Auto merge of #53604 - oli-obk:min_const_fn, r=Centril,varkor

Implement the `min_const_fn` feature gate

cc @RalfJung @eddyb

r? @Centril

implements the feature gate for #53555

I added a hack so the `const_fn` feature gate also enables the `min_const_fn` feature gate. This ensures that nightly users of `const_fn` don't have to touch their code at all.

The `min_const_fn` checks are run first, and if they succeeded, the `const_fn` checks are run additionally to ensure we didn't miss anything.

Author: bors

Diff for: src/liballoc/lib.rs

@@ -88,7 +88,8 @@
 #![feature(box_syntax)]
 #![feature(cfg_target_has_atomic)]
 #![feature(coerce_unsized)]
-#![feature(const_fn)]
+#![cfg_attr(stage0, feature(const_fn))]
+#![cfg_attr(not(stage0), feature(min_const_fn))]
 #![feature(core_intrinsics)]
 #![feature(custom_attribute)]
 #![feature(dropck_eyepatch)]

Diff for: src/liballoc/tests/lib.rs

@@ -11,7 +11,8 @@
 #![feature(allocator_api)]
 #![feature(alloc_system)]
 #![feature(box_syntax)]
-#![feature(const_fn)]
+#![cfg_attr(stage0, feature(const_fn))]
+#![cfg_attr(not(stage0), feature(min_const_fn))]
 #![feature(drain_filter)]
 #![feature(exact_size_is_empty)]
 #![feature(pattern)]

Diff for: src/libcore/mem.rs

@@ -285,6 +285,15 @@ pub fn forget<T>(t: T) {
 /// [alignment]: ./fn.align_of.html
 #[inline]
 #[stable(feature = "rust1", since = "1.0.0")]
+#[cfg(not(stage0))]
+pub const fn size_of<T>() -> usize {
+    intrinsics::size_of::<T>()
+}
+
+#[inline]
+#[stable(feature = "rust1", since = "1.0.0")]
+#[cfg(stage0)]
+/// Ceci n'est pas la documentation
 pub const fn size_of<T>() -> usize {
     unsafe { intrinsics::size_of::<T>() }
 }
@@ -334,6 +343,16 @@ pub fn size_of_val<T: ?Sized>(val: &T) -> usize {
 #[inline]
 #[stable(feature = "rust1", since = "1.0.0")]
 #[rustc_deprecated(reason = "use `align_of` instead", since = "1.2.0")]
+#[cfg(not(stage0))]
+pub fn min_align_of<T>() -> usize {
+    intrinsics::min_align_of::<T>()
+}
+
+#[inline]
+#[stable(feature = "rust1", since = "1.0.0")]
+#[rustc_deprecated(reason = "use `align_of` instead", since = "1.2.0")]
+#[cfg(stage0)]
+/// Ceci n'est pas la documentation
 pub fn min_align_of<T>() -> usize {
     unsafe { intrinsics::min_align_of::<T>() }
 }
@@ -376,6 +395,15 @@ pub fn min_align_of_val<T: ?Sized>(val: &T) -> usize {
 /// ```
 #[inline]
 #[stable(feature = "rust1", since = "1.0.0")]
+#[cfg(not(stage0))]
+pub const fn align_of<T>() -> usize {
+    intrinsics::min_align_of::<T>()
+}
+
+#[inline]
+#[stable(feature = "rust1", since = "1.0.0")]
+#[cfg(stage0)]
+/// Ceci n'est pas la documentation
 pub const fn align_of<T>() -> usize {
     unsafe { intrinsics::min_align_of::<T>() }
 }

Diff for: src/librustc/ich/impls_mir.rs

@@ -65,6 +65,7 @@ for mir::UnsafetyViolationKind {
 
         match *self {
             mir::UnsafetyViolationKind::General => {}
+            mir::UnsafetyViolationKind::MinConstFn => {}
             mir::UnsafetyViolationKind::ExternStatic(lint_node_id) |
             mir::UnsafetyViolationKind::BorrowPacked(lint_node_id) => {
                 lint_node_id.hash_stable(hcx, hasher);

Diff for: src/librustc/ich/impls_syntax.rs

@@ -130,7 +130,7 @@ impl_stable_hash_for!(struct ::syntax::attr::Stability {
     level,
     feature,
     rustc_depr,
-    rustc_const_unstable
+    const_stability
 });
 
 impl<'a> HashStable<StableHashingContext<'a>>
@@ -161,7 +161,6 @@ for ::syntax::attr::StabilityLevel {
 }
 
 impl_stable_hash_for!(struct ::syntax::attr::RustcDeprecation { since, reason });
-impl_stable_hash_for!(struct ::syntax::attr::RustcConstUnstable { feature });
 
 
 impl_stable_hash_for!(enum ::syntax::attr::IntType {

Diff for: src/librustc/lib.rs

@@ -42,7 +42,8 @@
 
 #![feature(box_patterns)]
 #![feature(box_syntax)]
-#![feature(const_fn)]
+#![cfg_attr(stage0, feature(const_fn))]
+#![cfg_attr(not(stage0), feature(min_const_fn))]
 #![feature(core_intrinsics)]
 #![feature(drain_filter)]
 #![cfg_attr(windows, feature(libc))]

