4 files changed, 403 insertions, 4 deletions

diff --git a/crates/ide_db/src/helpers/famous_defs_fixture.rs b/crates/ide_db/src/helpers/famous_defs_fixture.rs
new file mode 100644
index 000000000..5e88de64d
--- /dev/null
+++ b/crates/ide_db/src/helpers/famous_defs_fixture.rs
@@ -0,0 +1,120 @@
+//- /libcore.rs crate:core
+//! Signatures of traits, types and functions from the core lib for use in tests.
+pub mod convert {
+    pub trait From<T> {
+        fn from(t: T) -> Self;
+    }
+}
+
+pub mod default {
+    pub trait Default {
+        fn default() -> Self;
+    }
+}
+
+pub mod iter {
+    pub use self::traits::{collect::IntoIterator, iterator::Iterator};
+    mod traits {
+        pub(crate) mod iterator {
+            use crate::option::Option;
+            pub trait Iterator {
+                type Item;
+                fn next(&mut self) -> Option<Self::Item>;
+                fn by_ref(&mut self) -> &mut Self {
+                    self
+                }
+                fn take(self, n: usize) -> crate::iter::Take<Self> {
+                    crate::iter::Take { inner: self }
+                }
+            }
+
+            impl<I: Iterator> Iterator for &mut I {
+                type Item = I::Item;
+                fn next(&mut self) -> Option<I::Item> {
+                    (**self).next()
+                }
+            }
+        }
+        pub(crate) mod collect {
+            pub trait IntoIterator {
+                type Item;
+            }
+        }
+    }
+
+    pub use self::sources::*;
+    pub(crate) mod sources {
+        use super::Iterator;
+        use crate::option::Option::{self, *};
+        pub struct Repeat<A> {
+            element: A,
+        }
+
+        pub fn repeat<T>(elt: T) -> Repeat<T> {
+            Repeat { element: elt }
+        }
+
+        impl<A> Iterator for Repeat<A> {
+            type Item = A;
+
+            fn next(&mut self) -> Option<A> {
+                None
+            }
+        }
+    }
+
+    pub use self::adapters::*;
+    pub(crate) mod adapters {
+        use super::Iterator;
+        use crate::option::Option::{self, *};
+        pub struct Take<I> {
+            pub(crate) inner: I,
+        }
+        impl<I> Iterator for Take<I>
+        where
+            I: Iterator,
+        {
+            type Item = <I as Iterator>::Item;
+            fn next(&mut self) -> Option<<I as Iterator>::Item> {
+                None
+            }
+        }
+    }
+}
+
+pub mod ops {
+    #[lang = "fn"]
+    pub trait Fn<Args>: FnMut<Args> {
+        extern "rust-call" fn call(&self, args: Args) -> Self::Output;
+    }
+
+    #[lang = "fn_mut"]
+    pub trait FnMut<Args>: FnOnce<Args> {
+        extern "rust-call" fn call_mut(&mut self, args: Args) -> Self::Output;
+    }
+    #[lang = "fn_once"]
+    pub trait FnOnce<Args> {
+        #[lang = "fn_once_output"]
+        type Output;
+        extern "rust-call" fn call_once(self, args: Args) -> Self::Output;
+    }
+}
+
+pub mod option {
+    pub enum Option<T> {
+        None,
+        Some(T),
+    }
+}
+
+pub mod prelude {
+    pub use crate::{
+        convert::From,
+        default::Default,
+        iter::{IntoIterator, Iterator},
+        ops::{Fn, FnMut, FnOnce},
+        option::Option::{self, *},
+    };
+}
+#[prelude_import]
+pub use prelude::*;
diff --git a/crates/ide_db/src/helpers/import_assets.rs b/crates/ide_db/src/helpers/import_assets.rs
new file mode 100644
index 000000000..edc3da318
--- /dev/null
+++ b/crates/ide_db/src/helpers/import_assets.rs
@@ -0,0 +1,267 @@
+//! Look up accessible paths for items.
