Do not warn about shadowing in a destructuring assigment

When lowering a destructuring assignment from AST to HIR, the compiler
will reuse the same identifier name (namely `sym::lhs`) for all the
fields. The desugaring must be checked for to avoid a false positive 
of the `shadow_unrelated` lint.

Author: samueltardieu

clippy_lints/src/shadow.rs

@@ -8,7 +8,9 @@ use rustc_data_structures::fx::FxHashMap;
 use rustc_hir::def::Res;
 use rustc_hir::def_id::LocalDefId;
 use rustc_hir::hir_id::ItemLocalId;
-use rustc_hir::{Block, Body, BodyOwnerKind, Expr, ExprKind, HirId, LetExpr, Node, Pat, PatKind, QPath, UnOp};
+use rustc_hir::{
+    Block, Body, BodyOwnerKind, Expr, ExprKind, HirId, LetExpr, LocalSource, Node, Pat, PatKind, QPath, UnOp,
+};
 use rustc_lint::{LateContext, LateLintPass};
 use rustc_session::impl_lint_pass;
 use rustc_span::{Span, Symbol};
@@ -125,6 +127,14 @@ impl<'tcx> LateLintPass<'tcx> for Shadow {
             return;
         }
 
+        // Desugaring of a destructuring assignment may reuse the same identifier internally
+        if let Node::Pat(parent_pat) = cx.tcx.parent_hir_node(pat.hir_id)
+            && let Node::LetStmt(let_stmt) = cx.tcx.parent_hir_node(parent_pat.hir_id)
+            && let LocalSource::AssignDesugar(_) = let_stmt.source
+        {
+            return;
+        }
+
         let HirId { owner, local_id } = id;
         // get (or insert) the list of items for this owner and symbol
         let (ref mut data, scope_owner) = *self.bindings.last_mut().unwrap();

tests/ui/shadow.rs

@@ -167,4 +167,10 @@ fn issue13795(value: Issue13795) {
     //~^ shadow_same
 }
 
+fn issue14377() {
+    let a;
+    let b;
+    (a, b) = (0, 1);
+}
+
 fn main() {}