Diff for: src/librustc/middle/stability.rs

@@ -440,7 +440,7 @@ impl<'a, 'tcx> Index<'tcx> {
                     },
                     feature: Symbol::intern("rustc_private"),
                     rustc_depr: None,
-                    rustc_const_unstable: None,
+                    const_stability: None,
                 });
                 annotator.parent_stab = Some(stability);
             }

Diff for: src/librustc/mir/mod.rs

@@ -2430,6 +2430,8 @@ impl Location {
 #[derive(Copy, Clone, Debug, PartialEq, Eq, Hash, RustcEncodable, RustcDecodable)]
 pub enum UnsafetyViolationKind {
     General,
+    /// unsafety is not allowed at all in min const fn
+    MinConstFn,
     ExternStatic(ast::NodeId),
     BorrowPacked(ast::NodeId),
 }

Diff for: src/librustc/ty/context.rs

@@ -1099,6 +1099,37 @@ impl<'a, 'gcx, 'tcx> TyCtxt<'a, 'gcx, 'tcx> {
         local as usize == global as usize
     }
 
+    /// Returns true if this function must conform to `min_const_fn`
+    pub fn is_min_const_fn(self, def_id: DefId) -> bool {
+        if self.features().staged_api {
+            // some intrinsics are waved through if called inside the
+            // standard library. Users never need to call them directly
+            if let abi::Abi::RustIntrinsic = self.fn_sig(def_id).abi() {
+                assert!(!self.is_const_fn(def_id));
+                match &self.item_name(def_id).as_str()[..] {
+                    | "size_of"
+                    | "min_align_of"
+                    => return true,
+                    _ => {},
+                }
+            }
+            // 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 self.lookup_stability(def_id) {
+                // stable functions with unstable const fn aren't `min_const_fn`
+                Some(&attr::Stability { const_stability: Some(_), .. }) => false,
+                // unstable functions don't need to conform
+                Some(&attr::Stability { ref level, .. }) if level.is_unstable() => false,
+                // everything else needs to conform, because it would be callable from
+                // other `min_const_fn` functions
+                _ => true,
+            }
+        } else {
+            // users enabling the `const_fn` can do what they want
+            !self.sess.features_untracked().const_fn
+        }
+    }
+
     /// Create a type context and call the closure with a `TyCtxt` reference
     /// to the context. The closure enforces that the type context and any interned
     /// value (types, substs, etc.) can only be used while `ty::tls` has a valid

Diff for: src/librustc_mir/hair/cx/mod.rs

@@ -18,7 +18,6 @@ use hair::*;
 
 use rustc_data_structures::indexed_vec::Idx;
 use rustc::hir::def_id::{DefId, LOCAL_CRATE};
-use rustc::hir::map::blocks::FnLikeNode;
 use rustc::hir::Node;
 use rustc::middle::region;
 use rustc::infer::InferCtxt;
@@ -67,10 +66,7 @@ impl<'a, 'gcx, 'tcx> Cx<'a, 'gcx, 'tcx> {
         let constness = match body_owner_kind {
             hir::BodyOwnerKind::Const |
             hir::BodyOwnerKind::Static(_) => hir::Constness::Const,
-            hir::BodyOwnerKind::Fn => {
-                let fn_like = FnLikeNode::from_node(infcx.tcx.hir.get(src_id));
-                fn_like.map_or(hir::Constness::NotConst, |f| f.constness())
-            }
+            hir::BodyOwnerKind::Fn => hir::Constness::NotConst,
         };
 
         let attrs = tcx.hir.attrs(src_id);
@@ -83,7 +79,7 @@ impl<'a, 'gcx, 'tcx> Cx<'a, 'gcx, 'tcx> {
         // Respect -C overflow-checks.
         check_overflow |= tcx.sess.overflow_checks();
 
-        // Constants and const fn's always need overflow checks.
+        // Constants always need overflow checks.
         check_overflow |= constness == hir::Constness::Const;
 
         let lint_level = lint_level_for_hir_id(tcx, src_id);

Diff for: src/librustc_mir/lib.rs

@@ -23,7 +23,6 @@ Rust MIR: a lowered representation of Rust. Also: an experiment!
 #![feature(box_patterns)]
 #![feature(box_syntax)]
 #![feature(crate_visibility_modifier)]
-#![feature(const_fn)]
 #![feature(core_intrinsics)]
 #![feature(decl_macro)]
 #![cfg_attr(stage0, feature(macro_vis_matcher))]

