Add clippy like expr_or_init fn to rustc LateContext

compiler/rustc_lint/src/context.rs

@@ -1315,6 +1315,54 @@ impl<'tcx> LateContext<'tcx> {
                 tcx.try_normalize_erasing_regions(self.param_env, proj).ok()
             })
     }
+
+    /// If the given expression is a local binding, find the initializer expression.
+    /// If that initializer expression is another local or **outside** (`const`/`static`)
+    /// binding, find its initializer again.
+    ///
+    /// This process repeats as long as possible (but usually no more than once).
+    /// Type-check adjustments are not taken in account in this function.
+    ///
+    /// Examples:
+    /// ```
+    /// const ABC: i32 = 1;
+    /// //               ^ output
+    /// let def = ABC;
+    /// dbg!(def);
+    /// //   ^^^ input
+    ///
+    /// // or...
+    /// let abc = 1;
+    /// let def = abc + 2;
+    /// //        ^^^^^^^ output
+    /// dbg!(def);
+    /// //   ^^^ input
+    /// ```
+    pub fn expr_or_init<'a>(&self, mut expr: &'a hir::Expr<'tcx>) -> &'a hir::Expr<'tcx> {
+        expr = expr.peel_blocks();
+
+        while let hir::ExprKind::Path(ref qpath) = expr.kind
+            && let Some(parent_node) = match self.qpath_res(qpath, expr.hir_id) {
+                Res::Local(hir_id) => self.tcx.hir().find_parent(hir_id),
+                Res::Def(_, def_id) => self.tcx.hir().get_if_local(def_id),
+                _ => None,
+            }
+            && let Some(init) = match parent_node {
+                hir::Node::Expr(expr) => Some(expr),
+                hir::Node::Local(hir::Local { init, .. }) => *init,
+                hir::Node::Item(item) => match item.kind {
+                    hir::ItemKind::Const(.., body_id) | hir::ItemKind::Static(.., body_id) => {
+                        Some(self.tcx.hir().body(body_id).value)
+                    }
+                    _ => None
+                }
+                _ => None
+            }
+        {
+            expr = init.peel_blocks();
+        }
+        expr
+    }
 }
 
 impl<'tcx> abi::HasDataLayout for LateContext<'tcx> {