edit.insert(offset, buf),
             }
 
-            if wrap_in_block {
-                edit.insert(anchor_stmt.text_range().end(), " }");
+            if let Anchor::WrapInBlock(_) = anchor {
+                edit.insert(anchor.syntax().text_range().end(), " }");
             }
         },
     )
@@ -138,32 +129,48 @@ fn valid_target_expr(node: SyntaxNode) -> Option<ast::Expr> {
     }
 }
 
-/// Returns the syntax node which will follow the freshly extractd var
-/// and a boolean indicating whether we have to wrap it within a { } block
-/// to produce correct code.
-/// It can be a statement, the last in a block expression or a wanna be block
-/// expression like a lambda or match arm.
-fn anchor_stmt(expr: ast::Expr) -> Option<(SyntaxNode, bool)> {
-    expr.syntax().ancestors().find_map(|node| {
-        if let Some(expr) = node.parent().and_then(ast::BlockExpr::cast).and_then(|it| it.expr()) {
-            if expr.syntax() == &node {
-                mark::hit!(test_extract_var_last_expr);
-                return Some((node, false));
+enum Anchor {
+    Before(SyntaxNode),
+    Replace(ast::ExprStmt),
+    WrapInBlock(SyntaxNode),
+}
+
+impl Anchor {
+    fn from(to_extract: &ast::Expr) -> Option<Anchor> {
+        to_extract.syntax().ancestors().find_map(|node| {
+            if let Some(expr) =
+                node.parent().and_then(ast::BlockExpr::cast).and_then(|it| it.expr())
+            {
+                if expr.syntax() == &node {
+                    mark::hit!(test_extract_var_last_expr);
+                    return Some(Anchor::Before(node));
+                }
             }
-        }
 
-        if let Some(parent) = node.parent() {
-            if parent.kind() == MATCH_ARM || parent.kind() == LAMBDA_EXPR {
-                return Some((node, true));
+            if let Some(parent) = node.parent() {
+                if parent.kind() == MATCH_ARM || parent.kind() == LAMBDA_EXPR {
+                    return Some(Anchor::WrapInBlock(node));
+                }
             }
-        }
 
-        if ast::Stmt::cast(node.clone()).is_some() {
-            return Some((node, false));
-        }
+            if let Some(stmt) = ast::Stmt::cast(node.clone()) {
+                if let ast::Stmt::ExprStmt(stmt) = stmt {
+                    if stmt.expr().as_ref() == Some(to_extract) {
+                        return Some(Anchor::Replace(stmt));
+                    }
+                }
+                return Some(Anchor::Before(node));
+            }
+            None
+        })
+    }
 
-        None
-    })
+    fn syntax(&self) -> &SyntaxNode {
+        match self {
+            Anchor::Before(it) | Anchor::WrapInBlock(it) => it,
+            Anchor::Replace(stmt) => stmt.syntax(),
+        }
+    }
 }
 
 #[cfg(test)]
diff --git a/crates/ra_assists/src/handlers/fix_visibility.rs b/crates/ra_assists/src/handlers/fix_visibility.rs
index e212557c8..1d3ed3c6a 100644
--- a/crates/ra_assists/src/handlers/fix_visibility.rs
+++ b/crates/ra_assists/src/handlers/fix_visibility.rs
@@ -3,6 +3,7 @@ use ra_db::FileId;
 use ra_syntax::{ast, AstNode, TextRange, TextSize};
 
 use crate::{utils::vis_offset, AssistContext, AssistId, AssistKind, Assists};
+use ast::VisibilityOwner;
 
 // FIXME: this really should be a fix for diagnostic, rather than an assist.
 
@@ -48,7 +49,8 @@ fn add_vis_to_referenced_module_def(acc: &mut Assists, ctx: &AssistContext) -> O
         return None;
     };
 
-    let (offset, target, target_file, target_name) = target_data_for_def(ctx.db(), def)?;
+    let (offset, current_visibility, target, target_file, target_name) =
+        target_data_for_def(ctx.db(), def)?;
 
