1 files changed, 687 insertions, 0 deletions

diff --git a/crates/hir_def/src/find_path.rs b/crates/hir_def/src/find_path.rs
new file mode 100644
index 000000000..ac2c54ac5
--- /dev/null
+++ b/crates/hir_def/src/find_path.rs
@@ -0,0 +1,687 @@
+//! An algorithm to find a path to refer to a certain item.
+
+use hir_expand::name::{known, AsName, Name};
+use rustc_hash::FxHashSet;
+use test_utils::mark;
+
+use crate::{
+    db::DefDatabase,
+    item_scope::ItemInNs,
+    path::{ModPath, PathKind},
+    visibility::Visibility,
+    ModuleDefId, ModuleId,
+};
+
+// FIXME: handle local items
+
+/// Find a path that can be used to refer to a certain item. This can depend on
+/// *from where* you're referring to the item, hence the `from` parameter.
+pub fn find_path(db: &dyn DefDatabase, item: ItemInNs, from: ModuleId) -> Option<ModPath> {
+    let _p = profile::span("find_path");
+    find_path_inner(db, item, from, MAX_PATH_LEN)
+}
+
+const MAX_PATH_LEN: usize = 15;
+
+impl ModPath {
+    fn starts_with_std(&self) -> bool {
+        self.segments.first() == Some(&known::std)
+    }
+
+    // When std library is present, paths starting with `std::`
+    // should be preferred over paths starting with `core::` and `alloc::`
+    fn can_start_with_std(&self) -> bool {
+        let first_segment = self.segments.first();
+        first_segment == Some(&known::alloc) || first_segment == Some(&known::core)
+    }
+}
+
+fn find_path_inner(
+    db: &dyn DefDatabase,
+    item: ItemInNs,
+    from: ModuleId,
+    max_len: usize,
+) -> Option<ModPath> {
+    if max_len == 0 {
+        return None;
+    }
+
+    // Base cases:
+
+    // - if the item is already in scope, return the name under which it is
+    let def_map = db.crate_def_map(from.krate);
+    let from_scope: &crate::item_scope::ItemScope = &def_map.modules[from.local_id].scope;
+    if let Some((name, _)) = from_scope.name_of(item) {
+        return Some(ModPath::from_segments(PathKind::Plain, vec![name.clone()]));
+    }
+
+    // - if the item is the crate root, return `crate`
+    if item
+        == ItemInNs::Types(ModuleDefId::ModuleId(ModuleId {
+            krate: from.krate,
+            local_id: def_map.root,
+        }))
+    {
+        return Some(ModPath::from_segments(PathKind::Crate, Vec::new()));
+    }
+
+    // - if the item is the module we're in, use `self`
+    if item == ItemInNs::Types(from.into()) {
+        return Some(ModPath::from_segments(PathKind::Super(0), Vec::new()));
+    }
+
+    // - if the item is the parent module, use `super` (this is not used recursively, since `super::super` is ugly)
+    if let Some(parent_id) = def_map.modules[from.local_id].parent {
+        if item
+            == ItemInNs::Types(ModuleDefId::ModuleId(ModuleId {
+                krate: from.krate,
+                local_id: parent_id,
+            }))
+        {
+            return Some(ModPath::from_segments(PathKind::Super(1), Vec::new()));
+        }
+    }
+
+    // - if the item is the crate root of a dependency crate, return the name from the extern prelude
+    for (name, def_id) in &def_map.extern_prelude {
+        if item == ItemInNs::Types(*def_id) {
+            return Some(ModPath::from_segments(PathKind::Plain, vec![name.clone()]));
+        }
+    }
+
+    // - if the item is in the prelude, return the name from there
+    if let Some(prelude_module) = def_map.prelude {
+        let prelude_def_map = db.crate_def_map(prelude_module.krate);
+        let prelude_scope: &crate::item_scope::ItemScope =
+            &prelude_def_map.modules[prelude_module.local_id].scope;
+        if let Some((name, vis)) = prelude_scope.name_of(item) {
+            if vis.is_visible_from(db, from) {
+                return Some(ModPath::from_segments(PathKind::Plain, vec![name.clone()]));
+            }
+        }
+    }
+
+    // - if the item is a builtin, it's in scope
+    if let ItemInNs::Types(ModuleDefId::BuiltinType(builtin)) = item {
+        return Some(ModPath::from_segments(PathKind::Plain, vec![builtin.as_name()]));
+    }
+