+use either::Either;
+use hir::{AsAssocItem, AssocItemContainer, ModuleDef, Semantics};
+use rustc_hash::FxHashSet;
+use syntax::{ast, AstNode, SyntaxNode};
+
+use crate::{imports_locator, RootDatabase};
+
+use super::insert_use::InsertUseConfig;
+
+#[derive(Debug)]
+pub enum ImportCandidate {
+    // A path, qualified (`std::collections::HashMap`) or not (`HashMap`).
+    Path(PathImportCandidate),
+    /// A trait associated function (with no self parameter) or associated constant.
+    /// For 'test_mod::TestEnum::test_function', `ty` is the `test_mod::TestEnum` expression type
+    /// and `name` is the `test_function`
+    TraitAssocItem(TraitImportCandidate),
+    /// A trait method with self parameter.
+    /// For 'test_enum.test_method()', `ty` is the `test_enum` expression type
+    /// and `name` is the `test_method`
+    TraitMethod(TraitImportCandidate),
+}
+
+#[derive(Debug)]
+pub struct TraitImportCandidate {
+    pub ty: hir::Type,
+    pub name: ast::NameRef,
+}
+
+#[derive(Debug)]
+pub struct PathImportCandidate {
+    pub qualifier: Option<ast::Path>,
+    pub name: ast::NameRef,
+}
+
+#[derive(Debug)]
+pub struct ImportAssets {
+    import_candidate: ImportCandidate,
+    module_with_name_to_import: hir::Module,
+    syntax_under_caret: SyntaxNode,
+}
+
+impl ImportAssets {
+    pub fn for_method_call(
+        method_call: ast::MethodCallExpr,
+        sema: &Semantics<RootDatabase>,
+    ) -> Option<Self> {
+        let syntax_under_caret = method_call.syntax().to_owned();
+        let module_with_name_to_import = sema.scope(&syntax_under_caret).module()?;
+        Some(Self {
+            import_candidate: ImportCandidate::for_method_call(sema, &method_call)?,
+            module_with_name_to_import,
+            syntax_under_caret,
+        })
+    }
+
+    pub fn for_regular_path(
+        path_under_caret: ast::Path,
+        sema: &Semantics<RootDatabase>,
+    ) -> Option<Self> {
+        let syntax_under_caret = path_under_caret.syntax().to_owned();
+        if syntax_under_caret.ancestors().find_map(ast::Use::cast).is_some() {
+            return None;
+        }
+
+        let module_with_name_to_import = sema.scope(&syntax_under_caret).module()?;
+        Some(Self {
+            import_candidate: ImportCandidate::for_regular_path(sema, &path_under_caret)?,
+            module_with_name_to_import,
+            syntax_under_caret,
+        })
+    }
+
+    pub fn syntax_under_caret(&self) -> &SyntaxNode {
+        &self.syntax_under_caret
+    }
+
+    pub fn import_candidate(&self) -> &ImportCandidate {
+        &self.import_candidate
+    }
+
+    fn get_search_query(&self) -> &str {
+        match &self.import_candidate {
+            ImportCandidate::Path(candidate) => candidate.name.text(),
+            ImportCandidate::TraitAssocItem(candidate)
+            | ImportCandidate::TraitMethod(candidate) => candidate.name.text(),
+        }
+    }
+
+    pub fn search_for_imports(
+        &self,
+        sema: &Semantics<RootDatabase>,
+        config: &InsertUseConfig,
+    ) -> Vec<(hir::ModPath, hir::ItemInNs)> {
+        let _p = profile::span("import_assists::search_for_imports");
+        self.search_for(sema, Some(config.prefix_kind))
+    }
+
+    /// This may return non-absolute paths if a part of the returned path is already imported into scope.
+    #[allow(dead_code)]
+    pub fn search_for_relative_paths(
+        &self,
+        sema: &Semantics<RootDatabase>,
+    ) -> Vec<(hir::ModPath, hir::ItemInNs)> {
+        let _p = profile::span("import_assists::search_for_relative_paths");
+        self.search_for(sema, None)
+    }
+
+    fn search_for(
+        &self,
+        sema: &Semantics<RootDatabase>,
+        prefixed: Option<hir::PrefixKind>,
+    ) -> Vec<(hir::ModPath, hir::ItemInNs)> {
+        let db = sema.db;
+        let mut trait_candidates = FxHashSet::default();
+        let current_crate = self.module_with_name_to_import.krate();
+
+        let filter = |candidate: Either<hir::ModuleDef, hir::MacroDef>| {
+            trait_candidates.clear();
+            match &self.import_candidate {
+                ImportCandidate::TraitAssocItem(trait_candidate) => {
+                    let located_assoc_item = match candidate {
+                        Either::Left(ModuleDef::Function(located_function)) => {
+                            located_function.as_assoc_item(db)
+                        }
+                        Either::Left(ModuleDef::Const(located_const)) => {
+                            located_const.as_assoc_item(db)
+                        }
+                        _ => None,
+                    }
+                    .map(|assoc| assoc.container(db))
+                    .and_then(Self::assoc_to_trait)?;
+
+                    trait_candidates.insert(located_assoc_item.into());
+
+                    trait_candidate
+                        .ty
+                        .iterate_path_candidates(
+                            db,
+                            current_crate,
+                            &trait_candidates,
+                            None,
+                            |_, assoc| Self::assoc_to_trait(assoc.container(db)),
+                        )
+                        .map(ModuleDef::from)
+                        .map(Either::Left)
+                }
+                ImportCandidate::TraitMethod(trait_candidate) => {
+                    let located_assoc_item =
+                        if let Either::Left(ModuleDef::Function(located_function)) = candidate {
+                            located_function
+                                .as_assoc_item(db)
+                                .map(|assoc| assoc.container(db))
+                                .and_then(Self::assoc_to_trait)
+                        } else {
+                            None
+                        }?;
+
+                    trait_candidates.insert(located_assoc_item.into());
+
+                    trait_candidate
+                        .ty
+                        .iterate_method_candidates(
+                            db,
+                            current_crate,
+                            &trait_candidates,
+                            None,
+                            |_, function| {
+                                Self::assoc_to_trait(function.as_assoc_item(db)?.container(db))
+                            },
+                        )
+                        .map(ModuleDef::from)
+                        .map(Either::Left)
+                }
+                _ => Some(candidate),
+            }
+        };
+
+        let mut res = imports_locator::find_exact_imports(
+            sema,
+            current_crate,
+            self.get_search_query().to_string(),
+        )
+        .filter_map(filter)
+        .filter_map(|candidate| {
+            let item: hir::ItemInNs = candidate.either(Into::into, Into::into);
+            if let Some(prefix_kind) = prefixed {
+                self.module_with_name_to_import.find_use_path_prefixed(db, item, prefix_kind)
+            } else {
+                self.module_with_name_to_import.find_use_path(db, item)
+            }
+            .map(|path| (path, item))
+        })
+        .filter(|(use_path, _)| use_path.len() > 1)
+        .take(20)
+        .collect::<Vec<_>>();
+        res.sort_by_key(|(path, _)| path.clone());
+        res
+    }
+
+    fn assoc_to_trait(assoc: AssocItemContainer) -> Option<hir::Trait> {
+        if let AssocItemContainer::Trait(extracted_trait) = assoc {
+            Some(extracted_trait)
+        } else {
+            None
+        }
+    }
+}
+
+impl ImportCandidate {
+    fn for_method_call(
+        sema: &Semantics<RootDatabase>,
+        method_call: &ast::MethodCallExpr,
+    ) -> Option<Self> {
+        match sema.resolve_method_call(method_call) {
+            Some(_) => None,
+            None => Some(Self::TraitMethod(TraitImportCandidate {
+                ty: sema.type_of_expr(&method_call.receiver()?)?,
+                name: method_call.name_ref()?,
+            })),
+        }
+    }
+
+    fn for_regular_path(
+        sema: &Semantics<RootDatabase>,
+        path_under_caret: &ast::Path,
+    ) -> Option<Self> {
+        if sema.resolve_path(path_under_caret).is_some() {
+            return None;
+        }
+
+        let segment = path_under_caret.segment()?;
+        let candidate = if let Some(qualifier) = path_under_caret.qualifier() {
+            let qualifier_start = qualifier.syntax().descendants().find_map(ast::NameRef::cast)?;
+            let qualifier_start_path =
+                qualifier_start.syntax().ancestors().find_map(ast::Path::cast)?;
+            if let Some(qualifier_start_resolution) = sema.resolve_path(&qualifier_start_path) {
+                let qualifier_resolution = if qualifier_start_path == qualifier {
+                    qualifier_start_resolution
+                } else {
+                    sema.resolve_path(&qualifier)?
