Fix problem causing rusqlite compilation to OOM.

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

Author: nnethercote

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;
+ }
+ );
+}