+    // Recursive case:
+    // - if the item is an enum variant, refer to it via the enum
+    if let Some(ModuleDefId::EnumVariantId(variant)) = item.as_module_def_id() {
+        if let Some(mut path) = find_path(db, ItemInNs::Types(variant.parent.into()), from) {
+            let data = db.enum_data(variant.parent);
+            path.segments.push(data.variants[variant.local_id].name.clone());
+            return Some(path);
+        }
+        // If this doesn't work, it seems we have no way of referring to the
+        // enum; that's very weird, but there might still be a reexport of the
+        // variant somewhere
+    }
+
+    // - otherwise, look for modules containing (reexporting) it and import it from one of those
+
+    let crate_root = ModuleId { local_id: def_map.root, krate: from.krate };
+    let crate_attrs = db.attrs(crate_root.into());
+    let prefer_no_std = crate_attrs.by_key("no_std").exists();
+    let mut best_path = None;
+    let mut best_path_len = max_len;
+
+    if item.krate(db) == Some(from.krate) {
+        // Item was defined in the same crate that wants to import it. It cannot be found in any
+        // dependency in this case.
+
+        let local_imports = find_local_import_locations(db, item, from);
+        for (module_id, name) in local_imports {
+            if let Some(mut path) = find_path_inner(
+                db,
+                ItemInNs::Types(ModuleDefId::ModuleId(module_id)),
+                from,
+                best_path_len - 1,
+            ) {
+                path.segments.push(name);
+
+                let new_path = if let Some(best_path) = best_path {
+                    select_best_path(best_path, path, prefer_no_std)
+                } else {
+                    path
+                };
+                best_path_len = new_path.len();
+                best_path = Some(new_path);
+            }
+        }
+    } else {
+        // Item was defined in some upstream crate. This means that it must be exported from one,
+        // too (unless we can't name it at all). It could *also* be (re)exported by the same crate
+        // that wants to import it here, but we always prefer to use the external path here.
+
+        let crate_graph = db.crate_graph();
+        let extern_paths = crate_graph[from.krate].dependencies.iter().filter_map(|dep| {
+            let import_map = db.import_map(dep.crate_id);
+            import_map.import_info_for(item).and_then(|info| {
+                // Determine best path for containing module and append last segment from `info`.
+                let mut path = find_path_inner(
+                    db,
+                    ItemInNs::Types(ModuleDefId::ModuleId(info.container)),
+                    from,
+                    best_path_len - 1,
+                )?;
+                path.segments.push(info.path.segments.last().unwrap().clone());
+                Some(path)
+            })
+        });
+
+        for path in extern_paths {
+            let new_path = if let Some(best_path) = best_path {
+                select_best_path(best_path, path, prefer_no_std)
+            } else {
+                path
+            };
+            best_path = Some(new_path);
+        }
+    }
+
+    best_path
+}
+
+fn select_best_path(old_path: ModPath, new_path: ModPath, prefer_no_std: bool) -> ModPath {
+    if old_path.starts_with_std() && new_path.can_start_with_std() {
+        if prefer_no_std {
+            mark::hit!(prefer_no_std_paths);
+            new_path
+        } else {
+            mark::hit!(prefer_std_paths);
+            old_path
+        }
+    } else if new_path.starts_with_std() && old_path.can_start_with_std() {
+        if prefer_no_std {
+            mark::hit!(prefer_no_std_paths);
+            old_path
+        } else {
+            mark::hit!(prefer_std_paths);
+            new_path
+        }
+    } else if new_path.len() < old_path.len() {
+        new_path
+    } else {
+        old_path
+    }
+}
+
+/// Finds locations in `from.krate` from which `item` can be imported by `from`.