Diff for: src/librustc_mir/transform/check_unsafety.rs

@@ -28,6 +28,7 @@ use util;
 
 pub struct UnsafetyChecker<'a, 'tcx: 'a> {
     mir: &'a Mir<'tcx>,
+    min_const_fn: bool,
     source_scope_local_data: &'a IndexVec<SourceScope, SourceScopeLocalData>,
     violations: Vec<UnsafetyViolation>,
     source_info: SourceInfo,
@@ -38,12 +39,16 @@ pub struct UnsafetyChecker<'a, 'tcx: 'a> {
 }
 
 impl<'a, 'gcx, 'tcx> UnsafetyChecker<'a, 'tcx> {
-    fn new(mir: &'a Mir<'tcx>,
-           source_scope_local_data: &'a IndexVec<SourceScope, SourceScopeLocalData>,
-           tcx: TyCtxt<'a, 'tcx, 'tcx>,
-           param_env: ty::ParamEnv<'tcx>) -> Self {
+    fn new(
+        min_const_fn: bool,
+        mir: &'a Mir<'tcx>,
+        source_scope_local_data: &'a IndexVec<SourceScope, SourceScopeLocalData>,
+        tcx: TyCtxt<'a, 'tcx, 'tcx>,
+        param_env: ty::ParamEnv<'tcx>,
+    ) -> Self {
         Self {
             mir,
+            min_const_fn,
             source_scope_local_data,
             violations: vec![],
             source_info: SourceInfo {
@@ -269,14 +274,22 @@ impl<'a, 'tcx> UnsafetyChecker<'a, 'tcx> {
     fn register_violations(&mut self,
                            violations: &[UnsafetyViolation],
                            unsafe_blocks: &[(ast::NodeId, bool)]) {
+        if self.min_const_fn {
+            for violation in violations {
+                let mut violation = violation.clone();
+                violation.kind = UnsafetyViolationKind::MinConstFn;
+                if !self.violations.contains(&violation) {
+                    self.violations.push(violation)
+                }
+            }
+        }
         let within_unsafe = match self.source_scope_local_data[self.source_info.scope].safety {
             Safety::Safe => {
                 for violation in violations {
                     if !self.violations.contains(violation) {
                         self.violations.push(violation.clone())
                     }
                 }
-
                 false
             }
             Safety::BuiltinUnsafe | Safety::FnUnsafe => true,
@@ -369,6 +382,7 @@ fn unsafety_check_result<'a, 'tcx>(tcx: TyCtxt<'a, 'tcx, 'tcx>, def_id: DefId)
 
     let param_env = tcx.param_env(def_id);
     let mut checker = UnsafetyChecker::new(
+        tcx.is_const_fn(def_id) && tcx.is_min_const_fn(def_id),
         mir, source_scope_local_data, tcx, param_env);
     checker.visit_mir(mir);
 
@@ -478,6 +492,15 @@ pub fn check_unsafety<'a, 'tcx>(tcx: TyCtxt<'a, 'tcx, 'tcx>, def_id: DefId) {
                     .note(&details.as_str()[..])
                     .emit();
             }
+            UnsafetyViolationKind::MinConstFn => {
+                tcx.sess.struct_span_err(
+                    source_info.span,
+                    &format!("{} is unsafe and unsafe operations \
+                            are not allowed in const fn", description))
+                    .span_label(source_info.span, &description.as_str()[..])
+                    .note(&details.as_str()[..])
+                    .emit();
+            }
             UnsafetyViolationKind::ExternStatic(lint_node_id) => {
                 tcx.lint_node_note(SAFE_EXTERN_STATICS,
                               lint_node_id,

Diff for: src/librustc_mir/transform/mod.rs

@@ -36,6 +36,7 @@ pub mod elaborate_drops;
 pub mod add_call_guards;
 pub mod promote_consts;
 pub mod qualify_consts;
+mod qualify_min_const_fn;
 pub mod remove_noop_landing_pads;
 pub mod dump_mir;
 pub mod deaggregator;

Diff for: src/librustc_mir/transform/qualify_consts.rs

@@ -917,9 +917,7 @@ impl<'a, 'tcx> Visitor<'tcx> for Qualifier<'a, 'tcx, 'tcx> {
                         );
                     }
                 } else if let Some(&attr::Stability {
-                    rustc_const_unstable: Some(attr::RustcConstUnstable {
-                        feature: ref feature_name
-                    }),
+                    const_stability: Some(ref feature_name),
                 .. }) = self.tcx.lookup_stability(def_id) {
                     if
                         // feature-gate is not enabled,
@@ -1173,8 +1171,20 @@ impl MirPass for QualifyAndPromoteConstants {
             let (temps, candidates) = {
                 let mut qualifier = Qualifier::new(tcx, def_id, mir, mode);
                 if mode == Mode::ConstFn {
-                    // Enforce a constant-like CFG for `const fn`.
-                    qualifier.qualify_const();
+                    if tcx.is_min_const_fn(def_id) {
+                        // enforce `min_const_fn` for stable const fns
+                        use super::qualify_min_const_fn::is_min_const_fn;
+                        if let Err((span, err)) = is_min_const_fn(tcx, def_id, mir) {
+                            tcx.sess.span_err(span, &err);
+                        } else {
+                            // this should not produce any errors, but better safe than sorry
+                            // FIXME(#53819)
+                            qualifier.qualify_const();
+                        }
+                    } else {
+                        // Enforce a constant-like CFG for `const fn`.
+                        qualifier.qualify_const();
+                    }
                 } else {
                     while let Some((bb, data)) = qualifier.rpo.next() {
                         qualifier.visit_basic_block_data(bb, data);