Auto merge of #67216 - ecstatic-morse:const-loop, r=oli-obk

Enable `loop` and `while` in constants behind a feature flag

This PR is an initial implementation of #52000. It adds a `const_loop` feature gate, which allows `while` and `loop` expressions through both HIR and MIR const-checkers if enabled. `for` expressions remain forbidden by the HIR const-checker, since they desugar to a call to `IntoIterator::into_iter`, which will be rejected anyways.

`while` loops also require [`#![feature(const_if_match)]`](#66507), since they have a conditional built into them. The diagnostics from the HIR const checker will suggest this to the user.

r? @oli-obk
cc @rust-lang/wg-const-eval

Author: bors

src/librustc_error_codes/error_codes/E0744.md

@@ -3,7 +3,7 @@ Control-flow expressions are not allowed inside a const context.
 At the moment, `if` and `match`, as well as the looping constructs `for`,
 `while`, and `loop`, are forbidden inside a `const`, `static`, or `const fn`.
 
-```compile_fail,E0744
+```compile_fail,E0658
 const _: i32 = {
     let mut x = 0;
     loop {

src/librustc_feature/active.rs

@@ -52,6 +52,17 @@ macro_rules! declare_features {
             pub fn walk_feature_fields(&self, mut f: impl FnMut(&str, bool)) {
                 $(f(stringify!($feature), self.$feature);)+
             }
+
+            /// Is the given feature enabled?
+            ///
+            /// Panics if the symbol doesn't correspond to a declared feature.
+            pub fn enabled(&self, feature: Symbol) -> bool {
+                match feature {
+                    $( sym::$feature => self.$feature, )*
+
+                    _ => panic!("`{}` was not listed in `declare_features`", feature),
+                }
+            }
         }
     };
 }
@@ -525,6 +536,9 @@ declare_features! (
     /// Allows using `&mut` in constant functions.
     (active, const_mut_refs, "1.41.0", Some(57349), None),
 
+    /// Allows the use of `loop` and `while` in constants.
+    (active, const_loop, "1.41.0", Some(52000), None),
+
     // -------------------------------------------------------------------------
     // feature-group-end: actual feature gates
     // -------------------------------------------------------------------------

src/librustc_mir/transform/check_consts/ops.rs

@@ -170,6 +170,10 @@ impl NonConstOp for LiveDrop {
 #[derive(Debug)]
 pub struct Loop;
 impl NonConstOp for Loop {
+    fn feature_gate(tcx: TyCtxt<'_>) -> Option<bool> {
+        Some(tcx.features().const_loop)
+    }
+
     fn emit_error(&self, item: &Item<'_, '_>, span: Span) {
         // This should be caught by the HIR const-checker.
         item.tcx.sess.delay_span_bug(

src/librustc_mir/transform/qualify_min_const_fn.rs

@@ -390,8 +390,12 @@ fn check_terminator(
             cleanup: _,
         } => check_operand(tcx, cond, span, def_id, body),
 
+        | TerminatorKind::FalseUnwind { .. }
+        if feature_allowed(tcx, def_id, sym::const_loop)
+        => Ok(()),
+
         TerminatorKind::FalseUnwind { .. } => {
             Err((span, "loops are not allowed in const fn".into()))
-        },
+        }
     }
 }

src/librustc_passes/check_const.rs

@@ -13,11 +13,11 @@ use rustc::hir::map::Map;
 use rustc::hir;
 use rustc::ty::TyCtxt;
 use rustc::ty::query::Providers;
-use rustc_feature::Features;
+use rustc::session::config::nightly_options;
 use syntax::ast::Mutability;
 use syntax::feature_gate::feature_err;
 use syntax::span_err;
-use syntax_pos::{sym, Span};
+use syntax_pos::{sym, Span, Symbol};
 use rustc_error_codes::*;
 
 use std::fmt;
@@ -37,18 +37,31 @@ impl NonConstExpr {
         }
     }
 
-    /// Returns `true` if all feature gates required to enable this expression are turned on, or
-    /// `None` if there is no feature gate corresponding to this expression.
-    fn is_feature_gate_enabled(self, features: &Features) -> Option<bool> {
+    fn required_feature_gates(self) -> Option<&'static [Symbol]> {
         use hir::MatchSource::*;
-        match self {
+        use hir::LoopSource::*;
+
+        let gates: &[_] = match self {
             | Self::Match(Normal)
             | Self::Match(IfDesugar { .. })
             | Self::Match(IfLetDesugar { .. })
-            => Some(features.const_if_match),
+            => &[sym::const_if_match],
 