     let missing_visibility =
         if current_module.krate() == target_module.krate() { "pub(crate)" } else { "pub" };
@@ -61,8 +63,20 @@ fn add_vis_to_referenced_module_def(acc: &mut Assists, ctx: &AssistContext) -> O
     acc.add(AssistId("fix_visibility", AssistKind::QuickFix), assist_label, target, |builder| {
         builder.edit_file(target_file);
         match ctx.config.snippet_cap {
-            Some(cap) => builder.insert_snippet(cap, offset, format!("$0{} ", missing_visibility)),
-            None => builder.insert(offset, format!("{} ", missing_visibility)),
+            Some(cap) => match current_visibility {
+                Some(current_visibility) => builder.replace_snippet(
+                    cap,
+                    current_visibility.syntax().text_range(),
+                    format!("$0{}", missing_visibility),
+                ),
+                None => builder.insert_snippet(cap, offset, format!("$0{} ", missing_visibility)),
+            },
+            None => match current_visibility {
+                Some(current_visibility) => {
+                    builder.replace(current_visibility.syntax().text_range(), missing_visibility)
+                }
+                None => builder.insert(offset, format!("{} ", missing_visibility)),
+            },
         }
     })
 }
@@ -82,14 +96,14 @@ fn add_vis_to_referenced_record_field(acc: &mut Assists, ctx: &AssistContext) ->
     let target_module = parent.module(ctx.db());
 
     let in_file_source = record_field_def.source(ctx.db());
-    let (offset, target) = match in_file_source.value {
+    let (offset, current_visibility, target) = match in_file_source.value {
         hir::FieldSource::Named(it) => {
             let s = it.syntax();
-            (vis_offset(s), s.text_range())
+            (vis_offset(s), it.visibility(), s.text_range())
         }
         hir::FieldSource::Pos(it) => {
             let s = it.syntax();
-            (vis_offset(s), s.text_range())
+            (vis_offset(s), it.visibility(), s.text_range())
         }
     };
 
@@ -104,8 +118,20 @@ fn add_vis_to_referenced_record_field(acc: &mut Assists, ctx: &AssistContext) ->
     acc.add(AssistId("fix_visibility", AssistKind::QuickFix), assist_label, target, |builder| {
         builder.edit_file(target_file);
         match ctx.config.snippet_cap {
-            Some(cap) => builder.insert_snippet(cap, offset, format!("$0{} ", missing_visibility)),
-            None => builder.insert(offset, format!("{} ", missing_visibility)),
+            Some(cap) => match current_visibility {
+                Some(current_visibility) => builder.replace_snippet(
+                    cap,
+                    dbg!(current_visibility.syntax()).text_range(),
+                    format!("$0{}", missing_visibility),
+                ),
+                None => builder.insert_snippet(cap, offset, format!("$0{} ", missing_visibility)),
+            },
+            None => match current_visibility {
+                Some(current_visibility) => {
+                    builder.replace(current_visibility.syntax().text_range(), missing_visibility)
+                }
+                None => builder.insert(offset, format!("{} ", missing_visibility)),
+            },
         }
     })
 }
@@ -113,24 +139,30 @@ fn add_vis_to_referenced_record_field(acc: &mut Assists, ctx: &AssistContext) ->
 fn target_data_for_def(
     db: &dyn HirDatabase,
     def: hir::ModuleDef,
-) -> Option<(TextSize, TextRange, FileId, Option<hir::Name>)> {
+) -> Option<(TextSize, Option<ast::Visibility>, TextRange, FileId, Option<hir::Name>)> {
     fn offset_target_and_file_id<S, Ast>(
         db: &dyn HirDatabase,
         x: S,
-    ) -> (TextSize, TextRange, FileId)
+    ) -> (TextSize, Option<ast::Visibility>, TextRange, FileId)
     where
         S: HasSource<Ast = Ast>,
-        Ast: AstNode,
+        Ast: AstNode + ast::VisibilityOwner,
     {
         let source = x.source(db);
         let in_file_syntax = source.syntax();
         let file_id = in_file_syntax.file_id;
         let syntax = in_file_syntax.value;
-        (vis_offset(syntax), syntax.text_range(), file_id.original_file(db.upcast()))
+        let current_visibility = source.value.visibility();
+        (
+            vis_offset(syntax),
+            current_visibility,
+            syntax.text_range(),
+            file_id.original_file(db.upcast()),
+        )
     }
 
     let target_name;
-    let (offset, target, target_file) = match def {
+    let (offset, current_visibility, target, target_file) = match def {
         hir::ModuleDef::Function(f) => {
             target_name = Some(f.name(db));
             offset_target_and_file_id(db, f)
@@ -164,13 +196,13 @@ fn target_data_for_def(
             let in_file_source = m.declaration_source(db)?;
             let file_id = in_file_source.file_id.original_file(db.upcast());
             let syntax = in_file_source.value.syntax();
-            (vis_offset(syntax), syntax.text_range(), file_id)
+            (vis_offset(syntax), in_file_source.value.visibility(), syntax.text_range(), file_id)
         }
         // Enum variants can't be private, we can't modify builtin types
         hir::ModuleDef::EnumVariant(_) | hir::ModuleDef::BuiltinType(_) => return None,
     };
 
