Rollup merge of rust-lang#67781 - cjgillot:passes-const, r=oli-obk

Move `is_min_const_fn` query to librustc_mir.

The only two uses of the associated methods are in `librustc_mir` and
`librustdoc`. Please tell me if there is a better choice.

cc rust-lang#65031

src/librustc/ty/mod.rs

@@ -97,7 +97,6 @@ pub mod cast;
 #[macro_use]
 pub mod codec;
 pub mod _match;
-mod constness;
 mod erase_regions;
 pub mod error;
 pub mod fast_reject;
@@ -3318,7 +3317,6 @@ pub fn provide(providers: &mut ty::query::Providers<'_>) {
     context::provide(providers);
     erase_regions::provide(providers);
     layout::provide(providers);
-    constness::provide(providers);
     *providers = ty::query::Providers {
         asyncness,
         associated_item,

src/librustc_mir/const_eval.rs

@@ -9,10 +9,12 @@ use crate::interpret::{intern_const_alloc_recursive, ConstValue, InterpCx};
 
 mod error;
 mod eval_queries;
+mod fn_queries;
 mod machine;
 
 pub use error::*;
 pub use eval_queries::*;
+pub use fn_queries::*;
 pub use machine::*;
 
 /// Extracts a field of a (variant of a) const.

src/librustc_mir/const_eval/fn_queries.rs

@@ -0,0 +1,151 @@
+use rustc::hir::map::blocks::FnLikeNode;
+use rustc::ty::query::Providers;
+use rustc::ty::TyCtxt;
+use rustc_hir as hir;
+use rustc_hir::def_id::DefId;
+use rustc_span::symbol::Symbol;
+use rustc_target::spec::abi::Abi;
+use syntax::attr;
+
+/// Whether the `def_id` counts as const fn in your current crate, considering all active
+/// feature gates
+pub fn is_const_fn(tcx: TyCtxt<'_>, def_id: DefId) -> bool {
+    tcx.is_const_fn_raw(def_id)
+        && match is_unstable_const_fn(tcx, def_id) {
+            Some(feature_name) => {
+                // has a `rustc_const_unstable` attribute, check whether the user enabled the
+                // corresponding feature gate.
+                tcx.features().declared_lib_features.iter().any(|&(sym, _)| sym == feature_name)
+            }
+            // functions without const stability are either stable user written
+            // const fn or the user is using feature gates and we thus don't
+            // care what they do
+            None => true,
+        }
+}
+
+/// Whether the `def_id` is an unstable const fn and what feature gate is necessary to enable it
+pub fn is_unstable_const_fn(tcx: TyCtxt<'_>, def_id: DefId) -> Option<Symbol> {
+    if tcx.is_const_fn_raw(def_id) {
+        let const_stab = tcx.lookup_const_stability(def_id)?;
+        if const_stab.level.is_unstable() { Some(const_stab.feature) } else { None }
+    } else {
+        None
+    }
+}
+
+/// 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.span_err(
+                            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 provide(providers: &mut Providers<'_>) {
+    /// Const evaluability whitelist is here to check evaluability at the
+    /// top level beforehand.
+    fn is_const_intrinsic(tcx: TyCtxt<'_>, def_id: DefId) -> Option<bool> {
+        match tcx.fn_sig(def_id).abi() {
+            Abi::RustIntrinsic | Abi::PlatformIntrinsic => {
+                Some(tcx.lookup_const_stability(def_id).is_some())
+            }
+            _ => None,
+        }
+    }
+
+    /// Checks whether the function has a `const` modifier or, in case it is an intrinsic, whether
+    /// said intrinsic is on the whitelist for being const callable.
+    fn is_const_fn_raw(tcx: TyCtxt<'_>, def_id: DefId) -> bool {
+        let hir_id = tcx
+            .hir()
+            .as_local_hir_id(def_id)
+            .expect("Non-local call to local provider is_const_fn");
+
+        let node = tcx.hir().get(hir_id);
+
+        if let Some(whitelisted) = is_const_intrinsic(tcx, def_id) {
+            whitelisted
+        } else if let Some(fn_like) = FnLikeNode::from_node(node) {
+            fn_like.constness() == hir::Constness::Const
+        } else if let hir::Node::Ctor(_) = node {
+            true
+        } else {
+            false
+        }
+    }
+
+    fn is_promotable_const_fn(tcx: TyCtxt<'_>, def_id: DefId) -> bool {
+        is_const_fn(tcx, def_id)
+            && match tcx.lookup_const_stability(def_id) {
+                Some(stab) => {
+                    if cfg!(debug_assertions) && stab.promotable {
+                        let sig = tcx.fn_sig(def_id);
+                        assert_eq!(
+                            sig.unsafety(),
+                            hir::Unsafety::Normal,
+                            "don't mark const unsafe fns as promotable",
+                            // https://github.com/rust-lang/rust/pull/53851#issuecomment-418760682
+                        );
+                    }
+                    stab.promotable
+                }
+                None => false,
+            }
+    }
+
+    fn const_fn_is_allowed_fn_ptr(tcx: TyCtxt<'_>, def_id: DefId) -> bool {
+        is_const_fn(tcx, def_id)
+            && tcx
+                .lookup_const_stability(def_id)
+                .map(|stab| stab.allow_const_fn_ptr)
+                .unwrap_or(false)
+    }
+
+    *providers = Providers {
+        is_const_fn_raw,
+        is_promotable_const_fn,
+        const_fn_is_allowed_fn_ptr,
+        ..*providers
+    };
+}

