From 3cf2c3b943a5b373dbd333622c16f3d606fdb6ab Mon Sep 17 00:00:00 2001
From: Jonas Schievink <jonasschievink@gmail.com>
Date: Sat, 25 Apr 2020 16:24:44 +0200
Subject: Prefer core/alloc paths on #![no_std]

---
 crates/ra_hir_def/src/attr.rs      | 23 ++++++++---
 crates/ra_hir_def/src/find_path.rs | 78 ++++++++++++++++++++++++++++++++++----
 2 files changed, 87 insertions(+), 14 deletions(-)

diff --git a/crates/ra_hir_def/src/attr.rs b/crates/ra_hir_def/src/attr.rs
index 714a66b02..5a86af8ba 100644
--- a/crates/ra_hir_def/src/attr.rs
+++ b/crates/ra_hir_def/src/attr.rs
@@ -13,7 +13,8 @@ use ra_syntax::{
 use tt::Subtree;
 
 use crate::{
-    db::DefDatabase, path::ModPath, src::HasChildSource, src::HasSource, AdtId, AttrDefId, Lookup,
+    db::DefDatabase, nameres::ModuleSource, path::ModPath, src::HasChildSource, src::HasSource,
+    AdtId, AttrDefId, Lookup,
 };
 
 #[derive(Default, Debug, Clone, PartialEq, Eq)]
@@ -37,11 +38,19 @@ impl Attrs {
         match def {
             AttrDefId::ModuleId(module) => {
                 let def_map = db.crate_def_map(module.krate);
-                let src = match def_map[module.local_id].declaration_source(db) {
-                    Some(it) => it,
-                    None => return Attrs::default(),
-                };
-                Attrs::from_attrs_owner(db, src.as_ref().map(|it| it as &dyn AttrsOwner))
+                let mod_data = &def_map[module.local_id];
+                match mod_data.declaration_source(db) {
+                    Some(it) => {
+                        Attrs::from_attrs_owner(db, it.as_ref().map(|it| it as &dyn AttrsOwner))
+                    }
+                    None => Attrs::from_attrs_owner(
+                        db,
+                        mod_data.definition_source(db).as_ref().map(|src| match src {
+                            ModuleSource::SourceFile(file) => file as &dyn AttrsOwner,
+                            ModuleSource::Module(module) => module as &dyn AttrsOwner,
+                        }),
+                    ),
+                }
             }
             AttrDefId::FieldId(it) => {
                 let src = it.parent.child_source(db);
@@ -106,7 +115,9 @@ pub struct Attr {
 
 #[derive(Debug, Clone, PartialEq, Eq)]
 pub enum AttrInput {
+    /// `#[attr = "string"]`
     Literal(SmolStr),
+    /// `#[attr(subtree)]`
     TokenTree(Subtree),
 }
 
diff --git a/crates/ra_hir_def/src/find_path.rs b/crates/ra_hir_def/src/find_path.rs
index 81eff5bfe..70dcb03e6 100644
--- a/crates/ra_hir_def/src/find_path.rs
+++ b/crates/ra_hir_def/src/find_path.rs
@@ -19,7 +19,7 @@ impl ModPath {
 
     // When std library is present, paths starting with `std::`
     // should be preferred over paths starting with `core::` and `alloc::`
-    fn should_start_with_std(&self) -> bool {
+    fn can_start_with_std(&self) -> bool {
         self.segments
             .first()
             .filter(|&first_segment| {
@@ -132,6 +132,9 @@ fn find_path_inner(
     }
 
     // - 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 importable_locations = find_importable_locations(db, item, from);
     let mut best_path = None;
     let mut best_path_len = max_len;
@@ -147,21 +150,32 @@ fn find_path_inner(
         };
         path.segments.push(name);
 
-        let new_path =
-            if let Some(best_path) = best_path { select_best_path(best_path, path) } else { path };
+        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);
     }
     best_path
 }
 
-fn select_best_path(old_path: ModPath, new_path: ModPath) -> ModPath {
-    if old_path.starts_with_std() && new_path.should_start_with_std() {
+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() {
         tested_by!(prefer_std_paths);
-        old_path
-    } else if new_path.starts_with_std() && old_path.should_start_with_std() {
+        if prefer_no_std {
+            new_path
+        } else {
+            old_path
+        }
+    } else if new_path.starts_with_std() && old_path.can_start_with_std() {
         tested_by!(prefer_std_paths);
-        new_path
+        if prefer_no_std {
+            old_path
+        } else {
+            new_path
+        }
     } else if new_path.len() < old_path.len() {
         new_path
     } else {
@@ -512,6 +526,54 @@ mod tests {
         check_found_path(code, "std::sync::Arc");
     }
 
+    #[test]
+    fn prefer_alloc_paths_over_std() {
+        covers!(prefer_std_paths);
+        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_core_paths_over_std() {
+        covers!(prefer_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_shorter_paths_if_not_alloc() {
         let code = r#"
-- 
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