-    Some((offset, target, target_file, target_name))
+    Some((offset, current_visibility, target, target_file, target_name))
 }
 
 #[cfg(test)]
@@ -523,6 +555,34 @@ struct Bar;
     }
 
     #[test]
+    fn replaces_pub_crate_with_pub() {
+        check_assist(
+            fix_visibility,
+            r"
+//- /main.rs crate:a deps:foo
+foo::Bar<|>
+//- /lib.rs crate:foo
+pub(crate) struct Bar;
+",
+            r"$0pub struct Bar;
+",
+        );
+        check_assist(
+            fix_visibility,
+            r"
+//- /main.rs crate:a deps:foo
+fn main() {
+    foo::Foo { <|>bar: () };
+}
+//- /lib.rs crate:foo
+pub struct Foo { pub(crate) bar: () }
+",
+            r"pub struct Foo { $0pub bar: () }
+",
+        );
+    }
+
+    #[test]
     #[ignore]
     // FIXME handle reexports properly
     fn fix_visibility_of_reexport() {
diff --git a/crates/ra_hir_def/src/item_scope.rs b/crates/ra_hir_def/src/item_scope.rs
index beeb98559..8fee4b15e 100644
--- a/crates/ra_hir_def/src/item_scope.rs
+++ b/crates/ra_hir_def/src/item_scope.rs
@@ -36,6 +36,8 @@ pub struct ItemScope {
 
     defs: Vec<ModuleDefId>,
     impls: Vec<ImplId>,
+    /// Traits imported via `use Trait as _;`.
+    unnamed_trait_imports: FxHashMap<TraitId, Visibility>,
     /// Macros visible in current module in legacy textual scope
     ///
     /// For macros invoked by an unqualified identifier like `bar!()`, `legacy_macros` will be searched in first.
@@ -126,10 +128,13 @@ impl ItemScope {
     }
 
     pub(crate) fn traits<'a>(&'a self) -> impl Iterator<Item = TraitId> + 'a {
-        self.types.values().filter_map(|(def, _)| match def {
-            ModuleDefId::TraitId(t) => Some(*t),
-            _ => None,
-        })
+        self.types
+            .values()
+            .filter_map(|(def, _)| match def {
+                ModuleDefId::TraitId(t) => Some(*t),
+                _ => None,
+            })
+            .chain(self.unnamed_trait_imports.keys().copied())
     }
 
     pub(crate) fn define_def(&mut self, def: ModuleDefId) {
@@ -148,6 +153,14 @@ impl ItemScope {
         self.legacy_macros.insert(name, mac);
     }
 
+    pub(crate) fn unnamed_trait_vis(&self, tr: TraitId) -> Option<Visibility> {
+        self.unnamed_trait_imports.get(&tr).copied()
+    }
+
+    pub(crate) fn push_unnamed_trait(&mut self, tr: TraitId, vis: Visibility) {
+        self.unnamed_trait_imports.insert(tr, vis);
+    }
+
     pub(crate) fn push_res(&mut self, name: Name, def: PerNs) -> bool {
         let mut changed = false;
 
@@ -241,8 +254,12 @@ impl ItemScope {
         changed
     }
 
-    pub(crate) fn resolutions<'a>(&'a self) -> impl Iterator<Item = (Name, PerNs)> + 'a {
-        self.entries().map(|(name, res)| (name.clone(), res))
+    pub(crate) fn resolutions<'a>(&'a self) -> impl Iterator<Item = (Option<Name>, PerNs)> + 'a {
+        self.entries().map(|(name, res)| (Some(name.clone()), res)).chain(
+            self.unnamed_trait_imports
+                .iter()
+                .map(|(tr, vis)| (None, PerNs::types(ModuleDefId::TraitId(*tr), *vis))),
+        )
     }
 
     pub(crate) fn collect_legacy_macros(&self) -> FxHashMap<Name, MacroDefId> {
diff --git a/crates/ra_hir_def/src/nameres.rs b/crates/ra_hir_def/src/nameres.rs
index 5a9de3d3e..3d9b55a73 100644
--- a/crates/ra_hir_def/src/nameres.rs
+++ b/crates/ra_hir_def/src/nameres.rs
@@ -239,7 +239,7 @@ impl CrateDefMap {
             entries.sort_by_key(|(name, _)| name.clone());
 
