Organize intrinsics const evaluability checks

src/libcore/intrinsics.rs

@@ -935,6 +935,7 @@ extern "rust-intrinsic" {
     /// }
     /// ```
     #[stable(feature = "rust1", since = "1.0.0")]
+    #[rustc_const_unstable(feature = "const_transmute")]
     pub fn transmute<T, U>(e: T) -> U;
 
     /// Returns `true` if the actual type given as `T` requires drop

src/librustc/ty/constness.rs

@@ -3,6 +3,7 @@ use crate::hir::def_id::DefId;
 use crate::hir;
 use crate::ty::TyCtxt;
 use syntax_pos::symbol::{sym, Symbol};
+use rustc_target::spec::abi::Abi;
 use crate::hir::map::blocks::FnLikeNode;
 use syntax::attr;
 
@@ -68,14 +69,57 @@ impl<'tcx> TyCtxt<'tcx> {
 
 
 pub fn provide(providers: &mut Providers<'_>) {
-    /// only checks whether the function has a `const` modifier
+    fn is_const_intrinsic(tcx: TyCtxt<'_>, def_id: DefId) -> bool {
+        // Const evaluability whitelist is here to check evaluability at the
+        // top level beforehand.
+        match tcx.fn_sig(def_id).abi() {
+            Abi::RustIntrinsic |
+            Abi::PlatformIntrinsic => {
+                match &tcx.item_name(def_id).as_str()[..] {
+                    | "size_of"
+                    | "min_align_of"
+                    | "needs_drop"
+                    | "type_id"
+                    | "bswap"
+                    | "bitreverse"
+                    | "ctpop"
+                    | "cttz"
+                    | "cttz_nonzero"
+                    | "ctlz"
+                    | "ctlz_nonzero"
+                    | "overflowing_add"
+                    | "overflowing_sub"
+                    | "overflowing_mul"
+                    | "unchecked_shl"
+                    | "unchecked_shr"
+                    | "rotate_left"
+                    | "rotate_right"
+                    | "add_with_overflow"
+                    | "sub_with_overflow"
+                    | "mul_with_overflow"
+                    | "saturating_add"
+                    | "saturating_sub"
+                    | "transmute"
+                    => true,
+
+                    _ => false
+                }
+            }
+            _ => false
+        }
+    }
+
+    /// Checks whether the function has a `const` modifier and intrinsics can be promotable in it
     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(fn_like) = FnLikeNode::from_node(node) {
-            fn_like.constness() == hir::Constness::Const
+
+        if is_const_intrinsic(tcx, def_id) {
+            true
+        } 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 {

src/librustc_mir/transform/qualify_consts.rs

@@ -528,51 +528,12 @@ impl Qualif for IsNotPromotable {
         let fn_ty = callee.ty(cx.body, cx.tcx);
         match fn_ty.kind {
             ty::FnDef(def_id, _) => {
-                match cx.tcx.fn_sig(def_id).abi() {
-                    Abi::RustIntrinsic |
-                    Abi::PlatformIntrinsic => {
-                        assert!(!cx.tcx.is_const_fn(def_id));
-                        match &cx.tcx.item_name(def_id).as_str()[..] {
-                            | "size_of"
-                            | "min_align_of"
-                            | "needs_drop"
-                            | "type_id"
-                            | "bswap"
-                            | "bitreverse"
-                            | "ctpop"
-                            | "cttz"
-                            | "cttz_nonzero"
-                            | "ctlz"
-                            | "ctlz_nonzero"
-                            | "wrapping_add"
-                            | "wrapping_sub"
-                            | "wrapping_mul"
-                            | "unchecked_shl"
-                            | "unchecked_shr"
-                            | "rotate_left"
-                            | "rotate_right"
-                            | "add_with_overflow"
-                            | "sub_with_overflow"
-                            | "mul_with_overflow"
-                            | "saturating_add"
-                            | "saturating_sub"
-                            | "transmute"
-                            | "simd_insert"
-                            | "simd_extract"
-                            => return true,
-
-                            _ => {}
-                        }
-                    }
-                    _ => {
-                        let is_const_fn =
-                            cx.tcx.is_const_fn(def_id) ||
-                            cx.tcx.is_unstable_const_fn(def_id).is_some() ||
-                            cx.is_const_panic_fn(def_id);
-                        if !is_const_fn {
-                            return true;
-                        }
-                    }
+                let is_const_fn =
+                    cx.tcx.is_const_fn(def_id) ||
+                    cx.tcx.is_unstable_const_fn(def_id).is_some() ||
+                    cx.is_const_panic_fn(def_id);
+                if !is_const_fn {
+                    return true;
                 }
             }
             _ => return true,
@@ -923,6 +884,68 @@ impl<'a, 'tcx> Checker<'a, 'tcx> {
         }
     }
 
