Implement extern crate completion

crates/hir-def/src/resolver.rs

@@ -479,6 +479,10 @@ impl Resolver {
         res.map
     }
 
+    pub fn extern_crates_in_scope(&self) -> Vec<Name> {
+        self.module_scope.def_map.extern_prelude().map(|(name, _)| name.clone()).collect()
+    }
+
     pub fn traits_in_scope(&self, db: &dyn DefDatabase) -> FxHashSet<TraitId> {
         // FIXME(trait_alias): Trait alias brings aliased traits in scope! Note that supertraits of
         // aliased traits are NOT brought in scope (unless also aliased).

crates/hir/src/semantics.rs

@@ -1702,6 +1702,10 @@ impl SemanticsScope<'_> {
             |name, id| cb(name, id.into()),
         )
     }
+
+    pub fn extern_crates(&self) -> Vec<Name> {
+        self.resolver.extern_crates_in_scope()
+    }
 }
 
 #[derive(Debug)]

crates/ide-completion/src/completions.rs

@@ -20,6 +20,7 @@ pub(crate) mod r#type;
 pub(crate) mod use_;
 pub(crate) mod vis;
 pub(crate) mod env_vars;
+pub(crate) mod extern_crate;
 
 use std::iter;
 
@@ -737,6 +738,7 @@ pub(super) fn complete_name_ref(
                 }
             }
         }
+        NameRefKind::ExternCrate => extern_crate::complete_extern_crate(acc, ctx, nameref.as_ref()),
         NameRefKind::DotAccess(dot_access) => {
             flyimport::import_on_the_fly_dot(acc, ctx, dot_access);
             dot::complete_dot(acc, ctx, dot_access);

crates/ide-completion/src/completions/extern_crate.rs

@@ -0,0 +1,94 @@
+use ide_db::{FxHashSet, SymbolKind};
+use syntax::{
+    ast::{self, NameRef},
+    AstNode,
+};
+
+use crate::{context::CompletionContext, CompletionItem, CompletionItemKind};
+
+use super::Completions;
+
+pub(crate) fn complete_extern_crate(
+    acc: &mut Completions,
+    ctx: &CompletionContext<'_>,
+    name_ref: Option<&NameRef>,
+) {
+    let imported_extern_crates: FxHashSet<String> = ctx
+        .token
+        .parent_ancestors()
+        .find_map(ast::SourceFile::cast)
+        .map(|src_file| {
+            src_file
+                .syntax()
+                .children()
+                .into_iter()
+                .filter_map(ast::ExternCrate::cast)
+                .filter_map(|node| node.name_ref())
+                .map(|name| name.to_string())
+                .collect()
+        })
+        .unwrap_or_default();
+
+    let current_txt =
+        name_ref.as_ref().map(|name_ref| name_ref.to_string()).unwrap_or(String::new());
+
+    for name in ctx.scope.extern_crates() {
+        if (!current_txt.is_empty() && !name.to_smol_str().starts_with(&current_txt))
+            || imported_extern_crates.contains(name.to_smol_str().as_str())
+        {
+            continue;
+        }
+
+        CompletionItem::new(
+            CompletionItemKind::SymbolKind(SymbolKind::Module),
+            ctx.source_range(),
+            name.to_smol_str(),
+        )
+        .add_to(acc, ctx.db);
+    }
+}
+
+#[cfg(test)]
+mod test {
+    use crate::tests::completion_list_no_kw;
+
+    #[test]
+    fn can_complete_extern_crate() {
+        let case = r#"
+//- /lib.rs crate:other_crate_a
+// nothing here
+//- /other_crate_b.rs crate:other_crate_b
+pub mod good_mod{}
+//- /lib.rs crate:crate_c
+// nothing here
+//- /lib.rs crate:lib deps:other_crate_a,other_crate_b,crate_c extern-prelude:other_crate_a,crate_c
+extern crate crate_c;
+extern crate oth$0
+mod other_mod {}
+"#;
+
+        let completion_list = completion_list_no_kw(case);
+
+        assert_eq!("md other_crate_a\n".to_string(), completion_list);
+    }
+
+    #[test]
+    fn will_not_complete_existing_import() {
+        let case = r#"
+//- /lib.rs crate:other_crate_a
+// nothing here
+//- /lib.rs crate:crate_c
+// nothing here
+//- /lib.rs crate:other_crate_b
+//
+//- /lib.rs crate:lib deps:other_crate_a,other_crate_b,crate_c extern-prelude:other_crate_a,other_crate_b,crate_c
+extern crate other_crate_b;
+extern crate oth$0
+mod other_mod {}
+"#;
+
+        let completion_list = completion_list_no_kw(case);
+
+        assert_eq!("md other_crate_a\n".to_string(), completion_list);
+    }
+}

crates/ide-completion/src/context.rs

@@ -301,6 +301,7 @@ pub(super) enum NameRefKind {
         expr: ast::RecordExpr,
     },
     Pattern(PatternContext),
+    ExternCrate,
 }
 
 /// The identifier we are currently completing.

crates/ide-completion/src/context/analysis.rs

@@ -624,6 +624,10 @@ fn classify_name_ref(
                 });
                 return Some(make_res(kind));
             },
+            ast::ExternCrate(_) => {
+                let kind = NameRefKind::ExternCrate;
+                return Some(make_res(kind));
+            },
             ast::MethodCallExpr(method) => {
                 let receiver = find_opt_node_in_file(original_file, method.receiver());
                 let kind = NameRefKind::DotAccess(DotAccess {