explain ImportData::imported_module

Author: bvanjoi

compiler/rustc_resolve/src/imports.rs

@@ -174,7 +174,17 @@ pub(crate) struct ImportData<'ra> {
 
     pub parent_scope: ParentScope<'ra>,
     pub module_path: Vec<Segment>,
-    /// The resolution of `module_path`.
+    /// The resolution of `module_path`:
+    /// - It refers to `a::b` when the `module_path` is `::a::b::c`
+    ///   in 2018+ editions, where `a` must be an external crate.
+    /// - It refers to `crate::a::b` when the `module_path` is `::a::b::c`
+    ///   in 2015 edition, where `a` can be either an external crate or
+    ///   a module defined at crate root.
+    /// - It refers to `a::b` when the `module_path` is `a::b::c`, and
+    ///   `a` refers to the module defined in the scope where the use
+    ///   statement appears.
+    /// - It refers to the module or scope where the use statement is
+    ///   defined when the `module_path` only has one segment.
     pub imported_module: Cell<Option<ModuleOrUniformRoot<'ra>>>,
     pub vis: ty::Visibility,
 }