+                };
+                match qualifier_resolution {
+                    hir::PathResolution::Def(hir::ModuleDef::Adt(assoc_item_path)) => {
+                        ImportCandidate::TraitAssocItem(TraitImportCandidate {
+                            ty: assoc_item_path.ty(sema.db),
+                            name: segment.name_ref()?,
+                        })
+                    }
+                    _ => return None,
+                }
+            } else {
+                ImportCandidate::Path(PathImportCandidate {
+                    qualifier: Some(qualifier),
+                    name: qualifier_start,
+                })
+            }
+        } else {
+            ImportCandidate::Path(PathImportCandidate {
+                qualifier: None,
+                name: segment.syntax().descendants().find_map(ast::NameRef::cast)?,
+            })
+        };
+        Some(candidate)
+    }
+}
diff --git a/crates/ide_db/src/helpers/insert_use.rs b/crates/ide_db/src/helpers/insert_use.rs
index d6b498be3..877d4f1c7 100644
--- a/crates/ide_db/src/helpers/insert_use.rs
+++ b/crates/ide_db/src/helpers/insert_use.rs
@@ -15,6 +15,12 @@ use syntax::{
 };
 use test_utils::mark;
 
+#[derive(Clone, Copy, Debug, PartialEq, Eq)]
+pub struct InsertUseConfig {
+    pub merge: Option<MergeBehavior>,
+    pub prefix_kind: hir::PrefixKind,
+}
+
 #[derive(Debug, Clone)]
 pub enum ImportScope {
     File(ast::SourceFile),
@@ -97,7 +103,7 @@ pub fn insert_use<'a>(
 ) -> SyntaxRewriter<'a> {
     let _p = profile::span("insert_use");
     let mut rewriter = SyntaxRewriter::default();
-    let use_item = make::use_(make::use_tree(path.clone(), None, None, false));
+    let use_item = make::use_(None, make::use_tree(path.clone(), None, None, false));
     // merge into existing imports if possible
     if let Some(mb) = merge {
         for existing_use in scope.as_syntax_node().children().filter_map(ast::Use::cast) {
@@ -444,8 +450,14 @@ fn use_tree_path_cmp(a: &ast::Path, a_has_tl: bool, b: &ast::Path, b_has_tl: boo
 }
 
 fn path_segment_cmp(a: &ast::PathSegment, b: &ast::PathSegment) -> Ordering {
-    let a = a.name_ref();
-    let b = b.name_ref();
+    let a = a.kind().and_then(|kind| match kind {
+        PathSegmentKind::Name(name_ref) => Some(name_ref),
+        _ => None,
+    });
+    let b = b.kind().and_then(|kind| match kind {
+        PathSegmentKind::Name(name_ref) => Some(name_ref),
+        _ => None,
+    });
     a.as_ref().map(ast::NameRef::text).cmp(&b.as_ref().map(ast::NameRef::text))
 }
 
diff --git a/crates/ide_db/src/helpers/insert_use/tests.rs b/crates/ide_db/src/helpers/insert_use/tests.rs
index a603fe87f..4bbe66f1f 100644
--- a/crates/ide_db/src/helpers/insert_use/tests.rs
+++ b/crates/ide_db/src/helpers/insert_use/tests.rs
@@ -599,7 +599,7 @@ fn check(
 
     let rewriter = insert_use(&file, path, mb);
     let result = rewriter.rewrite(file.as_syntax_node()).to_string();
-    assert_eq_text!(&result, ra_fixture_after);
+    assert_eq_text!(ra_fixture_after, &result);
 }
 
 fn check_full(path: &str, ra_fixture_before: &str, ra_fixture_after: &str) {