             for (name, def) in entries {
-                format_to!(buf, "{}:", name);
+                format_to!(buf, "{}:", name.map_or("_".to_string(), |name| name.to_string()));
 
                 if def.types.is_some() {
                     buf.push_str(" t");
diff --git a/crates/ra_hir_def/src/nameres/collector.rs b/crates/ra_hir_def/src/nameres/collector.rs
index d85a86c0a..a030cab47 100644
--- a/crates/ra_hir_def/src/nameres/collector.rs
+++ b/crates/ra_hir_def/src/nameres/collector.rs
@@ -310,7 +310,7 @@ impl DefCollector<'_> {
         if export {
             self.update(
                 self.def_map.root,
-                &[(name, PerNs::macros(macro_, Visibility::Public))],
+                &[(Some(name), PerNs::macros(macro_, Visibility::Public))],
                 Visibility::Public,
                 ImportType::Named,
             );
@@ -336,7 +336,7 @@ impl DefCollector<'_> {
     fn define_proc_macro(&mut self, name: Name, macro_: MacroDefId) {
         self.update(
             self.def_map.root,
-            &[(name, PerNs::macros(macro_, Visibility::Public))],
+            &[(Some(name), PerNs::macros(macro_, Visibility::Public))],
             Visibility::Public,
             ImportType::Named,
         );
@@ -534,7 +534,7 @@ impl DefCollector<'_> {
                             let name = variant_data.name.clone();
                             let variant = EnumVariantId { parent: e, local_id };
                             let res = PerNs::both(variant.into(), variant.into(), vis);
-                            (name, res)
+                            (Some(name), res)
                         })
                         .collect::<Vec<_>>();
                     self.update(module_id, &resolutions, vis, ImportType::Glob);
@@ -550,15 +550,15 @@ impl DefCollector<'_> {
             match import.path.segments.last() {
                 Some(last_segment) => {
                     let name = match &import.alias {
-                        Some(ImportAlias::Alias(name)) => name.clone(),
-                        Some(ImportAlias::Underscore) => last_segment.clone(), // FIXME rust-analyzer#2736
-                        None => last_segment.clone(),
+                        Some(ImportAlias::Alias(name)) => Some(name.clone()),
+                        Some(ImportAlias::Underscore) => None,
+                        None => Some(last_segment.clone()),
                     };
                     log::debug!("resolved import {:?} ({:?}) to {:?}", name, import, def);
 
                     // extern crates in the crate root are special-cased to insert entries into the extern prelude: rust-lang/rust#54658
                     if import.is_extern_crate && module_id == self.def_map.root {
-                        if let Some(def) = def.take_types() {
+                        if let (Some(def), Some(name)) = (def.take_types(), name.as_ref()) {
                             self.def_map.extern_prelude.insert(name.clone(), def);
                         }
                     }
@@ -573,7 +573,7 @@ impl DefCollector<'_> {
     fn update(
         &mut self,
         module_id: LocalModuleId,
-        resolutions: &[(Name, PerNs)],
+        resolutions: &[(Option<Name>, PerNs)],
         vis: Visibility,
         import_type: ImportType,
     ) {
@@ -584,7 +584,7 @@ impl DefCollector<'_> {
     fn update_recursive(
         &mut self,
         module_id: LocalModuleId,
-        resolutions: &[(Name, PerNs)],
+        resolutions: &[(Option<Name>, PerNs)],
         // All resolutions are imported with this visibility; the visibilies in
         // the `PerNs` values are ignored and overwritten
         vis: Visibility,
@@ -595,15 +595,51 @@ impl DefCollector<'_> {
             // prevent stack overflows (but this shouldn't be possible)
             panic!("infinite recursion in glob imports!");
         }
-        let scope = &mut self.def_map.modules[module_id].scope;
         let mut changed = false;
+
         for (name, res) in resolutions {
-            changed |= scope.push_res_with_import(
-                &mut self.from_glob_import,
-                (module_id, name.clone()),
-                res.with_visibility(vis),
-                import_type,
-            );
+            match name {
+                Some(name) => {
+                    let scope = &mut self.def_map.modules[module_id].scope;
+                    changed |= scope.push_res_with_import(
+                        &mut self.from_glob_import,
+                        (module_id, name.clone()),
+                        res.with_visibility(vis),
+                        import_type,
+                    );
+                }
+                None => {
+                    let tr = match res.take_types() {
+                        Some(ModuleDefId::TraitId(tr)) => tr,
+                        Some(other) => {
+                            log::debug!("non-trait `_` import of {:?}", other);
+                            continue;
+                        }
+                        None => continue,
+                    };
+                    let old_vis = self.def_map.modules[module_id].scope.unnamed_trait_vis(tr);
+                    let should_update = match old_vis {
+                        None => true,
+                        Some(old_vis) => {
+                            let max_vis = old_vis.max(vis, &self.def_map).unwrap_or_else(|| {
+                                panic!("`Tr as _` imports with unrelated visibilities {:?} and {:?} (trait {:?})", old_vis, vis, tr);
+                            });
+
+                            if max_vis == old_vis {
+                                false
+                            } else {
+                                mark::hit!(upgrade_underscore_visibility);
+                                true
+                            }
+                        }
+                    };
+
+                    if should_update {
+                        changed = true;
+                        self.def_map.modules[module_id].scope.push_unnamed_trait(tr, vis);
+                    }
+                }
+            }
         }
 
