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diff --git a/crates/ra_hir_expand/src/eager.rs b/crates/ra_hir_expand/src/eager.rs
new file mode 100644
index 000000000..7fcdfab5a
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+++ b/crates/ra_hir_expand/src/eager.rs
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+//! Eager expansion related utils
+//!
+//! Here is a dump of a discussion from Vadim Petrochenkov about Eager Expansion and
+//! Its name resolution :
+//!
+//! > Eagerly expanded macros (and also macros eagerly expanded by eagerly expanded macros,
+//! > which actually happens in practice too!) are resolved at the location of the "root" macro
+//! > that performs the eager expansion on its arguments.
+//! > If some name cannot be resolved at the eager expansion time it's considered unresolved,
+//! > even if becomes available later (e.g. from a glob import or other macro).
+//!
+//! > Eagerly expanded macros don't add anything to the module structure of the crate and
+//! > don't build any speculative module structures, i.e. they are expanded in a "flat"
+//! > way even if tokens in them look like modules.
+//!
+//! > In other words, it kinda works for simple cases for which it was originally intended,
+//! > and we need to live with it because it's available on stable and widely relied upon.
+//!
+//!
+//! See the full discussion : https://rust-lang.zulipchat.com/#narrow/stream/131828-t-compiler/topic/Eager.20expansion.20of.20built-in.20macros
+
+use crate::{
+    ast::{self, AstNode},
+    db::AstDatabase,
+    EagerCallLoc, EagerMacroId, InFile, MacroCallId, MacroCallKind, MacroDefId, MacroDefKind,
+};
+
+use ra_parser::FragmentKind;
+use ra_syntax::{algo::replace_descendants, SyntaxElement, SyntaxNode};
+use std::{collections::HashMap, sync::Arc};
+
+pub fn expand_eager_macro(
+    db: &impl AstDatabase,
+    macro_call: InFile<ast::MacroCall>,
+    def: MacroDefId,
+    resolver: &dyn Fn(ast::Path) -> Option<MacroDefId>,
+) -> Option<EagerMacroId> {
+    let args = macro_call.value.token_tree()?;
+    let parsed_args = mbe::ast_to_token_tree(&args)?.0;
+    let parsed_args = mbe::token_tree_to_syntax_node(&parsed_args, FragmentKind::Expr).ok()?.0;
+    let result = eager_macro_recur(db, macro_call.with_value(parsed_args.syntax_node()), resolver)?;
+
+    let subtree = to_subtree(&result)?;
+
+    if let MacroDefKind::BuiltInEager(eager) = def.kind {
+        let (subtree, fragment) = eager.expand(&subtree).ok()?;
+        let eager =
+            EagerCallLoc { def, fragment, subtree: Arc::new(subtree), file_id: macro_call.file_id };
+
+        Some(db.intern_eager_expansion(eager))
+    } else {
+        None
+    }
+}
+
+fn to_subtree(node: &SyntaxNode) -> Option<tt::Subtree> {
+    let mut subtree = mbe::syntax_node_to_token_tree(node)?.0;
+    subtree.delimiter = None;
+    Some(subtree)
+}
+
+fn lazy_expand(
+    db: &impl AstDatabase,
+    def: &MacroDefId,
+    macro_call: InFile<ast::MacroCall>,
+) -> Option<InFile<SyntaxNode>> {
+    let ast_id = db.ast_id_map(macro_call.file_id).ast_id(&macro_call.value);
+
+    let id: MacroCallId =
+        def.as_lazy_macro(db, MacroCallKind::FnLike(macro_call.with_value(ast_id))).into();
+
+    db.parse_or_expand(id.as_file()).map(|node| InFile::new(id.as_file(), node))
+}
+
+fn eager_macro_recur(
+    db: &impl AstDatabase,
+    curr: InFile<SyntaxNode>,
+    macro_resolver: &dyn Fn(ast::Path) -> Option<MacroDefId>,
+) -> Option<SyntaxNode> {
+    let mut original = curr.value.clone();
+
+    let children = curr.value.descendants().filter_map(ast::MacroCall::cast);
+    let mut replaces: HashMap<SyntaxElement, SyntaxElement> = HashMap::default();
+
+    // Collect replacement
+    for child in children {
+        let def: MacroDefId = macro_resolver(child.path()?)?;
+        let insert = match def.kind {
+            MacroDefKind::BuiltInEager(_) => {
+                let id: MacroCallId =
+                    expand_eager_macro(db, curr.with_value(child.clone()), def, macro_resolver)?
+                        .into();
+                db.parse_or_expand(id.as_file())?
+            }
+            MacroDefKind::Declarative
+            | MacroDefKind::BuiltIn(_)
+            | MacroDefKind::BuiltInDerive(_) => {
+                let expanded = lazy_expand(db, &def, curr.with_value(child.clone()))?;
+                // replace macro inside
+                eager_macro_recur(db, expanded, macro_resolver)?
+            }
+        };
+
+        replaces.insert(child.syntax().clone().into(), insert.into());
+    }
+
+    if !replaces.is_empty() {
+        original = replace_descendants(&original, |n| replaces.get(n).cloned());
+    }
+
+    Some(original)
+}