Rollup merge of #84547 - RalfJung:max_const_fn, r=oli-obk

Get rid of is_min_const_fn

This removes the last trace of the min_const_fn mechanism by making the unsafety checker agnostic about whether something is a min or "non-min" const fn. It seems this distinction was used to disallow some features inside `const fn`, but that is the responsibility of the const checker, not of the unsafety checker. No test seems to even notice this change in the unsafety checker so I guess we are good...

r? `@oli-obk`
Cc https://github.com/rust-lang/rust/issues/84510

3 files changed, 18 insertions, 102 deletions

diff --git a/compiler/rustc_middle/src/mir/query.rs b/compiler/rustc_middle/src/mir/query.rs
index ad3baccf154..fdd874c6f68 100644
--- a/compiler/rustc_middle/src/mir/query.rs
+++ b/compiler/rustc_middle/src/mir/query.rs
@@ -19,10 +19,8 @@ use super::{Field, SourceInfo};
 
 #[derive(Copy, Clone, PartialEq, TyEncodable, TyDecodable, HashStable, Debug)]
 pub enum UnsafetyViolationKind {
-    /// Only permitted in regular `fn`s, prohibited in `const fn`s.
+    /// Unsafe operation outside `unsafe`.
     General,
-    /// Permitted both in `const fn`s and regular `fn`s.
-    GeneralAndConstFn,
     /// Unsafe operation in an `unsafe fn` but outside an `unsafe` block.
     /// Has to be handled as a lint for backwards compatibility.
     UnsafeFn,
diff --git a/compiler/rustc_mir/src/const_eval/fn_queries.rs b/compiler/rustc_mir/src/const_eval/fn_queries.rs
index 8c18dfcb8d0..40419a4d201 100644
--- a/compiler/rustc_mir/src/const_eval/fn_queries.rs
+++ b/compiler/rustc_mir/src/const_eval/fn_queries.rs
@@ -1,4 +1,3 @@
-use rustc_attr as attr;
 use rustc_hir as hir;
 use rustc_hir::def_id::{DefId, LocalDefId};
 use rustc_middle::hir::map::blocks::FnLikeNode;
@@ -34,54 +33,6 @@ pub fn is_unstable_const_fn(tcx: TyCtxt<'_>, def_id: DefId) -> Option<Symbol> {
     }
 }
 
-/// Returns `true` if this function must conform to `min_const_fn`
-pub fn is_min_const_fn(tcx: TyCtxt<'_>, def_id: DefId) -> bool {
-    // Bail out if the signature doesn't contain `const`
-    if !tcx.is_const_fn_raw(def_id) {
-        return false;
-    }
-
-    if tcx.features().staged_api {
-        // In order for a libstd function to be considered min_const_fn
-        // it needs to be stable and have no `rustc_const_unstable` attribute.
-        match tcx.lookup_const_stability(def_id) {
-            // `rustc_const_unstable` functions don't need to conform.
-            Some(&attr::ConstStability { ref level, .. }) if level.is_unstable() => false,
-            None => {
-                if let Some(stab) = tcx.lookup_stability(def_id) {
-                    if stab.level.is_stable() {
-                        tcx.sess.delay_span_bug(
-                            tcx.def_span(def_id),
-                            "stable const functions must have either `rustc_const_stable` or \
-                             `rustc_const_unstable` attribute",
-                        );
-                        // While we errored above, because we don't know if we need to conform, we
-                        // err on the "safe" side and require min_const_fn.
-                        true
-                    } else {
-                        // Unstable functions need not conform to min_const_fn.
-                        false
-                    }
-                } else {
-                    // Internal functions are forced to conform to min_const_fn.
-                    // Annotate the internal function with a const stability attribute if
-                    // you need to use unstable features.
-                    // Note: this is an arbitrary choice that does not affect stability or const
-                    // safety or anything, it just changes whether we need to annotate some
-                    // internal functions with `rustc_const_stable` or with `rustc_const_unstable`
-                    true
-                }
-            }
-            // Everything else needs to conform, because it would be callable from
-            // other `min_const_fn` functions.
-            _ => true,
-        }
-    } else {
-        // users enabling the `const_fn` feature gate can do what they want
-        !tcx.features().const_fn
-    }
-}
-
 pub fn is_parent_const_impl_raw(tcx: TyCtxt<'_>, hir_id: hir::HirId) -> bool {
     let parent_id = tcx.hir().get_parent_did(hir_id);
     if !parent_id.is_top_level_module() { is_const_impl_raw(tcx, parent_id) } else { false }
diff --git a/compiler/rustc_mir/src/transform/check_unsafety.rs b/compiler/rustc_mir/src/transform/check_unsafety.rs
index 09da9b2e4d6..955be8cc81e 100644
--- a/compiler/rustc_mir/src/transform/check_unsafety.rs
+++ b/compiler/rustc_mir/src/transform/check_unsafety.rs
@@ -15,13 +15,10 @@ use rustc_session::lint::Level;
 
 use std::ops::Bound;
 