src/librustc_mir/lib.rs

@@ -52,6 +52,7 @@ use rustc::ty::query::Providers;
 
 pub fn provide(providers: &mut Providers<'_>) {
     borrow_check::provide(providers);
+    const_eval::provide(providers);
     shim::provide(providers);
     transform::provide(providers);
     monomorphize::partitioning::provide(providers);

src/librustc_mir/transform/check_consts/validation.rs

@@ -21,6 +21,7 @@ use super::ops::{self, NonConstOp};
 use super::qualifs::{self, HasMutInterior, NeedsDrop};
 use super::resolver::FlowSensitiveAnalysis;
 use super::{is_lang_panic_fn, ConstKind, Item, Qualif};
+use crate::const_eval::{is_const_fn, is_unstable_const_fn};
 use crate::dataflow::{self as old_dataflow, generic as dataflow};
 
 pub type IndirectlyMutableResults<'mir, 'tcx> =
@@ -173,7 +174,7 @@ impl Validator<'a, 'mir, 'tcx> {
         let Item { tcx, body, def_id, const_kind, .. } = *self.item;
 
         let use_min_const_fn_checks = (const_kind == Some(ConstKind::ConstFn)
-            && tcx.is_min_const_fn(def_id))
+            && crate::const_eval::is_min_const_fn(tcx, def_id))
             && !tcx.sess.opts.debugging_opts.unleash_the_miri_inside_of_you;
 
         if use_min_const_fn_checks {
@@ -560,13 +561,13 @@ impl Visitor<'tcx> for Validator<'_, 'mir, 'tcx> {
                 };
 
                 // At this point, we are calling a function whose `DefId` is known...
-                if self.tcx.is_const_fn(def_id) {
+                if is_const_fn(self.tcx, def_id) {
                     return;
                 }
 
                 if is_lang_panic_fn(self.tcx, def_id) {
                     self.check_op(ops::Panic);
-                } else if let Some(feature) = self.tcx.is_unstable_const_fn(def_id) {
+                } else if let Some(feature) = is_unstable_const_fn(self.tcx, def_id) {
                     // Exempt unstable const fns inside of macros with
                     // `#[allow_internal_unstable]`.
                     if !self.span.allows_unstable(feature) {

src/librustc_mir/transform/check_unsafety.rs

@@ -14,6 +14,7 @@ use rustc_span::symbol::{sym, Symbol};
 
 use std::ops::Bound;
 
+use crate::const_eval::{is_const_fn, is_min_const_fn};
 use crate::util;
 
 use rustc_error_codes::*;
@@ -523,7 +524,7 @@ fn unsafety_check_result(tcx: TyCtxt<'_>, def_id: DefId) -> UnsafetyCheckResult
     let id = tcx.hir().as_local_hir_id(def_id).unwrap();
     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(def_id), tcx.is_min_const_fn(def_id)),
+        hir::BodyOwnerKind::Fn => (is_const_fn(tcx, def_id), is_min_const_fn(tcx, def_id)),
         hir::BodyOwnerKind::Const | hir::BodyOwnerKind::Static(_) => (true, false),
     };
     let mut checker = UnsafetyChecker::new(const_context, min_const_fn, body, tcx, param_env);