Fix problem causing rusqlite compilation to OOM.

This makes the expression re-parsing more like how it's originally done
in `parse_nonterminal`.

compiler/rustc_parse/src/parser/expr.rs

@@ -1397,7 +1397,9 @@ impl<'a> Parser<'a> {
         } else if let Some(expr) = self.eat_metavar_seq_with_matcher(
             |mv_kind| matches!(mv_kind, MetaVarKind::Expr { .. }),
             |this| {
-                let expr = this.parse_expr();
+                // Force collection (as opposed to just `parse_expr`) is required to avoid the
+                // attribute duplication seen in #138478.
+                let expr = this.parse_expr_force_collect();
                 // FIXME(nnethercote) Sometimes with expressions we get a trailing comma, possibly
                 // related to the FIXME in `collect_tokens_for_expr`. Examples are the multi-line
                 // `assert_eq!` calls involving arguments annotated with `#[rustfmt::skip]` in

tests/ui/attributes/dont-dup-expr-attrs.rs

@@ -0,0 +1,133 @@
+//@ check-pass
+//
+// During development of #124141 at one point expression on attributes were
+// being duplicated and `m1` caused an exponential blowup that caused OOM.
+// The number of recursive calls depends on the number of doc comments on the
+// expr block. On each recursive call, the `#[allow(deprecated)]` attribute(s) on
+// the `0` somehow get duplicated, resulting in 1, 2, 4, 8, ... identical
+// attributes.
+//
+// After the fix, the code compiles quickly and normally.
+
+macro_rules! m1 {
+    ($(#[$meta:meta])* { $e:expr }) => {
+        m1! { expr: { $e }, unprocessed: [$(#[$meta])*] }
+    };
+
+    (expr: { $e:expr }, unprocessed: [ #[$meta:meta] $($metas:tt)* ]) => {
+        m1! { expr: { $e }, unprocessed: [ $($metas)* ] }
+    };
+
+    (expr: { $e:expr }, unprocessed: []) => {
+        { $e }
+    }
+}
+
+macro_rules! m2 {
+    ($(#[$meta:meta])* { $e:stmt }) => {
+        m2! { stmt: { $e }, unprocessed: [$(#[$meta])*] }
+    };
+
+    (stmt: { $e:stmt }, unprocessed: [ #[$meta:meta] $($metas:tt)* ]) => {
+        m2! { stmt: { $e }, unprocessed: [ $($metas)* ] }
+    };
+
+    (stmt: { $e:stmt }, unprocessed: []) => {
+        { $e }
+    }
+}
+
+macro_rules! m3 {
+    ($(#[$meta:meta])* { $e:item }) => {
+        m3! { item: { $e }, unprocessed: [$(#[$meta])*] }
+    };
+
+    (item: { $e:item }, unprocessed: [ #[$meta:meta] $($metas:tt)* ]) => {
+        m3! { item: { $e }, unprocessed: [ $($metas)* ] }
+    };
+
+    (item: { $e:item }, unprocessed: []) => {
+        { $e }
+    }
+}
+
+fn main() {
+    // Each additional doc comment line doubles the compile time.
+    m1!(
+        /// a1
+        /// a2
+        /// a3
+        /// a4
+        /// a5
+        /// a6
+        /// a7
+        /// a8
+        /// a9
+        /// a10
+        /// a11
+        /// a12
+        /// a13
+        /// a14
+        /// a15
+        /// a16
+        /// a17
+        /// a18
+        /// a19
+        /// a20
+        {
+            #[allow(deprecated)] 0
+        }
+    );
+
+    m2!(
+        /// a1
+        /// a2
+        /// a3
+        /// a4
+        /// a5
+        /// a6
+        /// a7
+        /// a8
+        /// a9
+        /// a10
+        /// a11
+        /// a12
+        /// a13
+        /// a14
+        /// a15
+        /// a16
+        /// a17
+        /// a18
+        /// a19
+        /// a20
+        {
+            #[allow(deprecated)] let x = 5
+        }
+    );
+
+    m3!(
+        /// a1
+        /// a2
+        /// a3
+        /// a4
+        /// a5
+        /// a6
+        /// a7
+        /// a8
+        /// a9
+        /// a10
+        /// a11
+        /// a12
+        /// a13
+        /// a14
+        /// a15
+        /// a16
+        /// a17
+        /// a18
+        /// a19
+        /// a20
+        {
+            #[allow(deprecated)] struct S;
+        }
+    );
+}