Rollup merge of #71688 - ecstatic-morse:const-downcast, r=oli-obk

Allow `Downcast` projections unconditionally in const-checking

`ProjectionElem::Downcast` sounds scary, but it's really just the projection we use to access a particular enum variant. They usually appear in the lowering of a `match` statement, so they have been associated with control flow in const-checking, but they don't do any control flow by themselves. We already have a HIR pass that looks for `if` and `match` (even ones that have 1 or fewer reachable branches). That pass is double-checked by a MIR pass that looks for `SwitchInt`s and `FakeRead`s for match scrutinees. In my opinion, there's no need to look for `Downcast` as well.

r? @oli-obk

src/librustc_mir/transform/check_consts/ops.rs

@@ -53,15 +53,6 @@ pub trait NonConstOp: std::fmt::Debug {
     }
 }
 
-/// A `Downcast` projection.
-#[derive(Debug)]
-pub struct Downcast;
-impl NonConstOp for Downcast {
-    fn feature_gate() -> Option<Symbol> {
-        Some(sym::const_if_match)
-    }
-}
-
 /// A function call where the callee is a pointer.
 #[derive(Debug)]
 pub struct FnCallIndirect;

src/librustc_mir/transform/check_consts/validation.rs

@@ -472,6 +472,7 @@ impl Visitor<'tcx> for Validator<'mir, 'tcx> {
             }
 
             ProjectionElem::ConstantIndex { .. }
+            | ProjectionElem::Downcast(..)
             | ProjectionElem::Subslice { .. }
             | ProjectionElem::Field(..)
             | ProjectionElem::Index(_) => {
@@ -484,10 +485,6 @@ impl Visitor<'tcx> for Validator<'mir, 'tcx> {
                     _ => {}
                 }
             }
-
-            ProjectionElem::Downcast(..) => {
-                self.check_op(ops::Downcast);
-            }
         }
     }
 

src/librustc_mir/transform/qualify_min_const_fn.rs

@@ -286,11 +286,6 @@ fn check_place(
     while let &[ref proj_base @ .., elem] = cursor {
         cursor = proj_base;
         match elem {
-            ProjectionElem::Downcast(..) if !feature_allowed(tcx, def_id, sym::const_if_match) => {
-                return Err((span, "`match` or `if let` in `const fn` is unstable".into()));
-            }
-            ProjectionElem::Downcast(_symbol, _variant_index) => {}
-
             ProjectionElem::Field(..) => {
                 let base_ty = Place::ty_from(place.local, &proj_base, body, tcx).ty;
                 if let Some(def) = base_ty.ty_adt_def() {
@@ -303,6 +298,7 @@ fn check_place(
                 }
             }
             ProjectionElem::ConstantIndex { .. }
+            | ProjectionElem::Downcast(..)
             | ProjectionElem::Subslice { .. }
             | ProjectionElem::Deref
             | ProjectionElem::Index(_) => {}

src/test/compile-fail/consts/const-fn-error.rs

@@ -10,7 +10,6 @@ const fn f(x: usize) -> usize {
         //~| ERROR E0658
         //~| ERROR E0080
         //~| ERROR E0744
-        //~| ERROR E0019
         sum += i;
     }
     sum

src/test/compile-fail/issue-52443.rs

@@ -11,6 +11,5 @@ fn main() {
     //~| ERROR calls in constants are limited to constant functions
     //~| ERROR references in constants may only refer to immutable values
     //~| ERROR calls in constants are limited to constant functions
-    //~| ERROR constant contains unimplemented expression type
     //~| ERROR evaluation of constant value failed
 }

src/test/ui/consts/control-flow/feature-gate-const-if-match.rs

@@ -113,6 +113,5 @@ fn main() { //[if_match]~ ERROR fatal error triggered by #[rustc_error]
         //[stock]~^ ERROR `match` is not allowed in a `const`
         if let Some(x) = Some(x) { x } else { 1 }
         //[stock]~^ ERROR `if` is not allowed in a `const`
-        //[stock]~| ERROR constant contains unimplemented expression type
     }];
 }

src/test/ui/consts/control-flow/feature-gate-const-if-match.stock.stderr

@@ -237,13 +237,6 @@ LL |     const MATCH: i32 = match 0 { 1 => 2, _ => 0 };
    = note: see issue #49146 <https://github.com/rust-lang/rust/issues/49146> for more information
    = help: add `#![feature(const_if_match)]` to the crate attributes to enable
 
-error[E0019]: constant contains unimplemented expression type
-  --> $DIR/feature-gate-const-if-match.rs:114:21
-   |
-LL |         if let Some(x) = Some(x) { x } else { 1 }
-   |                     ^
+error: aborting due to 24 previous errors
 
-error: aborting due to 25 previous errors
-
-Some errors have detailed explanations: E0019, E0658.
-For more information about an error, try `rustc --explain E0019`.
+For more information about this error, try `rustc --explain E0658`.

src/test/ui/or-patterns/feature-gate-const-fn.rs

@@ -30,8 +30,6 @@ fn main() {
         let x = Ok(3);
         let Ok(y) | Err(y) = x;
         //~^ ERROR or-pattern is not allowed in a `const`
-        //~| ERROR constant contains unimplemented expression type
-        //~| ERROR constant contains unimplemented expression type
         2
     }];
 }

src/test/ui/or-patterns/feature-gate-const-fn.stderr

@@ -52,19 +52,6 @@ LL |         let Ok(y) | Err(y) = x;
    = note: see issue #49146 <https://github.com/rust-lang/rust/issues/49146> for more information
    = help: add `#![feature(const_if_match)]` to the crate attributes to enable
 
-error[E0019]: constant contains unimplemented expression type
-  --> $DIR/feature-gate-const-fn.rs:31:25
-   |
-LL |         let Ok(y) | Err(y) = x;
-   |                         ^
+error: aborting due to 6 previous errors
 
-error[E0019]: constant contains unimplemented expression type
-  --> $DIR/feature-gate-const-fn.rs:31:16
-   |
-LL |         let Ok(y) | Err(y) = x;
-   |                ^
-
-error: aborting due to 8 previous errors
-
-Some errors have detailed explanations: E0019, E0658.
-For more information about an error, try `rustc --explain E0019`.
+For more information about this error, try `rustc --explain E0658`.