-use crate::const_eval::is_min_const_fn;
-
 pub struct UnsafetyChecker<'a, 'tcx> {
     body: &'a Body<'tcx>,
     body_did: LocalDefId,
     const_context: bool,
-    min_const_fn: bool,
     violations: Vec<UnsafetyViolation>,
     source_info: SourceInfo,
     tcx: TyCtxt<'tcx>,
@@ -34,21 +31,15 @@ pub struct UnsafetyChecker<'a, 'tcx> {
 impl<'a, 'tcx> UnsafetyChecker<'a, 'tcx> {
     fn new(
         const_context: bool,
-        min_const_fn: bool,
         body: &'a Body<'tcx>,
         body_did: LocalDefId,
         tcx: TyCtxt<'tcx>,
         param_env: ty::ParamEnv<'tcx>,
     ) -> Self {
-        // sanity check
-        if min_const_fn {
-            assert!(const_context);
-        }
         Self {
             body,
             body_did,
             const_context,
-            min_const_fn,
             violations: vec![],
             source_info: SourceInfo::outermost(body.span),
             tcx,
@@ -84,7 +75,7 @@ impl<'a, 'tcx> Visitor<'tcx> for UnsafetyChecker<'a, 'tcx> {
                 let sig = func_ty.fn_sig(self.tcx);
                 if let hir::Unsafety::Unsafe = sig.unsafety() {
                     self.require_unsafe(
-                        UnsafetyViolationKind::GeneralAndConstFn,
+                        UnsafetyViolationKind::General,
                         UnsafetyViolationDetails::CallToUnsafeFunction,
                     )
                 }
@@ -134,7 +125,7 @@ impl<'a, 'tcx> Visitor<'tcx> for UnsafetyChecker<'a, 'tcx> {
                     match self.tcx.layout_scalar_valid_range(def.did) {
                         (Bound::Unbounded, Bound::Unbounded) => {}
                         _ => self.require_unsafe(
-                            UnsafetyViolationKind::GeneralAndConstFn,
+                            UnsafetyViolationKind::General,
                             UnsafetyViolationDetails::InitializingTypeWith,
                         ),
                     }
@@ -213,7 +204,7 @@ impl<'a, 'tcx> Visitor<'tcx> for UnsafetyChecker<'a, 'tcx> {
                 let base_ty = base.ty(self.body, self.tcx).ty;
                 if base_ty.is_unsafe_ptr() {
                     self.require_unsafe(
-                        UnsafetyViolationKind::GeneralAndConstFn,
+                        UnsafetyViolationKind::General,
                         UnsafetyViolationDetails::DerefOfRawPointer,
                     )
                 }
@@ -258,7 +249,7 @@ impl<'a, 'tcx> Visitor<'tcx> for UnsafetyChecker<'a, 'tcx> {
                         );
                     if !nodrop {
                         self.require_unsafe(
-                            UnsafetyViolationKind::GeneralAndConstFn,
+                            UnsafetyViolationKind::General,
                             UnsafetyViolationDetails::AssignToDroppingUnionField,
                         );
                     } else {
@@ -266,7 +257,7 @@ impl<'a, 'tcx> Visitor<'tcx> for UnsafetyChecker<'a, 'tcx> {
                     }
                 } else {
                     self.require_unsafe(
-                        UnsafetyViolationKind::GeneralAndConstFn,
+                        UnsafetyViolationKind::General,
                         UnsafetyViolationDetails::AccessToUnionField,
                     )
                 }
@@ -277,6 +268,9 @@ impl<'a, 'tcx> Visitor<'tcx> for UnsafetyChecker<'a, 'tcx> {
 