+    /// Check fn in const context with supplied feature gates
+    fn check_fn_in_const_ctx(&mut self, def_id: DefId) -> () {
+        // Only in non-const functions it is perfectly fine to call any
+        // function, even ones whose constness is unstable. Everywhere else
+        // we need to check the appropriate feature gates.
+        if self.mode.requires_const_checking() {
+            let unleash_miri = self
+                .tcx
+                .sess
+                .opts
+                .debugging_opts
+                .unleash_the_miri_inside_of_you;
+            if self.tcx.is_const_fn(def_id) || unleash_miri {
+                // stable const fns or unstable const fns
+                // with their feature gate active
+                // FIXME(eddyb) move stability checks from `is_const_fn` here.
+            } else if self.is_const_panic_fn(def_id) {
+                // Check the const_panic feature gate.
+                // FIXME: cannot allow this inside `allow_internal_unstable`
+                // because that would make `panic!` insta stable in constants,
+                // since the macro is marked with the attribute.
+                if !self.tcx.features().const_panic {
+                    // Don't allow panics in constants without the feature gate.
+                    emit_feature_err(
+                        &self.tcx.sess.parse_sess,
+                        sym::const_panic,
+                        self.span,
+                        GateIssue::Language,
+                        &format!("panicking in {}s is unstable", self.mode),
+                    );
+                }
+            } else if let Some(feature)
+            = self.tcx.is_unstable_const_fn(def_id) {
+                // Check `#[unstable]` const fns or `#[rustc_const_unstable]`
+                // functions without the feature gate active in this crate in
+                // order to report a better error message than the one below.
+                if !self.span.allows_unstable(feature) {
+                    let mut err = self.tcx.sess.struct_span_err(self.span,
+                        &format!("`{}` is not yet stable as a const fn",
+                            self.tcx.def_path_str(def_id)));
+                    if nightly_options::is_nightly_build() {
+                        help!(&mut err,
+                              "add `#![feature({})]` to the \
+                               crate attributes to enable",
+                              feature);
+                    }
+                    err.emit();
+                }
+            } else {
+                let mut err = struct_span_err!(
+                    self.tcx.sess,
+                    self.span,
+                    E0015,
+                    "calls in {}s are limited to constant functions, \
+                     tuple structs and tuple variants",
+                    self.mode,
+                );
+                err.emit();
+            }
+        }
+    }
+
     /// Check a whole const, static initializer or const fn.
     fn check_const(&mut self) -> (u8, &'tcx BitSet<Local>) {
         debug!("const-checking {} {:?}", self.mode, self.def_id);
@@ -1320,92 +1343,20 @@ impl<'a, 'tcx> Visitor<'tcx> for Checker<'a, 'tcx> {
                 ty::FnDef(def_id, _) => {
                     callee_def_id = Some(def_id);
                     match self.tcx.fn_sig(def_id).abi() {
-                        Abi::RustIntrinsic |
                         Abi::PlatformIntrinsic => {
-                            assert!(!self.tcx.is_const_fn(def_id));
                             match &self.tcx.item_name(def_id).as_str()[..] {
-                                // special intrinsic that can be called diretly without an intrinsic
-                                // feature gate needs a language feature gate
-                                "transmute" => {
-                                    if self.mode.requires_const_checking() {
-                                        // const eval transmute calls only with the feature gate
-                                        if !self.tcx.features().const_transmute {
-                                            emit_feature_err(
-                                                &self.tcx.sess.parse_sess, sym::const_transmute,
-                                                self.span, GateIssue::Language,
-                                                &format!("The use of std::mem::transmute() \
-                                                is gated in {}s", self.mode));
-                                        }
-                                    }
-                                }
-
                                 name if name.starts_with("simd_shuffle") => {
                                     is_shuffle = true;
                                 }
-
-                                // no need to check feature gates, intrinsics are only callable
-                                // from the libstd or with forever unstable feature gates
-                                _ => {}
-                            }
-                        }
-                        _ => {
-                            // In normal functions no calls are feature-gated.
-                            if self.mode.requires_const_checking() {
-                                let unleash_miri = self
-                                    .tcx
-                                    .sess
-                                    .opts
-                                    .debugging_opts
-                                    .unleash_the_miri_inside_of_you;
-                                if self.tcx.is_const_fn(def_id) || unleash_miri {
-                                    // stable const fns or unstable const fns
-                                    // with their feature gate active
-                                    // FIXME(eddyb) move stability checks from `is_const_fn` here.
-                                } else if self.is_const_panic_fn(def_id) {
-                                    // Check the const_panic feature gate.
-                                    // FIXME: cannot allow this inside `allow_internal_unstable`
-                                    // because that would make `panic!` insta stable in constants,
-                                    // since the macro is marked with the attribute.
-                                    if !self.tcx.features().const_panic {
-                                        // Don't allow panics in constants without the feature gate.
-                                        emit_feature_err(
-                                            &self.tcx.sess.parse_sess,
-                                            sym::const_panic,
-                                            self.span,
-                                            GateIssue::Language,
-                                            &format!("panicking in {}s is unstable", self.mode),
-                                        );
-                                    }
-                                } else if let Some(feature)
-                                              = self.tcx.is_unstable_const_fn(def_id) {
-                                    // Check `#[unstable]` const fns or `#[rustc_const_unstable]`
-                                    // functions without the feature gate active in this crate in
-                                    // order to report a better error message than the one below.
-                                    if !self.span.allows_unstable(feature) {
-                                        let mut err = self.tcx.sess.struct_span_err(self.span,
-                                            &format!("`{}` is not yet stable as a const fn",
-                                                    self.tcx.def_path_str(def_id)));
-                                        if nightly_options::is_nightly_build() {
-                                            help!(&mut err,
-                                                  "add `#![feature({})]` to the \
-                                                   crate attributes to enable",
-                                                  feature);
-                                        }
-                                        err.emit();
-                                    }
-                                } else {
-                                    let mut err = struct_span_err!(
-                                        self.tcx.sess,
-                                        self.span,
-                                        E0015,
-                                        "calls in {}s are limited to constant functions, \
-                                         tuple structs and tuple variants",
-                                        self.mode,
-                                    );
-                                    err.emit();
+                                _ => {
+                                    // check for other platform intrinsics which
+                                    // may not be usable with their unstable feature gates
+                                    // in const fn.
+                                    self.check_fn_in_const_ctx(def_id)
                                 }
                             }
                         }
+                        _ => self.check_fn_in_const_ctx(def_id)
                     }
                 }
                 ty::FnPtr(_) => {

