Recover dyn and impl after for<...>

compiler/rustc_parse/messages.ftl

@@ -739,6 +739,9 @@ parse_trailing_vert_not_allowed = a trailing `|` is not allowed in an or-pattern
 parse_trait_alias_cannot_be_auto = trait aliases cannot be `auto`
 parse_trait_alias_cannot_be_unsafe = trait aliases cannot be `unsafe`
 
+parse_transpose_dyn_or_impl = `for<...>` expected after `{$kw}`, not before
+    .suggestion = move `{$kw}` before the `for<...>`
+
 parse_type_ascription_removed =
     if you meant to annotate an expression with a type, the type ascription syntax has been removed, see issue #101728 <https://github.com/rust-lang/rust/issues/101728>
 

compiler/rustc_parse/src/errors.rs

@@ -2828,3 +2828,23 @@ pub(crate) struct GenericArgsInPatRequireTurbofishSyntax {
     )]
     pub suggest_turbofish: Span,
 }
+
+#[derive(Diagnostic)]
+#[diag(parse_transpose_dyn_or_impl)]
+pub(crate) struct TransposeDynOrImpl<'a> {
+    #[primary_span]
+    pub span: Span,
+    pub kw: &'a str,
+    #[subdiagnostic]
+    pub sugg: TransposeDynOrImplSugg<'a>,
+}
+
+#[derive(Subdiagnostic)]
+#[multipart_suggestion(parse_suggestion, applicability = "machine-applicable")]
+pub(crate) struct TransposeDynOrImplSugg<'a> {
+    #[suggestion_part(code = "")]
+    pub removal_span: Span,
+    #[suggestion_part(code = "{kw} ")]
+    pub insertion_span: Span,
+    pub kw: &'a str,
+}

compiler/rustc_parse/src/parser/ty.rs

@@ -287,6 +287,7 @@ fn parse_ty_common(
             // Function pointer type
             self.parse_ty_bare_fn(lo, ThinVec::new(), None, recover_return_sign)?
         } else if self.check_keyword(kw::For) {
+            let for_span = self.token.span;
             // Function pointer type or bound list (trait object type) starting with a poly-trait.
             //   `for<'lt> [unsafe] [extern "ABI"] fn (&'lt S) -> T`
             //   `for<'lt> Trait1<'lt> + Trait2 + 'a`
@@ -299,9 +300,42 @@ fn parse_ty_common(
                     recover_return_sign,
                 )?
             } else {
-                let path = self.parse_path(PathStyle::Type)?;
-                let parse_plus = allow_plus == AllowPlus::Yes && self.check_plus();
-                self.parse_remaining_bounds_path(lifetime_defs, path, lo, parse_plus)?
+                // Try to recover `for<'a> dyn Trait` or `for<'a> impl Trait`.
+                if self.may_recover()
+                    && (self.eat_keyword_noexpect(kw::Impl) || self.eat_keyword_noexpect(kw::Dyn))
+                {
+                    let kw = self.prev_token.ident().unwrap().0.name;
+                    let mut err = self.sess.create_err(errors::TransposeDynOrImpl {
+                        span: self.prev_token.span,
+                        kw: kw.as_str(),
+                        sugg: errors::TransposeDynOrImplSugg {
+                            removal_span: self.prev_token.span.with_hi(self.token.span.lo()),
+                            insertion_span: for_span.shrink_to_lo(),
+                            kw: kw.as_str(),
+                        },
+                    });
+                    let path = self.parse_path(PathStyle::Type)?;
+                    let parse_plus = allow_plus == AllowPlus::Yes && self.check_plus();
+                    let kind =
+                        self.parse_remaining_bounds_path(lifetime_defs, path, lo, parse_plus)?;
+                    // Take the parsed bare trait object and turn it either
+                    // into a `dyn` object or an `impl Trait`.
+                    let kind = match (kind, kw) {
+                        (TyKind::TraitObject(bounds, _), kw::Dyn) => {
+                            TyKind::TraitObject(bounds, TraitObjectSyntax::Dyn)
+                        }
+                        (TyKind::TraitObject(bounds, _), kw::Impl) => {
+                            TyKind::ImplTrait(ast::DUMMY_NODE_ID, bounds)
+                        }
+                        _ => return Err(err),
+                    };
+                    err.emit();
+                    kind
+                } else {
+                    let path = self.parse_path(PathStyle::Type)?;
+                    let parse_plus = allow_plus == AllowPlus::Yes && self.check_plus();
+                    self.parse_remaining_bounds_path(lifetime_defs, path, lo, parse_plus)?
+                }
             }
         } else if self.eat_keyword(kw::Impl) {
             self.parse_impl_ty(&mut impl_dyn_multi)?

tests/ui/parser/recover-hrtb-before-dyn-impl-kw.rs

@@ -0,0 +1,9 @@
+trait Trait {}
+
+fn test(_: &for<'a> dyn Trait) {}
+//~^ ERROR `for<...>` expected after `dyn`, not before
+
+fn test2(_: for<'a> impl Trait) {}
+//~^ ERROR `for<...>` expected after `impl`, not before
+
+fn main() {}

tests/ui/parser/recover-hrtb-before-dyn-impl-kw.stderr

@@ -0,0 +1,26 @@
+error: `for<...>` expected after `dyn`, not before
+  --> $DIR/recover-hrtb-before-dyn-impl-kw.rs:3:21
+   |
+LL | fn test(_: &for<'a> dyn Trait) {}
+   |                     ^^^
+   |
+help: move `dyn` before the `for<...>`
+   |
+LL - fn test(_: &for<'a> dyn Trait) {}
+LL + fn test(_: &dyn for<'a> Trait) {}
+   |
+
+error: `for<...>` expected after `impl`, not before
+  --> $DIR/recover-hrtb-before-dyn-impl-kw.rs:6:21
+   |
+LL | fn test2(_: for<'a> impl Trait) {}
+   |                     ^^^^
+   |
+help: move `impl` before the `for<...>`
+   |
+LL - fn test2(_: for<'a> impl Trait) {}
+LL + fn test2(_: impl for<'a> Trait) {}
+   |
+
+error: aborting due to 2 previous errors
+