Add inline_const feature flag

compiler/rustc_ast_passes/src/feature_gate.rs

@@ -629,6 +629,7 @@ pub fn check_crate(krate: &ast::Crate, sess: &Session) {
     gate_all!(const_trait_bound_opt_out, "`?const` on trait bounds is experimental");
     gate_all!(const_trait_impl, "const trait impls are experimental");
     gate_all!(half_open_range_patterns, "half-open range patterns are unstable");
+    gate_all!(inline_const, "inline-const is experimental");
 
     // All uses of `gate_all!` below this point were added in #65742,
     // and subsequently disabled (with the non-early gating readded).

compiler/rustc_feature/src/active.rs

@@ -598,6 +598,9 @@ declare_features! (
     /// Allows `#[instruction_set(_)]` attribute
     (active, isa_attribute, "1.48.0", Some(74727), None),
 
+    /// Allow anonymous constants from an inline `const` block
+    (active, inline_const, "1.49.0", Some(76001), None),
+
     // -------------------------------------------------------------------------
     // feature-group-end: actual feature gates
     // -------------------------------------------------------------------------

compiler/rustc_span/src/symbol.rs

@@ -593,6 +593,7 @@ symbols! {
         infer_static_outlives_requirements,
         inlateout,
         inline,
+        inline_const,
         inout,
         instruction_set,
         intel,

src/test/ui/feature-gate-inline_const.rs

@@ -0,0 +1,6 @@
+fn main() {
+    let _ = const {
+        //~^ ERROR expected expression, found keyword `const`
+        true
+    };
+}

src/test/ui/feature-gate-inline_const.stderr

@@ -0,0 +1,8 @@
+error: expected expression, found keyword `const`
+  --> $DIR/feature-gate-inline_const.rs:2:13
+   |
+LL |     let _ = const {
+   |             ^^^^^ expected expression
+
+error: aborting due to previous error
+