         if !changed {
@@ -950,7 +986,7 @@ impl ModCollector<'_, '_> {
                         .unwrap_or(Visibility::Public);
                     self.def_collector.update(
                         self.module_id,
-                        &[(name.clone(), PerNs::from_def(id, vis, has_constructor))],
+                        &[(Some(name.clone()), PerNs::from_def(id, vis, has_constructor))],
                         vis,
                         ImportType::Named,
                     )
@@ -1057,7 +1093,7 @@ impl ModCollector<'_, '_> {
         self.def_collector.def_map.modules[self.module_id].scope.define_def(def);
         self.def_collector.update(
             self.module_id,
-            &[(name, PerNs::from_def(def, vis, false))],
+            &[(Some(name), PerNs::from_def(def, vis, false))],
             vis,
             ImportType::Named,
         );
diff --git a/crates/ra_hir_def/src/nameres/tests.rs b/crates/ra_hir_def/src/nameres/tests.rs
index 205d3528b..839b1de57 100644
--- a/crates/ra_hir_def/src/nameres/tests.rs
+++ b/crates/ra_hir_def/src/nameres/tests.rs
@@ -558,3 +558,133 @@ mod b {
         "#]],
     );
 }
+
+#[test]
+fn underscore_import() {
+    check(
+        r#"
+//- /main.rs
+use tr::Tr as _;
+use tr::Tr2 as _;
+
+mod tr {
+    pub trait Tr {}
+    pub trait Tr2 {}
+}
+    "#,
+        expect![[r#"
+            crate
+            _: t
+            _: t
+            tr: t
+
+            crate::tr
+            Tr: t
+            Tr2: t
+        "#]],
+    );
+}
+
+#[test]
+fn underscore_reexport() {
+    check(
+        r#"
+//- /main.rs
+mod tr {
+    pub trait PubTr {}
+    pub trait PrivTr {}
+}
+mod reex {
+    use crate::tr::PrivTr as _;
+    pub use crate::tr::PubTr as _;
+}
+use crate::reex::*;
+    "#,
+        expect![[r#"
+            crate
+            _: t
+            reex: t
+            tr: t
+
+            crate::tr
+            PrivTr: t
+            PubTr: t
+
+            crate::reex
+            _: t
+            _: t
+        "#]],
+    );
+}
+
+#[test]
+fn underscore_pub_crate_reexport() {
+    mark::check!(upgrade_underscore_visibility);
+    check(
+        r#"
+//- /main.rs crate:main deps:lib
+use lib::*;
+
+//- /lib.rs crate:lib
+use tr::Tr as _;
+pub use tr::Tr as _;
+
+mod tr {
+    pub trait Tr {}
+}
+    "#,
+        expect![[r#"
+            crate
+            _: t
+        "#]],
+    );
+}
+
+#[test]
+fn underscore_nontrait() {
+    check(
+        r#"
+//- /main.rs
+mod m {
+    pub struct Struct;
+    pub enum Enum {}
+    pub const CONST: () = ();
+}
+use crate::m::{Struct as _, Enum as _, CONST as _};
+    "#,
+        expect![[r#"
+            crate
+            m: t
+
+            crate::m
+            CONST: v
+            Enum: t
+            Struct: t v
+        "#]],
+    );
+}
+
+#[test]
+fn underscore_name_conflict() {
+    check(
+        r#"
+//- /main.rs
+struct Tr;
+
+use tr::Tr as _;
+
+mod tr {
+    pub trait Tr {}
+}
+    "#,
+        expect![[r#"
+            crate
+            _: t
+            Tr: t v
+            tr: t
+
+            crate::tr
+            Tr: t
+        "#]],
+    );
+}
diff --git a/crates/ra_hir_def/src/visibility.rs b/crates/ra_hir_def/src/visibility.rs
index 8136cb50c..1abffb4c3 100644
--- a/crates/ra_hir_def/src/visibility.rs
+++ b/crates/ra_hir_def/src/visibility.rs
@@ -5,6 +5,7 @@ use ra_syntax::ast;
 
