[BETA]: parse array lengths without stripping const blocks

compiler/rustc_parse/src/parser/expr.rs

@@ -1622,16 +1622,8 @@ impl<'a> Parser<'a> {
             let first_expr = self.parse_expr()?;
             if self.eat(exp!(Semi)) {
                 // Repeating array syntax: `[ 0; 512 ]`
-                let count = if self.eat_keyword(exp!(Const)) {
-                    // While we could just disambiguate `Direct` from `AnonConst` by
-                    // treating all const block exprs as `AnonConst`, that would
-                    // complicate the DefCollector and likely all other visitors.
-                    // So we strip the const blockiness and just store it as a block
-                    // in the AST with the extra disambiguator on the AnonConst
-                    self.parse_mgca_const_block(false)?
-                } else {
-                    self.parse_expr_anon_const(|this, expr| this.mgca_direct_lit_hack(expr))?
-                };
+                let count =
+                    self.parse_expr_anon_const(|this, expr| this.mgca_direct_lit_hack(expr))?;
                 self.expect(close)?;
                 ExprKind::Repeat(first_expr, count)
             } else if self.eat(exp!(Comma)) {

compiler/rustc_parse/src/parser/ty.rs

@@ -658,16 +658,8 @@ impl<'a> Parser<'a> {
         };
 
         let ty = if self.eat(exp!(Semi)) {
-            let mut length = if self.eat_keyword(exp!(Const)) {
-                // While we could just disambiguate `Direct` from `AnonConst` by
-                // treating all const block exprs as `AnonConst`, that would
-                // complicate the DefCollector and likely all other visitors.
-                // So we strip the const blockiness and just store it as a block
-                // in the AST with the extra disambiguator on the AnonConst
-                self.parse_mgca_const_block(false)?
-            } else {
-                self.parse_expr_anon_const(|this, expr| this.mgca_direct_lit_hack(expr))?
-            };
+            let mut length =
+                self.parse_expr_anon_const(|this, expr| this.mgca_direct_lit_hack(expr))?;
 
             if let Err(e) = self.expect(exp!(CloseBracket)) {
                 // Try to recover from `X<Y, ...>` when `X::<Y, ...>` works

src/tools/rustfmt/tests/source/issue-6788.rs

@@ -0,0 +1,7 @@
+fn foo() {
+    let a = [(); const { let x = 1; x }];
+}
+
+fn foo() {
+    let x = [(); const { 1 }];
+}

src/tools/rustfmt/tests/target/issue-6788.rs

@@ -0,0 +1,10 @@
+fn foo() {
+    let a = [(); const {
+        let x = 1;
+        x
+    }];
+}
+
+fn foo() {
+    let x = [(); const { 1 }];
+}