Allow reading non-mutable statics in const prop

src/librustc_mir/interpret/machine.rs

@@ -194,10 +194,9 @@ pub trait Machine<'mir, 'tcx>: Sized {
         frame.locals[local].access()
     }
 
-    /// Called before a `StaticKind::Static` value is read.
-    fn before_eval_static(
-        _ecx: &InterpCx<'mir, 'tcx, Self>,
-        _place_static: &mir::Static<'tcx>,
+    /// Called before a `StaticKind::Static` value is accessed.
+    fn before_access_static(
+        _allocation: &Allocation,
     ) -> InterpResult<'tcx> {
         Ok(())
     }

src/librustc_mir/interpret/memory.rs

@@ -462,6 +462,8 @@ impl<'mir, 'tcx, M: Machine<'mir, 'tcx>> Memory<'mir, 'tcx, M> {
                     // