Add feature gates for for and ? in consts

Author: jonas-schievink

compiler/rustc_feature/src/active.rs

@@ -677,6 +677,12 @@ declare_features! (
     /// Allows `#[derive(Default)]` and `#[default]` on enums.
     (active, derive_default_enum, "1.56.0", Some(86985), None),
 
+    /// Allows `for _ in _` loops in const contexts.
+    (active, const_for, "1.55.0", None, None),
+
+    /// Allows the `?` operator in const contexts.
+    (active, const_try, "1.55.0", None, None),
+
     // -------------------------------------------------------------------------
     // feature-group-end: actual feature gates
     // -------------------------------------------------------------------------

compiler/rustc_passes/src/check_const.rs

@@ -40,13 +40,14 @@ impl NonConstExpr {
         use hir::MatchSource::*;
 
         let gates: &[_] = match self {
-            // A `for` loop's desugaring contains a call to `IntoIterator::into_iter`,
-            // so they are not yet allowed.
-            // Likewise, `?` desugars to a call to `Try::into_result`.
-            Self::Loop(ForLoop) | Self::Match(ForLoopDesugar | TryDesugar | AwaitDesugar) => {
+            Self::Match(AwaitDesugar) => {
                 return None;
             }
 
+            Self::Loop(ForLoop) | Self::Match(ForLoopDesugar) => &[sym::const_for],
+
+            Self::Match(TryDesugar) => &[sym::const_try],
+
             Self::Match(IfLetGuardDesugar) => bug!("`if let` guard outside a `match` expression"),
 
             // All other expressions are allowed.

compiler/rustc_span/src/symbol.rs

@@ -410,6 +410,7 @@ symbols! {
         const_fn_transmute,
         const_fn_union,
         const_fn_unsize,
+        const_for,
         const_format_args,
         const_generic_defaults,
         const_generics,
@@ -432,6 +433,7 @@ symbols! {
         const_trait_bound_opt_out,
         const_trait_impl,
         const_transmute,
+        const_try,
         constant,
         constructor,
         contents,

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();
+};