Auto merge of rust-lang#87237 - jonas-schievink:const-for-and-try, r=oli-obk

Add feature gates for `for` and `?` in consts

These operations seems *relatively* straightforward to support, and only seem to be blocked on `impl const Trait`.

I have included a working test for `const_try`, but `const_for` is currently unusable without reimplementing *every single* defaulted `Iterator` method, so I didn't do that.

(both features still need tracking issues before this is merged)

Author: bors

compiler/rustc_error_codes/src/error_codes/E0744.md

@@ -2,25 +2,15 @@ An unsupported expression was used inside a const context.
 
 Erroneous code example:
 
-```compile_fail,E0744
+```compile_fail,edition2018,E0744
 const _: i32 = {
-    let mut x = 0;
-
-    for i in 0..4 { // error!
-        x += i;
-    }
+    async { 0 }.await
 };
 ```
 
-At the moment, `for` loops, `.await`, and the `Try` operator (`?`) are forbidden
-inside a `const`, `static`, or `const fn`.
+At the moment, `.await` is forbidden inside a `const`, `static`, or `const fn`.
 
 This may be allowed at some point in the future, but the implementation is not
-yet complete. See the tracking issues for [`async`] and [`?`] in `const fn`, and
-(to support `for` loops in `const fn`) the tracking issues for [`impl const
-Trait for Ty`] and [`&mut T`] in `const fn`.
+yet complete. See the tracking issue for [`async`] in `const fn`.
 
 [`async`]: https://github.com/rust-lang/rust/issues/69431
-[`?`]: https://github.com/rust-lang/rust/issues/74935
-[`impl const Trait for Ty`]: https://github.com/rust-lang/rust/issues/67792
-[`&mut T`]: https://github.com/rust-lang/rust/issues/57349

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, "1.56.0", Some(87575), None),
+
+    /// Allows the `?` operator in const contexts.
+    (active, const_try, "1.56.0", Some(74935), 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-fn-error.rs

@@ -7,7 +7,7 @@ const fn f(x: usize) -> usize {
         //~| ERROR calls in constant functions
         //~| ERROR calls in constant functions
         //~| ERROR E0080
-        //~| ERROR E0744
+        //~| ERROR `for` is not allowed in a `const fn`
         sum += i;
     }
     sum

src/test/ui/consts/const-fn-error.stderr

@@ -1,4 +1,4 @@
-error[E0744]: `for` is not allowed in a `const fn`
+error[E0658]: `for` is not allowed in a `const fn`
   --> $DIR/const-fn-error.rs:5:5
    |
 LL | /     for i in 0..x {
@@ -9,6 +9,9 @@ LL | |
 LL | |         sum += i;
 LL | |     }
    | |_____^
+   |
+   = note: see issue #87575 <https://github.com/rust-lang/rust/issues/87575> for more information
+   = help: add `#![feature(const_for)]` to the crate attributes to enable
 
 error[E0015]: calls in constant functions are limited to constant functions, tuple structs and tuple variants
   --> $DIR/const-fn-error.rs:5:14
@@ -45,5 +48,5 @@ LL |     let a : [i32; f(X)];
 
 error: aborting due to 5 previous errors
 
-Some errors have detailed explanations: E0015, E0080, E0658, E0744.
+Some errors have detailed explanations: E0015, E0080, E0658.
 For more information about an error, try `rustc --explain E0015`.

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 +1,12 @@
+error[E0658]: `for` is not allowed in a `const`
+  --> $DIR/const-for-feature-gate.rs:4:5
+   |
+LL |     for _ in 0..5 {}
+   |     ^^^^^^^^^^^^^^^^
+   |
+   = note: see issue #87575 <https://github.com/rust-lang/rust/issues/87575> for more information
+   = 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-for.rs

@@ -0,0 +1,10 @@
+#![feature(const_for)]
+#![feature(const_mut_refs)]
+
+const _: () = {
+    for _ in 0..5 {}
+    //~^ error: calls in constants are limited to
+    //~| error: calls in constants are limited to
+};
+
+fn main() {}

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

@@ -0,0 +1,15 @@
+error[E0015]: calls in constants are limited to constant functions, tuple structs and tuple variants
+  --> $DIR/const-for.rs:5:14
+   |
+LL |     for _ in 0..5 {}
+   |              ^^^^
+
+error[E0015]: calls in constants are limited to constant functions, tuple structs and tuple variants
+  --> $DIR/const-for.rs:5:14
+   |
+LL |     for _ in 0..5 {}
+   |              ^^^^
+
+error: aborting due to 2 previous errors
+
+For more information about this error, try `rustc --explain E0015`.