Handle parenthesised infer args

compiler/rustc_ast/src/ast.rs

@@ -288,6 +288,7 @@ impl ParenthesizedArgs {
     }
 }
 
+use crate::AstDeref;
 pub use crate::node_id::{CRATE_NODE_ID, DUMMY_NODE_ID, NodeId};
 
 /// Modifiers on a trait bound like `~const`, `?` and `!`.
@@ -2166,6 +2167,14 @@ impl Ty {
         }
         final_ty
     }
+
+    pub fn is_maybe_parenthesised_infer(&self) -> bool {
+        match &self.kind {
+            TyKind::Infer => true,
+            TyKind::Paren(inner) => inner.ast_deref().is_maybe_parenthesised_infer(),
+            _ => false,
+        }
+    }
 }
 
 #[derive(Clone, Encodable, Decodable, Debug)]

compiler/rustc_ast_lowering/src/lib.rs

@@ -1084,17 +1084,22 @@ impl<'a, 'hir> LoweringContext<'a, 'hir> {
         match arg {
             ast::GenericArg::Lifetime(lt) => GenericArg::Lifetime(self.lower_lifetime(lt)),
             ast::GenericArg::Type(ty) => {
-                match &ty.kind {
+                // We cannot just match on `TyKind::Infer` as `(_)` is represented as
-                    TyKind::Infer => {
+                // `TyKind::Paren(TyKind::Infer)` and should also be lowered to `GenericArg::Infer`
+                if ty.is_maybe_parenthesised_infer() {
                     return GenericArg::Infer(hir::InferArg {
                         hir_id: self.lower_node_id(ty.id),
                         span: self.lower_span(ty.span),
                     });
                 }
+
+                match &ty.kind {
                     // We parse const arguments as path types as we cannot distinguish them during
                     // parsing. We try to resolve that ambiguity by attempting resolution in both the
                     // type and value namespaces. If we resolved the path in the value namespace, we
                     // transform it into a generic const argument.
+                    //
+                    // FIXME: Should we be handling `(PATH_TO_CONST)`?
                     TyKind::Path(None, path) => {
                         if let Some(res) = self
                             .resolver

tests/ui/const-generics/generic_arg_infer/parend_infer.rs

@@ -0,0 +1,12 @@
+//@ check-pass
+//@ revisions: gate nogate
+#![cfg_attr(gate, feature(generic_arg_infer))]
+
+fn main() {
+    // AST Types preserve parens for pretty printing reasons. This means
+    // that this is parsed as a `TyKind::Paren(TyKind::Infer)`. Generic
+    // arg lowering therefore needs to take into account not just `TyKind::Infer`
+    // but `TyKind::Infer` wrapped in arbitrarily many `TyKind::Paren`.
+    let a: Vec<(_)> = vec![1_u8];
+    let a: Vec<(((((_)))))> = vec![1_u8];
+}