 use crate::{
     db::DefDatabase,
+    nameres::CrateDefMap,
     path::{ModPath, PathKind},
     ModuleId,
 };
@@ -115,7 +116,7 @@ impl Visibility {
 
     pub(crate) fn is_visible_from_def_map(
         self,
-        def_map: &crate::nameres::CrateDefMap,
+        def_map: &CrateDefMap,
         from_module: crate::LocalModuleId,
     ) -> bool {
         let to_module = match self {
@@ -129,4 +130,42 @@ impl Visibility {
         });
         ancestors.any(|m| m == to_module.local_id)
     }
+
+    /// Returns the most permissive visibility of `self` and `other`.
+    ///
+    /// If there is no subset relation between `self` and `other`, returns `None` (ie. they're only
+    /// visible in unrelated modules).
+    pub(crate) fn max(self, other: Visibility, def_map: &CrateDefMap) -> Option<Visibility> {
+        match (self, other) {
+            (Visibility::Module(_), Visibility::Public)
+            | (Visibility::Public, Visibility::Module(_))
+            | (Visibility::Public, Visibility::Public) => Some(Visibility::Public),
+            (Visibility::Module(mod_a), Visibility::Module(mod_b)) => {
+                if mod_a.krate != mod_b.krate {
+                    return None;
+                }
+
+                let mut a_ancestors = std::iter::successors(Some(mod_a.local_id), |m| {
+                    let parent_id = def_map[*m].parent?;
+                    Some(parent_id)
+                });
+                let mut b_ancestors = std::iter::successors(Some(mod_b.local_id), |m| {
+                    let parent_id = def_map[*m].parent?;
+                    Some(parent_id)
+                });
+
+                if a_ancestors.any(|m| m == mod_b.local_id) {
+                    // B is above A
+                    return Some(Visibility::Module(mod_b));
+                }
+
+                if b_ancestors.any(|m| m == mod_a.local_id) {
+                    // A is above B
+                    return Some(Visibility::Module(mod_a));
+                }
+
+                None
+            }
+        }
+    }
 }
diff --git a/crates/ra_hir_ty/src/tests/traits.rs b/crates/ra_hir_ty/src/tests/traits.rs
index d3c4d3f2a..526e61caf 100644
--- a/crates/ra_hir_ty/src/tests/traits.rs
+++ b/crates/ra_hir_ty/src/tests/traits.rs
@@ -3089,3 +3089,25 @@ fn test() {
         "#,
     );
 }
+
+#[test]
+fn underscore_import() {
+    check_types(
+        r#"
+mod tr {
+    pub trait Tr {
+        fn method(&self) -> u8 { 0 }
+    }
+}
+
+struct Tr;
+impl crate::tr::Tr for Tr {}
+
+use crate::tr::Tr as _;
+fn test() {
+    Tr.method();
+  //^^^^^^^^^^^ u8
+}
+    "#,
+    );
+}
diff --git a/crates/ra_prof/Cargo.toml b/crates/ra_prof/Cargo.toml
index b3d52985a..6c214501e 100644
--- a/crates/ra_prof/Cargo.toml
+++ b/crates/ra_prof/Cargo.toml
@@ -13,18 +13,12 @@ doctest = false
 ra_arena = { path = "../ra_arena" }
 once_cell = "1.3.1"
 backtrace = { version = "0.3.44", optional = true }
-mimalloc = { version = "0.1.19", default-features = false, optional = true }
-
-[target.'cfg(not(target_env = "msvc"))'.dependencies]
-jemallocator = { version = "0.3.2", optional = true }
-jemalloc-ctl = { version = "0.3.3", optional = true }
+cfg-if = "0.1.10"
+libc = "0.2.73"
 
 [features]
-jemalloc = [ "jemallocator", "jemalloc-ctl" ]
 cpu_profiler = []
 