 impl<'a, 'tcx> UnsafetyChecker<'a, 'tcx> {
     fn require_unsafe(&mut self, kind: UnsafetyViolationKind, details: UnsafetyViolationDetails) {
+        // Violations can turn out to be `UnsafeFn` during analysis, but they should not start out as such.
+        assert_ne!(kind, UnsafetyViolationKind::UnsafeFn);
+
         let source_info = self.source_info;
         let lint_root = self.body.source_scopes[self.source_info.scope]
             .local_data
@@ -304,8 +298,7 @@ impl<'a, 'tcx> UnsafetyChecker<'a, 'tcx> {
             Safety::Safe => {
                 for violation in violations {
                     match violation.kind {
-                        UnsafetyViolationKind::GeneralAndConstFn
-                        | UnsafetyViolationKind::General => {}
+                        UnsafetyViolationKind::General => {}
                         UnsafetyViolationKind::UnsafeFn => {
                             bug!("`UnsafetyViolationKind::UnsafeFn` in an `Safe` context")
                         }
@@ -334,29 +327,6 @@ impl<'a, 'tcx> UnsafetyChecker<'a, 'tcx> {
                 if !violations.is_empty() {
                     self.used_unsafe.insert(hir_id);
                 }
-                // only some unsafety is allowed in const fn
-                if self.min_const_fn {
-                    for violation in violations {
-                        match violation.kind {
-                            // these unsafe things are stable in const fn
-                            UnsafetyViolationKind::GeneralAndConstFn => {}
-                            // these things are forbidden in const fns
-                            UnsafetyViolationKind::General => {
-                                let mut violation = *violation;
-                                // const fns don't need to be backwards compatible and can
-                                // emit these violations as a hard error instead of a backwards
-                                // compat lint
-                                violation.kind = UnsafetyViolationKind::General;
-                                if !self.violations.contains(&violation) {
-                                    self.violations.push(violation)
-                                }
-                            }
-                            UnsafetyViolationKind::UnsafeFn => bug!(
-                                "`UnsafetyViolationKind::UnsafeFn` in an `ExplicitUnsafe` context"
-                            ),
-                        }
-                    }
-                }
                 true
             }
         };
@@ -394,7 +364,7 @@ impl<'a, 'tcx> UnsafetyChecker<'a, 'tcx> {
                             } else {
                                 continue;
                             };
-                            self.require_unsafe(UnsafetyViolationKind::GeneralAndConstFn, details);
+                            self.require_unsafe(UnsafetyViolationKind::General, details);
                         }
                     }
                 }
@@ -412,7 +382,7 @@ impl<'a, 'tcx> UnsafetyChecker<'a, 'tcx> {
         // Is `callee_features` a subset of `calling_features`?
         if !callee_features.iter().all(|feature| self_features.contains(feature)) {
             self.require_unsafe(
-                UnsafetyViolationKind::GeneralAndConstFn,
+                UnsafetyViolationKind::General,
                 UnsafetyViolationDetails::CallToFunctionWith,
             )
         }
@@ -494,15 +464,12 @@ fn unsafety_check_result<'tcx>(
     let param_env = tcx.param_env(def.did);
 
     let id = tcx.hir().local_def_id_to_hir_id(def.did);
-    let (const_context, min_const_fn) = match tcx.hir().body_owner_kind(id) {
-        hir::BodyOwnerKind::Closure => (false, false),
-        hir::BodyOwnerKind::Fn => {
-            (tcx.is_const_fn_raw(def.did.to_def_id()), is_min_const_fn(tcx, def.did.to_def_id()))
-        }
-        hir::BodyOwnerKind::Const | hir::BodyOwnerKind::Static(_) => (true, false),
+    let const_context = match tcx.hir().body_owner_kind(id) {
+        hir::BodyOwnerKind::Closure => false,
+        hir::BodyOwnerKind::Fn => tcx.is_const_fn_raw(def.did.to_def_id()),
+        hir::BodyOwnerKind::Const | hir::BodyOwnerKind::Static(_) => true,
     };
-    let mut checker =
-        UnsafetyChecker::new(const_context, min_const_fn, body, def.did, tcx, param_env);
+    let mut checker = UnsafetyChecker::new(const_context, body, def.did, tcx, param_env);
     checker.visit_body(&body);
 
     check_unused_unsafe(tcx, def.did, &checker.used_unsafe, &mut checker.inherited_blocks);
@@ -577,7 +544,7 @@ pub fn check_unsafety(tcx: TyCtxt<'_>, def_id: LocalDefId) {
             if unsafe_op_in_unsafe_fn_allowed(tcx, lint_root) { " function or" } else { "" };
 
         match kind {
-            UnsafetyViolationKind::GeneralAndConstFn | UnsafetyViolationKind::General => {
+            UnsafetyViolationKind::General => {
                 // once
                 struct_span_err!(
                     tcx.sess,