+fn find_local_import_locations(
+    db: &dyn DefDatabase,
+    item: ItemInNs,
+    from: ModuleId,
+) -> Vec<(ModuleId, Name)> {
+    let _p = profile::span("find_local_import_locations");
+
+    // `from` can import anything below `from` with visibility of at least `from`, and anything
+    // above `from` with any visibility. That means we do not need to descend into private siblings
+    // of `from` (and similar).
+
+    let def_map = db.crate_def_map(from.krate);
+
+    // Compute the initial worklist. We start with all direct child modules of `from` as well as all
+    // of its (recursive) parent modules.
+    let data = &def_map.modules[from.local_id];
+    let mut worklist = data
+        .children
+        .values()
+        .map(|child| ModuleId { krate: from.krate, local_id: *child })
+        .collect::<Vec<_>>();
+    let mut parent = data.parent;
+    while let Some(p) = parent {
+        worklist.push(ModuleId { krate: from.krate, local_id: p });
+        parent = def_map.modules[p].parent;
+    }
+
+    let mut seen: FxHashSet<_> = FxHashSet::default();
+
+    let mut locations = Vec::new();
+    while let Some(module) = worklist.pop() {
+        if !seen.insert(module) {
+            continue; // already processed this module
+        }
+
+        let ext_def_map;
+        let data = if module.krate == from.krate {
+            &def_map[module.local_id]
+        } else {
+            // The crate might reexport a module defined in another crate.
+            ext_def_map = db.crate_def_map(module.krate);
+            &ext_def_map[module.local_id]
+        };
+
+        if let Some((name, vis)) = data.scope.name_of(item) {
+            if vis.is_visible_from(db, from) {
+                let is_private = if let Visibility::Module(private_to) = vis {
+                    private_to.local_id == module.local_id
+                } else {
+                    false
+                };
+                let is_original_def = if let Some(module_def_id) = item.as_module_def_id() {
+                    data.scope.declarations().any(|it| it == module_def_id)
+                } else {
+                    false
+                };
+
+                // Ignore private imports. these could be used if we are
+                // in a submodule of this module, but that's usually not
+                // what the user wants; and if this module can import
+                // the item and we're a submodule of it, so can we.
+                // Also this keeps the cached data smaller.
+                if !is_private || is_original_def {
+                    locations.push((module, name.clone()));
+                }
+            }
+        }
+
+        // Descend into all modules visible from `from`.
+        for (_, per_ns) in data.scope.entries() {
+            if let Some((ModuleDefId::ModuleId(module), vis)) = per_ns.take_types_vis() {
+                if vis.is_visible_from(db, from) {
+                    worklist.push(module);
+                }
+            }
+        }
+    }
+
+    locations
+}
+
+#[cfg(test)]
+mod tests {
+    use base_db::fixture::WithFixture;
+    use hir_expand::hygiene::Hygiene;
+    use syntax::ast::AstNode;
+    use test_utils::mark;
+
+    use crate::test_db::TestDB;
+
+    use super::*;
+
+    /// `code` needs to contain a cursor marker; checks that `find_path` for the
+    /// item the `path` refers to returns that same path when called from the
+    /// module the cursor is in.