 # Uncomment to enable for the whole crate graph
 # default = [ "backtrace" ]
-# default = [ "jemalloc" ]
-# default = [ "mimalloc" ]
 # default = [ "cpu_profiler" ]
diff --git a/crates/ra_prof/src/lib.rs b/crates/ra_prof/src/lib.rs
index b54531b4e..ba5609703 100644
--- a/crates/ra_prof/src/lib.rs
+++ b/crates/ra_prof/src/lib.rs
@@ -13,16 +13,6 @@ pub use crate::{
     memory_usage::{Bytes, MemoryUsage},
 };
 
-// We use jemalloc mainly to get heap usage statistics, actual performance
-// difference is not measures.
-#[cfg(all(feature = "jemalloc", not(target_env = "msvc")))]
-#[global_allocator]
-static ALLOC: jemallocator::Jemalloc = jemallocator::Jemalloc;
-
-#[cfg(all(feature = "mimalloc"))]
-#[global_allocator]
-static ALLOC: mimalloc::MiMalloc = mimalloc::MiMalloc;
-
 /// Prints backtrace to stderr, useful for debugging.
 #[cfg(feature = "backtrace")]
 pub fn print_backtrace() {
diff --git a/crates/ra_prof/src/memory_usage.rs b/crates/ra_prof/src/memory_usage.rs
index 9768f656c..ee79ec3ee 100644
--- a/crates/ra_prof/src/memory_usage.rs
+++ b/crates/ra_prof/src/memory_usage.rs
@@ -1,26 +1,25 @@
 //! FIXME: write short doc here
-
 use std::fmt;
 
+use cfg_if::cfg_if;
+
 pub struct MemoryUsage {
     pub allocated: Bytes,
     pub resident: Bytes,
 }
 
 impl MemoryUsage {
-    #[cfg(all(feature = "jemalloc", not(target_env = "msvc")))]
     pub fn current() -> MemoryUsage {
-        jemalloc_ctl::epoch::advance().unwrap();
-        MemoryUsage {
-            allocated: Bytes(jemalloc_ctl::stats::allocated::read().unwrap()),
-            resident: Bytes(jemalloc_ctl::stats::resident::read().unwrap()),
+        cfg_if! {
+            if #[cfg(target_os = "linux")] {
+                // Note: This is incredibly slow.
+                let alloc = unsafe { libc::mallinfo() }.uordblks as u32 as usize;
+                MemoryUsage { allocated: Bytes(alloc), resident: Bytes(0) }
+            } else {
+                MemoryUsage { allocated: Bytes(0), resident: Bytes(0) }
+            }
         }
     }
-
-    #[cfg(any(not(feature = "jemalloc"), target_env = "msvc"))]
-    pub fn current() -> MemoryUsage {
-        MemoryUsage { allocated: Bytes(0), resident: Bytes(0) }
-    }
 }
 
 impl fmt::Display for MemoryUsage {
diff --git a/crates/rust-analyzer/Cargo.toml b/crates/rust-analyzer/Cargo.toml
index 5eb2d0bb7..023c104d1 100644
--- a/crates/rust-analyzer/Cargo.toml
+++ b/crates/rust-analyzer/Cargo.toml
@@ -29,6 +29,7 @@ serde = { version = "1.0.106", features = ["derive"] }
 serde_json = "1.0.48"
 threadpool = "1.7.1"
 rayon = "1.3.1"
+mimalloc = { version = "0.1.19", default-features = false, optional = true }
 
 stdx = { path = "../stdx" }
 
@@ -62,7 +63,3 @@ expect = { path = "../expect" }
 test_utils = { path = "../test_utils" }
 mbe = { path = "../ra_mbe", package = "ra_mbe" }
 tt = { path = "../ra_tt", package = "ra_tt" }
-
-[features]
-jemalloc = [ "ra_prof/jemalloc" ]
-mimalloc = [ "ra_prof/mimalloc" ]
diff --git a/crates/rust-analyzer/src/bin/args.rs b/crates/rust-analyzer/src/bin/args.rs
index 3210416ee..741a2a951 100644
--- a/crates/rust-analyzer/src/bin/args.rs
+++ b/crates/rust-analyzer/src/bin/args.rs
@@ -166,7 +166,7 @@ USAGE:
 FLAGS:
     -o, --only              Only analyze items matching this path
     -h, --help              Prints help information
-        --memory-usage      Collect memory usage statistics (requires `--features jemalloc`)
+        --memory-usage      Collect memory usage statistics
         --randomize         Randomize order in which crates, modules, and items are processed
         --parallel          Run type inference in parallel
         --load-output-dirs  Load OUT_DIR values by running `cargo check` before analysis
@@ -221,7 +221,7 @@ USAGE:
 