src/libsyntax/feature_gate/active.rs

@@ -429,10 +429,6 @@ declare_features! (
     /// Allows using `#[doc(keyword = "...")]`.
     (active, doc_keyword, "1.28.0", Some(51315), None),
 
-    /// Allows reinterpretation of the bits of a value of one type as another
-    /// type during const eval.
-    (active, const_transmute, "1.29.0", Some(53605), None),
-
     /// Allows using `try {...}` expressions.
     (active, try_blocks, "1.29.0", Some(31436), None),
 

src/test/ui/consts/const-eval/const-eval-intrinsic-promotion.rs

@@ -0,0 +1,5 @@
+#![feature(core_intrinsics)]
+fn main() {
+    let x: &'static usize =
+        &std::intrinsics::size_of::<i32>(); //~ ERROR temporary value dropped while borrowed
+}

src/test/ui/consts/const-eval/const-eval-intrinsic-promotion.stderr

@@ -0,0 +1,13 @@
+error[E0716]: temporary value dropped while borrowed
+  --> $DIR/const-eval-intrinsic-promotion.rs:4:10
+   |
+LL |     let x: &'static usize =
+   |            -------------- type annotation requires that borrow lasts for `'static`
+LL |         &std::intrinsics::size_of::<i32>();
+   |          ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^ creates a temporary which is freed while still in use
+LL | }
+   | - temporary value is freed at the end of this statement
+
+error: aborting due to previous error
+
+For more information about this error, try `rustc --explain E0716`.