-            _ => None,
-        }
+            | Self::Loop(Loop)
+            => &[sym::const_loop],
+
+            | Self::Loop(While)
+            | Self::Loop(WhileLet)
+            | Self::Match(WhileDesugar)
+            | Self::Match(WhileLetDesugar)
+            => &[sym::const_loop, sym::const_if_match],
+
+            // A `for` loop's desugaring contains a call to `IntoIterator::into_iter`,
+            // so they are not yet allowed with `#![feature(const_loop)]`.
+            _ => return None,
+        };
+
+        Some(gates)
     }
 }
 
@@ -120,11 +133,15 @@ impl<'tcx> CheckConstVisitor<'tcx> {
 
     /// Emits an error when an unsupported expression is found in a const context.
     fn const_check_violated(&self, expr: NonConstExpr, span: Span) {
-        match expr.is_feature_gate_enabled(self.tcx.features()) {
+        let features = self.tcx.features();
+        let required_gates = expr.required_feature_gates();
+        match required_gates {
             // Don't emit an error if the user has enabled the requisite feature gates.
-            Some(true) => return,
+            Some(gates) if gates.iter().all(|&g| features.enabled(g)) => return,
 
-            // Users of `-Zunleash-the-miri-inside-of-you` must use feature gates when possible.
+            // `-Zunleash-the-miri-inside-of-you` only works for expressions that don't have a
+            // corresponding feature gate. This encourages nightly users to use feature gates when
+            // possible.
             None if self.tcx.sess.opts.debugging_opts.unleash_the_miri_inside_of_you => {
                 self.tcx.sess.span_warn(span, "skipping const checks");
                 return;
@@ -135,15 +152,47 @@ impl<'tcx> CheckConstVisitor<'tcx> {
 
         let const_kind = self.const_kind
             .expect("`const_check_violated` may only be called inside a const context");
-
         let msg = format!("`{}` is not allowed in a `{}`", expr.name(), const_kind);
-        match expr {
-            | NonConstExpr::Match(hir::MatchSource::Normal)
-            | NonConstExpr::Match(hir::MatchSource::IfDesugar { .. })
-            | NonConstExpr::Match(hir::MatchSource::IfLetDesugar { .. })
-            => feature_err(&self.tcx.sess.parse_sess, sym::const_if_match, span, &msg).emit(),
 
-            _ => span_err!(self.tcx.sess, span, E0744, "{}", msg),
+        let required_gates = required_gates.unwrap_or(&[]);
+        let missing_gates: Vec<_> = required_gates
+            .iter()
+            .copied()
+            .filter(|&g| !features.enabled(g))
+            .collect();
+
+        match missing_gates.as_slice() {
+            &[] => span_err!(self.tcx.sess, span, E0744, "{}", msg),
+
+            // If the user enabled `#![feature(const_loop)]` but not `#![feature(const_if_match)]`,
+            // explain why their `while` loop is being rejected.
+            &[gate @ sym::const_if_match] if required_gates.contains(&sym::const_loop) => {
+                feature_err(&self.tcx.sess.parse_sess, gate, span, &msg)
+                    .note("`#![feature(const_loop)]` alone is not sufficient, \
+                           since this loop expression contains an implicit conditional")
+                    .emit();
+            }
+
+            &[missing_primary, ref missing_secondary @ ..] => {
+                let mut err = feature_err(&self.tcx.sess.parse_sess, missing_primary, span, &msg);
+
+                // If multiple feature gates would be required to enable this expression, include
+                // them as help messages. Don't emit a separate error for each missing feature gate.
+                //
+                // FIXME(ecstaticmorse): Maybe this could be incorporated into `feature_err`? This
+                // is a pretty narrow case, however.
+                if nightly_options::is_nightly_build() {
+                    for gate in missing_secondary {
+                        let note = format!(
+                            "add `#![feature({})]` to the crate attributes to enable",
+                            gate,
+                        );
+                        err.help(&note);
+                    }
+                }
+
+                err.emit();
+            }
         }
     }
 

src/librustc_passes/lib.rs

@@ -8,6 +8,7 @@
 
 #![feature(in_band_lifetimes)]
 #![feature(nll)]
+#![feature(slice_patterns)]
 
