Add feature gates for for and ? in consts

src/test/ui/consts/const-for-feature-gate.rs

@@ -0,0 +1,8 @@
+// gate-test-const_for
+
+const _: () = {
+    for _ in 0..5 {}
+    //~^ error: `for` is not allowed in a `const`
+};
+
+fn main() {}

src/test/ui/consts/const-for-feature-gate.stderr

@@ -0,0 +1,11 @@
+error[E0658]: `for` is not allowed in a `const`
+  --> $DIR/const-for-feature-gate.rs:4:5
+   |
+LL |     for _ in 0..5 {}
+   |     ^^^^^^^^^^^^^^^^
+   |
+   = help: add `#![feature(const_for)]` to the crate attributes to enable
+
+error: aborting due to previous error
+
+For more information about this error, try `rustc --explain E0658`.

src/test/ui/consts/const-try-feature-gate.rs

@@ -0,0 +1,9 @@
+// gate-test-const_try
+
+const fn t() -> Option<()> {
+    Some(())?;
+    //~^ error: `?` is not allowed in a `const fn`
+    None
+}
+
+fn main() {}

src/test/ui/consts/const-try-feature-gate.stderr

@@ -0,0 +1,11 @@
+error[E0658]: `?` is not allowed in a `const fn`
+  --> $DIR/const-try-feature-gate.rs:4:5
+   |
+LL |     Some(())?;
+   |     ^^^^^^^^^
+   |
+   = help: add `#![feature(const_try)]` to the crate attributes to enable
+
+error: aborting due to previous error
+
+For more information about this error, try `rustc --explain E0658`.

src/test/ui/consts/const-try.rs

@@ -0,0 +1,39 @@
+// check-pass
+
+// Demonstrates what's needed to make use of `?` in const contexts.
+
+#![crate_type = "lib"]
+#![feature(try_trait_v2)]
+#![feature(const_trait_impl)]
+#![feature(const_try)]
+
+use std::ops::{ControlFlow, FromResidual, Try};
+
+struct TryMe;
+struct Error;
+
+impl const FromResidual<Error> for TryMe {
+    fn from_residual(residual: Error) -> Self {
+        TryMe
+    }
+}
+
+impl const Try for TryMe {
+    type Output = ();
+    type Residual = Error;
+    fn from_output(output: Self::Output) -> Self {
+        TryMe
+    }
+    fn branch(self) -> ControlFlow<Self::Residual, Self::Output> {
+        ControlFlow::Break(Error)
+    }
+}
+
+const fn t() -> TryMe {
+    TryMe?;
+    TryMe
+}
+
+const _: () = {
+    t();
+};