Rollup merge of #82579 - osa1:issue82566, r=estebank

Fix turbofish recovery with multiple generic args

This consists of two commits, each can be individually reviewed.

- First commit fixes the issue in [this comment](https://github.com/rust-lang/rust/issues/82566#issuecomment-786924466).
- Second commit fixes #82566

---

r? ````@estebank````

compiler/rustc_parse/src/parser/diagnostics.rs

@@ -662,7 +662,7 @@ impl<'a> Parser<'a> {
                     let x = self.parse_seq_to_before_end(
                         &token::Gt,
                         SeqSep::trailing_allowed(token::Comma),
-                        |p| p.parse_ty(),
+                        |p| p.parse_generic_arg(),
                     );
                     match x {
                         Ok((_, _, false)) => {

compiler/rustc_parse/src/parser/path.rs

@@ -545,7 +545,7 @@ impl<'a> Parser<'a> {
 
     /// Parse a generic argument in a path segment.
     /// This does not include constraints, e.g., `Item = u8`, which is handled in `parse_angle_arg`.
-    fn parse_generic_arg(&mut self) -> PResult<'a, Option<GenericArg>> {
+    pub(super) fn parse_generic_arg(&mut self) -> PResult<'a, Option<GenericArg>> {
         let start = self.token.span;
         let arg = if self.check_lifetime() && self.look_ahead(1, |t| !t.is_like_plus()) {
             // Parse lifetime argument.

compiler/rustc_parse/src/parser/ty.rs

@@ -360,12 +360,20 @@ impl<'a> Parser<'a> {
             }
             Err(err) => return Err(err),
         };
+
         let ty = if self.eat(&token::Semi) {
-            TyKind::Array(elt_ty, self.parse_anon_const_expr()?)
+            let mut length = self.parse_anon_const_expr()?;
+            if let Err(e) = self.expect(&token::CloseDelim(token::Bracket)) {
+                // Try to recover from `X<Y, ...>` when `X::<Y, ...>` works
+                self.check_mistyped_turbofish_with_multiple_type_params(e, &mut length.value)?;
+                self.expect(&token::CloseDelim(token::Bracket))?;
+            }
+            TyKind::Array(elt_ty, length)
         } else {
+            self.expect(&token::CloseDelim(token::Bracket))?;
             TyKind::Slice(elt_ty)
         };
-        self.expect(&token::CloseDelim(token::Bracket))?;
+
         Ok(ty)
     }
 

src/test/ui/suggestions/issue-82566-1.rs

@@ -0,0 +1,21 @@
+struct T1<const X1: usize>;
+struct T2<const X1: usize, const X2: usize>;
+struct T3<const X1: usize, const X2: usize, const X3: usize>;
+
+impl T1<1> {
+    const C: () = ();
+}
+
+impl T2<1, 2> {
+    const C: () = ();
+}
+
+impl T3<1, 2, 3> {
+    const C: () = ();
+}
+
+fn main() {
+    T1<1>::C; //~ ERROR: comparison operators cannot be chained
+    T2<1, 2>::C; //~ ERROR: expected one of `.`, `;`, `?`, `}`, or an operator, found `,`
+    T3<1, 2, 3>::C; //~ ERROR: expected one of `.`, `;`, `?`, `}`, or an operator, found `,`
+}

src/test/ui/suggestions/issue-82566-1.stderr

@@ -0,0 +1,35 @@
+error: comparison operators cannot be chained
+  --> $DIR/issue-82566-1.rs:18:7
+   |
+LL |     T1<1>::C;
+   |       ^ ^
+   |
+help: use `::<...>` instead of `<...>` to specify type or const arguments
+   |
+LL |     T1::<1>::C;
+   |       ^^
+
+error: expected one of `.`, `;`, `?`, `}`, or an operator, found `,`
+  --> $DIR/issue-82566-1.rs:19:9
+   |
+LL |     T2<1, 2>::C;
+   |         ^ expected one of `.`, `;`, `?`, `}`, or an operator
+   |
+help: use `::<...>` instead of `<...>` to specify type or const arguments
+   |
+LL |     T2::<1, 2>::C;
+   |       ^^
+
+error: expected one of `.`, `;`, `?`, `}`, or an operator, found `,`
+  --> $DIR/issue-82566-1.rs:20:9
+   |
+LL |     T3<1, 2, 3>::C;
+   |         ^ expected one of `.`, `;`, `?`, `}`, or an operator
+   |
+help: use `::<...>` instead of `<...>` to specify type or const arguments
+   |
+LL |     T3::<1, 2, 3>::C;
+   |       ^^
+
+error: aborting due to 3 previous errors
+

src/test/ui/suggestions/issue-82566-2.rs

@@ -0,0 +1,31 @@
+struct Foo1<const N1: usize>;
+struct Foo2<const N1: usize, const N2: usize>;
+struct Foo3<const N1: usize, const N2: usize, const N3: usize>;
+
+impl<const N1: usize> Foo1<N1> {
+    const SUM: usize = N1;
+}
+
+impl<const N1: usize, const N2: usize> Foo2<N1, N2> {
+    const SUM: usize = N1 + N2;
+}
+
+impl<const N1: usize, const N2: usize, const N3: usize> Foo3<N1, N2, N3> {
+    const SUM: usize = N1 + N2 + N3;
+}
+
+fn foo1() -> [(); Foo1<10>::SUM] { //~ ERROR: comparison operators cannot be chained
+    todo!()
+}
+
+fn foo2() -> [(); Foo2<10, 20>::SUM] {
+    //~^ ERROR: expected one of `.`, `?`, `]`, or an operator, found `,`
+    todo!()
+}
+
+fn foo3() -> [(); Foo3<10, 20, 30>::SUM] {
+    //~^ ERROR: expected one of `.`, `?`, `]`, or an operator, found `,`
+    todo!()
+}
+
+fn main() {}

src/test/ui/suggestions/issue-82566-2.stderr

@@ -0,0 +1,35 @@
+error: comparison operators cannot be chained
+  --> $DIR/issue-82566-2.rs:17:23
+   |
+LL | fn foo1() -> [(); Foo1<10>::SUM] {
+   |                       ^  ^
+   |
+help: use `::<...>` instead of `<...>` to specify type or const arguments
+   |
+LL | fn foo1() -> [(); Foo1::<10>::SUM] {
+   |                       ^^
+
+error: expected one of `.`, `?`, `]`, or an operator, found `,`
+  --> $DIR/issue-82566-2.rs:21:26
+   |
+LL | fn foo2() -> [(); Foo2<10, 20>::SUM] {
+   |                          ^ expected one of `.`, `?`, `]`, or an operator
+   |
+help: use `::<...>` instead of `<...>` to specify type or const arguments
+   |
+LL | fn foo2() -> [(); Foo2::<10, 20>::SUM] {
+   |                       ^^
+
+error: expected one of `.`, `?`, `]`, or an operator, found `,`
+  --> $DIR/issue-82566-2.rs:26:26
+   |
+LL | fn foo3() -> [(); Foo3<10, 20, 30>::SUM] {
+   |                          ^ expected one of `.`, `?`, `]`, or an operator
+   |
+help: use `::<...>` instead of `<...>` to specify type or const arguments
+   |
+LL | fn foo3() -> [(); Foo3::<10, 20, 30>::SUM] {
+   |                       ^^
+
+error: aborting due to 3 previous errors
+