 FLAGS:
     -h, --help          Prints help information
-    --memory-usage      Collect memory usage statistics (requires `--features jemalloc`)
+    --memory-usage      Collect memory usage statistics
     --load-output-dirs  Load OUT_DIR values by running `cargo check` before analysis
     --with-proc-macro   Use ra-proc-macro-srv for proc-macro expanding
     -v, --verbose
diff --git a/crates/rust-analyzer/src/bin/main.rs b/crates/rust-analyzer/src/bin/main.rs
index 408892eab..a473c9165 100644
--- a/crates/rust-analyzer/src/bin/main.rs
+++ b/crates/rust-analyzer/src/bin/main.rs
@@ -16,6 +16,10 @@ use vfs::AbsPathBuf;
 
 use crate::args::HelpPrinted;
 
+#[cfg(all(feature = "mimalloc"))]
+#[global_allocator]
+static ALLOC: mimalloc::MiMalloc = mimalloc::MiMalloc;
+
 fn main() -> Result<()> {
     setup_logging()?;
     let args = match args::Args::parse()? {
diff --git a/docs/dev/README.md b/docs/dev/README.md
index 3af01cd6b..2bb2f75bd 100644
--- a/docs/dev/README.md
+++ b/docs/dev/README.md
@@ -397,13 +397,7 @@ To log all communication between the server and the client, there are two choice
 
 There are also two VS Code commands which might be of interest:
 
-* `Rust Analyzer: Status` shows some memory-usage statistics. To take full
-  advantage of it, you need to compile rust-analyzer with jemalloc support:
-  ```
-  $ cargo install --path crates/rust-analyzer --force --features jemalloc
-  ```
-
-  There's an alias for this: `cargo xtask install --server --jemalloc`.
+* `Rust Analyzer: Status` shows some memory-usage statistics.
 
 * `Rust Analyzer: Syntax Tree` shows syntax tree of the current file/selection.
 
diff --git a/xtask/src/dist.rs b/xtask/src/dist.rs
index b8f68027c..c198c0907 100644
--- a/xtask/src/dist.rs
+++ b/xtask/src/dist.rs
@@ -57,8 +57,6 @@ fn dist_server() -> Result<()> {
         env::set_var("CC", "clang");
         run!(
             "cargo build --manifest-path ./crates/rust-analyzer/Cargo.toml --bin rust-analyzer --release"
-            // We'd want to add, but that requires setting the right linker somehow
-            // --features=jemalloc
         )?;
     } else {
         run!("cargo build --manifest-path ./crates/rust-analyzer/Cargo.toml --bin rust-analyzer --release")?;
diff --git a/xtask/src/install.rs b/xtask/src/install.rs
index a0dc0c9c2..b25a6e301 100644
--- a/xtask/src/install.rs
+++ b/xtask/src/install.rs
@@ -24,7 +24,6 @@ pub struct ServerOpt {
 
 pub enum Malloc {
     System,
-    Jemalloc,
     Mimalloc,
 }
 
@@ -138,7 +137,6 @@ fn install_server(opts: ServerOpt) -> Result<()> {
 
     let malloc_feature = match opts.malloc {
         Malloc::System => "",
-        Malloc::Jemalloc => "--features jemalloc",
         Malloc::Mimalloc => "--features mimalloc",
     };
     let res = run!("cargo install --path crates/rust-analyzer --locked --force {}", malloc_feature);
diff --git a/xtask/src/main.rs b/xtask/src/main.rs
index 399ff7204..53d3ce3e7 100644
--- a/xtask/src/main.rs
+++ b/xtask/src/main.rs
@@ -45,7 +45,6 @@ USAGE:
 FLAGS:
         --client-code    Install only VS Code plugin
         --server         Install only the language server
-        --jemalloc       Use jemalloc for server
         --mimalloc       Use mimalloc for server
     -h, --help           Prints help information
         "
@@ -62,15 +61,8 @@ FLAGS:
                 return Ok(());
             }
 
-            let malloc = match (args.contains("--jemalloc"), args.contains("--mimalloc")) {
-                (false, false) => Malloc::System,
-                (true, false) => Malloc::Jemalloc,
-                (false, true) => Malloc::Mimalloc,
-                (true, true) => {
-                    eprintln!("error: Cannot use both `--jemalloc` and `--mimalloc`");
-                    return Ok(());
-                }
-            };
+            let malloc =
+                if args.contains("--mimalloc") { Malloc::Mimalloc } else { Malloc::System };
 
             args.finish()?;