fix: Stop wrapping ConstParam's default values in ConstArg

This was causing ConstParam::default_val to always return None for block
expressions.

CONST_ARG@24..29
  BLOCK_EXPR@24..29
    ...

crates/parser/src/grammar/generic_args.rs

@@ -72,28 +72,24 @@ fn lifetime_arg(p: &mut Parser) {
     m.complete(p, LIFETIME_ARG);
 }
 
-// test const_arg
-// type T = S<92>;
-pub(super) fn const_arg(p: &mut Parser) {
-    let m = p.start();
+pub(super) fn const_arg_content(p: &mut Parser) {
+    // The tests in here are really for `const_arg`, which wraps the content
+    // CONST_ARG.
     match p.current() {
         // test const_arg_block
         // type T = S<{90 + 2}>;
         T!['{'] => {
             expressions::block_expr(p);
-            m.complete(p, CONST_ARG);
         }
         // test const_arg_literal
         // type T = S<"hello", 0xdeadbeef>;
         k if k.is_literal() => {
             expressions::literal(p);
-            m.complete(p, CONST_ARG);
         }
         // test const_arg_bool_literal
         // type T = S<true>;
         T![true] | T![false] => {
             expressions::literal(p);
-            m.complete(p, CONST_ARG);
         }
         // test const_arg_negative_number
         // type T = S<-92>;
@@ -102,19 +98,25 @@ pub(super) fn const_arg(p: &mut Parser) {
             p.bump(T![-]);
             expressions::literal(p);
             lm.complete(p, PREFIX_EXPR);
-            m.complete(p, CONST_ARG);
         }
         // test const_arg_path
-        // struct S<const N: u32 = u32::MAX>;
+        // type T = S<u32::MAX>;
         _ => {
             let lm = p.start();
             paths::use_path(p);
             lm.complete(p, PATH_EXPR);
-            m.complete(p, CONST_ARG);
         }
     }
 }
 
+// test const_arg
+// type T = S<92>;
+pub(super) fn const_arg(p: &mut Parser) {
+    let m = p.start();
+    const_arg_content(p);
+    m.complete(p, CONST_ARG);
+}
+
 fn type_arg(p: &mut Parser) {
     let m = p.start();
     types::type_(p);

crates/parser/src/grammar/generic_params.rs

@@ -79,10 +79,13 @@ fn const_param(p: &mut Parser, m: Marker) {
     }
 
     if p.at(T![=]) {
-        // test const_param_defaults
+        // test const_param_default_literal
         // struct A<const N: i32 = -1>;
         p.bump(T![=]);
-        generic_args::const_arg(p);
+
+        // test const_param_default_expression
+        // struct A<const N: i32 = { 1 }>;
+        generic_args::const_arg_content(p);
     }
 
     m.complete(p, CONST_PARAM);

crates/parser/test_data/parser/inline/ok/0188_const_arg_path.rast

@@ -1,37 +1,30 @@
 SOURCE_FILE
-  STRUCT
-    STRUCT_KW "struct"
+  TYPE_ALIAS
+    TYPE_KW "type"
     WHITESPACE " "
     NAME
-      IDENT "S"
-    GENERIC_PARAM_LIST
-      L_ANGLE "<"
-      CONST_PARAM
-        CONST_KW "const"
-        WHITESPACE " "
-        NAME
-          IDENT "N"
-        COLON ":"
-        WHITESPACE " "
-        PATH_TYPE
-          PATH
-            PATH_SEGMENT
-              NAME_REF
-                IDENT "u32"
-        WHITESPACE " "
-        EQ "="
-        WHITESPACE " "
-        CONST_ARG
-          PATH_EXPR
-            PATH
-              PATH
-                PATH_SEGMENT
-                  NAME_REF
-                    IDENT "u32"
-              COLON2 "::"
-              PATH_SEGMENT
-                NAME_REF
-                  IDENT "MAX"
-      R_ANGLE ">"
+      IDENT "T"
+    WHITESPACE " "
+    EQ "="
+    WHITESPACE " "
+    PATH_TYPE
+      PATH
+        PATH_SEGMENT
+          NAME_REF
+            IDENT "S"
+          GENERIC_ARG_LIST
+            L_ANGLE "<"
+            TYPE_ARG
+              PATH_TYPE
+                PATH
+                  PATH
+                    PATH_SEGMENT
+                      NAME_REF
+                        IDENT "u32"
+                  COLON2 "::"
+                  PATH_SEGMENT
+                    NAME_REF
+                      IDENT "MAX"
+            R_ANGLE ">"
     SEMICOLON ";"
   WHITESPACE "\n"

crates/parser/test_data/parser/inline/ok/0188_const_arg_path.rs

@@ -1 +1 @@
-struct S<const N: u32 = u32::MAX>;
+type T = S<u32::MAX>;

crates/parser/test_data/parser/inline/ok/0199_const_param_default_expression.rast

@@ -0,0 +1,34 @@
+SOURCE_FILE
+  STRUCT
+    STRUCT_KW "struct"
+    WHITESPACE " "
+    NAME
+      IDENT "A"
+    GENERIC_PARAM_LIST
+      L_ANGLE "<"
+      CONST_PARAM
+        CONST_KW "const"
+        WHITESPACE " "
+        NAME
+          IDENT "N"
+        COLON ":"
+        WHITESPACE " "
+        PATH_TYPE
+          PATH
+            PATH_SEGMENT
+              NAME_REF
+                IDENT "i32"
+        WHITESPACE " "
+        EQ "="
+        WHITESPACE " "
+        BLOCK_EXPR
+          STMT_LIST
+            L_CURLY "{"
+            WHITESPACE " "
+            LITERAL
+              INT_NUMBER "1"
+            WHITESPACE " "
+            R_CURLY "}"
+      R_ANGLE ">"
+    SEMICOLON ";"
+  WHITESPACE "\n"

crates/parser/test_data/parser/inline/ok/0199_const_param_default_expression.rs

@@ -0,0 +1 @@
+struct A<const N: i32 = { 1 }>;

crates/parser/test_data/parser/inline/ok/0200_const_param_default_literal.rast

@@ -21,11 +21,10 @@ SOURCE_FILE
         WHITESPACE " "
         EQ "="
         WHITESPACE " "
-        CONST_ARG
-          PREFIX_EXPR
-            MINUS "-"
-            LITERAL
-              INT_NUMBER "1"
+        PREFIX_EXPR
+          MINUS "-"
+          LITERAL
+            INT_NUMBER "1"
       R_ANGLE ">"
     SEMICOLON ";"
   WHITESPACE "\n"