+    fn check_found_path(ra_fixture: &str, path: &str) {
+        let (db, pos) = TestDB::with_position(ra_fixture);
+        let module = db.module_for_file(pos.file_id);
+        let parsed_path_file = syntax::SourceFile::parse(&format!("use {};", path));
+        let ast_path =
+            parsed_path_file.syntax_node().descendants().find_map(syntax::ast::Path::cast).unwrap();
+        let mod_path = ModPath::from_src(ast_path, &Hygiene::new_unhygienic()).unwrap();
+
+        let crate_def_map = db.crate_def_map(module.krate);
+        let resolved = crate_def_map
+            .resolve_path(
+                &db,
+                module.local_id,
+                &mod_path,
+                crate::item_scope::BuiltinShadowMode::Module,
+            )
+            .0
+            .take_types()
+            .unwrap();
+
+        let found_path = find_path(&db, ItemInNs::Types(resolved), module);
+
+        assert_eq!(found_path, Some(mod_path));
+    }
+
+    #[test]
+    fn same_module() {
+        let code = r#"
+            //- /main.rs
+            struct S;
+            <|>
+        "#;
+        check_found_path(code, "S");
+    }
+
+    #[test]
+    fn enum_variant() {
+        let code = r#"
+            //- /main.rs
+            enum E { A }
+            <|>
+        "#;
+        check_found_path(code, "E::A");
+    }
+
+    #[test]
+    fn sub_module() {
+        let code = r#"
+            //- /main.rs
+            mod foo {
+                pub struct S;
+            }
+            <|>
+        "#;
+        check_found_path(code, "foo::S");
+    }
+
+    #[test]
+    fn super_module() {
+        let code = r#"
+            //- /main.rs
+            mod foo;
+            //- /foo.rs
+            mod bar;
+            struct S;
+            //- /foo/bar.rs
+            <|>
+        "#;
+        check_found_path(code, "super::S");
+    }
+
+    #[test]
+    fn self_module() {
+        let code = r#"
+            //- /main.rs
+            mod foo;
+            //- /foo.rs
+            <|>
+        "#;
+        check_found_path(code, "self");
+    }
+
+    #[test]
+    fn crate_root() {
+        let code = r#"
+            //- /main.rs
+            mod foo;
+            //- /foo.rs
+            <|>
+        "#;
+        check_found_path(code, "crate");
+    }
+
+    #[test]
+    fn same_crate() {
+        let code = r#"
+            //- /main.rs
+            mod foo;
+            struct S;
+            //- /foo.rs
+            <|>
+        "#;
+        check_found_path(code, "crate::S");
+    }
+
+    #[test]
+    fn different_crate() {
+        let code = r#"
+            //- /main.rs crate:main deps:std
+            <|>
+            //- /std.rs crate:std
+            pub struct S;
+        "#;
+        check_found_path(code, "std::S");
+    }
+
+    #[test]
+    fn different_crate_renamed() {
+        let code = r#"
+            //- /main.rs crate:main deps:std
+            extern crate std as std_renamed;
+            <|>
+            //- /std.rs crate:std
+            pub struct S;
+        "#;
+        check_found_path(code, "std_renamed::S");
+    }
+
+    #[test]
+    fn partially_imported() {
+        // Tests that short paths are used even for external items, when parts of the path are
+        // already in scope.
+        check_found_path(
+            r#"
+            //- /main.rs crate:main deps:syntax
+
+            use syntax::ast;
+            <|>
+
+            //- /lib.rs crate:syntax
+            pub mod ast {
+                pub enum ModuleItem {
+                    A, B, C,
+                }
+            }
+        "#,
+            "ast::ModuleItem",
+        );
+
+        check_found_path(
+            r#"
+            //- /main.rs crate:main deps:syntax
+
+            <|>
+
+            //- /lib.rs crate:syntax
+            pub mod ast {
+                pub enum ModuleItem {
+                    A, B, C,
+                }
+            }
+        "#,
+            "syntax::ast::ModuleItem",
+        );
+    }
+
+    #[test]
+    fn same_crate_reexport() {
+        let code = r#"
+            //- /main.rs
+            mod bar {
+                mod foo { pub(super) struct S; }
+                pub(crate) use foo::*;
+            }
+            <|>
+        "#;
+        check_found_path(code, "bar::S");
+    }
+
+    #[test]
+    fn same_crate_reexport_rename() {
+        let code = r#"
+            //- /main.rs
+            mod bar {
+                mod foo { pub(super) struct S; }
+                pub(crate) use foo::S as U;