 #![recursion_limit="256"]
 

src/libsyntax_pos/symbol.rs

@@ -213,6 +213,7 @@ symbols! {
         const_indexing,
         const_in_array_repeat_expressions,
         const_let,
+        const_loop,
         const_mut_refs,
         const_panic,
         const_raw_ptr_deref,

src/test/ui/closures/issue-52437.stderr

@@ -4,11 +4,14 @@ error: invalid label name `'static`
 LL |     [(); &(&'static: loop { |x| {}; }) as *const _ as usize]
    |             ^^^^^^^
 
-error[E0744]: `loop` is not allowed in a `const`
+error[E0658]: `loop` is not allowed in a `const`
   --> $DIR/issue-52437.rs:2:13
    |
 LL |     [(); &(&'static: loop { |x| {}; }) as *const _ as usize]
    |             ^^^^^^^^^^^^^^^^^^^^^^^^^
+   |
+   = note: for more information, see https://github.com/rust-lang/rust/issues/52000
+   = help: add `#![feature(const_loop)]` to the crate attributes to enable
 
 error[E0282]: type annotations needed
   --> $DIR/issue-52437.rs:2:30
@@ -18,5 +21,5 @@ LL |     [(); &(&'static: loop { |x| {}; }) as *const _ as usize]
 
 error: aborting due to 3 previous errors
 
-Some errors have detailed explanations: E0282, E0744.
+Some errors have detailed explanations: E0282, E0658.
 For more information about an error, try `rustc --explain E0282`.

src/test/ui/consts/const-eval/infinite_loop.stderr

@@ -1,4 +1,4 @@
-error[E0744]: `while` is not allowed in a `const`
+error[E0658]: `while` is not allowed in a `const`
   --> $DIR/infinite_loop.rs:7:9
    |
 LL | /         while n != 0 {
@@ -8,6 +8,10 @@ LL | |
 LL | |
 LL | |         }
    | |_________^
+   |
+   = note: for more information, see https://github.com/rust-lang/rust/issues/52000
+   = help: add `#![feature(const_loop)]` to the crate attributes to enable
+   = help: add `#![feature(const_if_match)]` to the crate attributes to enable
 
 error[E0658]: `if` is not allowed in a `const`
   --> $DIR/infinite_loop.rs:9:17
@@ -39,5 +43,5 @@ LL |             n = if n % 2 == 0 { n/2 } else { 3*n + 1 };
 
 error: aborting due to 3 previous errors
 
-Some errors have detailed explanations: E0080, E0658, E0744.
+Some errors have detailed explanations: E0080, E0658.
 For more information about an error, try `rustc --explain E0080`.

src/test/ui/consts/const-eval/issue-52442.stderr

@@ -1,8 +1,11 @@
-error[E0744]: `loop` is not allowed in a `const`
+error[E0658]: `loop` is not allowed in a `const`
   --> $DIR/issue-52442.rs:2:14
    |
 LL |     [();  { &loop { break } as *const _ as usize } ];
    |              ^^^^^^^^^^^^^^
+   |
+   = note: for more information, see https://github.com/rust-lang/rust/issues/52000
+   = help: add `#![feature(const_loop)]` to the crate attributes to enable
 
 error[E0658]: casting pointers to integers in constants is unstable
   --> $DIR/issue-52442.rs:2:13
@@ -21,5 +24,5 @@ LL |     [();  { &loop { break } as *const _ as usize } ];
 
 error: aborting due to 3 previous errors
 
-Some errors have detailed explanations: E0080, E0658, E0744.
+Some errors have detailed explanations: E0080, E0658.
 For more information about an error, try `rustc --explain E0080`.