Auto merge of #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)

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`.

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 +1,12 @@
+error[E0658]: `?` is not allowed in a `const fn`
+  --> $DIR/const-try-feature-gate.rs:4:5
+   |
+LL |     Some(())?;
+   |     ^^^^^^^^^
+   |
+   = note: see issue #74935 <https://github.com/rust-lang/rust/issues/74935> for more information
+   = 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();
+};

src/test/ui/consts/control-flow/loop.stderr

@@ -1,19 +1,25 @@
-error[E0744]: `for` is not allowed in a `const`
+error[E0658]: `for` is not allowed in a `const`
   --> $DIR/loop.rs:53:5
    |
 LL | /     for i in 0..4 {
 LL | |         x += 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[E0744]: `for` is not allowed in a `const`
+error[E0658]: `for` is not allowed in a `const`
   --> $DIR/loop.rs:57:5
    |
 LL | /     for i in 0..4 {
 LL | |         x += 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: aborting due to 2 previous errors
 
-For more information about this error, try `rustc --explain E0744`.
+For more information about this error, try `rustc --explain E0658`.

src/test/ui/consts/control-flow/try.stderr

@@ -1,9 +1,12 @@
-error[E0744]: `?` is not allowed in a `const fn`
+error[E0658]: `?` is not allowed in a `const fn`
   --> $DIR/try.rs:6:5
    |
 LL |     x?;
    |     ^^
+   |
+   = note: see issue #74935 <https://github.com/rust-lang/rust/issues/74935> for more information
+   = 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 E0744`.
+For more information about this error, try `rustc --explain E0658`.

src/test/ui/issues/issue-50582.stderr

@@ -1,8 +1,11 @@
-error[E0744]: `for` is not allowed in a `const`
+error[E0658]: `for` is not allowed in a `const`
   --> $DIR/issue-50582.rs:2:20
    |
 LL |     Vec::<[(); 1 + for x in 0..1 {}]>::new();
    |                    ^^^^^^^^^^^^^^^^
+   |
+   = 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[E0277]: cannot add `()` to `{integer}`
   --> $DIR/issue-50582.rs:2:18
@@ -14,5 +17,5 @@ LL |     Vec::<[(); 1 + for x in 0..1 {}]>::new();
 
 error: aborting due to 2 previous errors
 
-Some errors have detailed explanations: E0277, E0744.
+Some errors have detailed explanations: E0277, E0658.
 For more information about an error, try `rustc --explain E0277`.

src/test/ui/issues/issue-50585.stderr

@@ -1,8 +1,11 @@
-error[E0744]: `for` is not allowed in a `const`
+error[E0658]: `for` is not allowed in a `const`
   --> $DIR/issue-50585.rs:2:18
    |
 LL |     |y: Vec<[(); for x in 0..2 {}]>| {};
    |                  ^^^^^^^^^^^^^^^^
+   |
+   = 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[E0308]: mismatched types
   --> $DIR/issue-50585.rs:2:18
@@ -12,5 +15,5 @@ LL |     |y: Vec<[(); for x in 0..2 {}]>| {};
 
 error: aborting due to 2 previous errors
 
-Some errors have detailed explanations: E0308, E0744.
+Some errors have detailed explanations: E0308, E0658.
 For more information about an error, try `rustc --explain E0308`.

src/test/ui/never_type/issue-52443.stderr

@@ -6,11 +6,14 @@ LL |     [(); {while true {break}; 0}];
    |
    = note: `#[warn(while_true)]` on by default
 
-error[E0744]: `for` is not allowed in a `const`
+error[E0658]: `for` is not allowed in a `const`
   --> $DIR/issue-52443.rs:9:12
    |
 LL |     [(); { for _ in 0usize.. {}; 0}];
    |            ^^^^^^^^^^^^^^^^^^^^
+   |
+   = 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[E0308]: mismatched types
   --> $DIR/issue-52443.rs:2:10
@@ -56,5 +59,5 @@ LL |     [(); { for _ in 0usize.. {}; 0}];
 
 error: aborting due to 6 previous errors; 1 warning emitted
 
-Some errors have detailed explanations: E0015, E0308, E0658, E0744.
+Some errors have detailed explanations: E0015, E0308, E0658.
 For more information about an error, try `rustc --explain E0015`.

src/test/ui/rfc-2632-const-trait-impl/hir-const-check.stderr

@@ -1,9 +1,12 @@
-error[E0744]: `?` is not allowed in a `const fn`
+error[E0658]: `?` is not allowed in a `const fn`
   --> $DIR/hir-const-check.rs:11:9
    |
 LL |         Some(())?;
    |         ^^^^^^^^^
+   |
+   = note: see issue #74935 <https://github.com/rust-lang/rust/issues/74935> for more information
+   = 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 E0744`.
+For more information about this error, try `rustc --explain E0658`.