+            }
+            <|>
+        "#;
+        check_found_path(code, "bar::U");
+    }
+
+    #[test]
+    fn different_crate_reexport() {
+        let code = r#"
+            //- /main.rs crate:main deps:std
+            <|>
+            //- /std.rs crate:std deps:core
+            pub use core::S;
+            //- /core.rs crate:core
+            pub struct S;
+        "#;
+        check_found_path(code, "std::S");
+    }
+
+    #[test]
+    fn prelude() {
+        let code = r#"
+            //- /main.rs crate:main deps:std
+            <|>
+            //- /std.rs crate:std
+            pub mod prelude { pub struct S; }
+            #[prelude_import]
+            pub use prelude::*;
+        "#;
+        check_found_path(code, "S");
+    }
+
+    #[test]
+    fn enum_variant_from_prelude() {
+        let code = r#"
+            //- /main.rs crate:main deps:std
+            <|>
+            //- /std.rs crate:std
+            pub mod prelude {
+                pub enum Option<T> { Some(T), None }
+                pub use Option::*;
+            }
+            #[prelude_import]
+            pub use prelude::*;
+        "#;
+        check_found_path(code, "None");
+        check_found_path(code, "Some");
+    }
+
+    #[test]
+    fn shortest_path() {
+        let code = r#"
+            //- /main.rs
+            pub mod foo;
+            pub mod baz;
+            struct S;
+            <|>
+            //- /foo.rs
+            pub mod bar { pub struct S; }
+            //- /baz.rs
+            pub use crate::foo::bar::S;
+        "#;
+        check_found_path(code, "baz::S");
+    }
+
+    #[test]
+    fn discount_private_imports() {
+        let code = r#"
+            //- /main.rs
+            mod foo;
+            pub mod bar { pub struct S; }
+            use bar::S;
+            //- /foo.rs
+            <|>
+        "#;
+        // crate::S would be shorter, but using private imports seems wrong
+        check_found_path(code, "crate::bar::S");
+    }
+
+    #[test]
+    fn import_cycle() {
+        let code = r#"
+            //- /main.rs
+            pub mod foo;
+            pub mod bar;
+            pub mod baz;
+            //- /bar.rs
+            <|>
+            //- /foo.rs
+            pub use super::baz;
+            pub struct S;
+            //- /baz.rs
+            pub use super::foo;
+        "#;
+        check_found_path(code, "crate::foo::S");
+    }
+
+    #[test]
+    fn prefer_std_paths_over_alloc() {
+        mark::check!(prefer_std_paths);
+        let code = r#"
+        //- /main.rs crate:main deps:alloc,std
+        <|>
+
+        //- /std.rs crate:std deps:alloc
+        pub mod sync {
+            pub use alloc::sync::Arc;
+        }
+
+        //- /zzz.rs crate:alloc
+        pub mod sync {
+            pub struct Arc;
+        }
+        "#;
+        check_found_path(code, "std::sync::Arc");
+    }
+
+    #[test]
+    fn prefer_core_paths_over_std() {
+        mark::check!(prefer_no_std_paths);
+        let code = r#"
+        //- /main.rs crate:main deps:core,std
+        #![no_std]
+
+        <|>
+
+        //- /std.rs crate:std deps:core
+
+        pub mod fmt {
+            pub use core::fmt::Error;
+        }
+
+        //- /zzz.rs crate:core
+
+        pub mod fmt {
+            pub struct Error;
+        }
+        "#;
+        check_found_path(code, "core::fmt::Error");
+    }
+
+    #[test]
+    fn prefer_alloc_paths_over_std() {
+        let code = r#"
+        //- /main.rs crate:main deps:alloc,std
+        #![no_std]
+
+        <|>
+
+        //- /std.rs crate:std deps:alloc
+
+        pub mod sync {
+            pub use alloc::sync::Arc;
+        }
+
+        //- /zzz.rs crate:alloc
+
+        pub mod sync {
+            pub struct Arc;
+        }
+        "#;
+        check_found_path(code, "alloc::sync::Arc");
+    }
+
+    #[test]
+    fn prefer_shorter_paths_if_not_alloc() {
+        let code = r#"
+        //- /main.rs crate:main deps:megaalloc,std
+        <|>
+
+        //- /std.rs crate:std deps:megaalloc
+        pub mod sync {
+            pub use megaalloc::sync::Arc;
+        }
+
+        //- /zzz.rs crate:megaalloc
+        pub struct Arc;
+        "#;
+        check_found_path(code, "megaalloc::Arc");
+    }
+
+    #[test]
+    fn builtins_are_in_scope() {
+        let code = r#"
+        //- /main.rs
+        <|>
+
+        pub mod primitive {
+            pub use u8;
+        }
+        "#;
+        check_found_path(code, "u8");
+        check_found_path(code, "u16");
+    }
+}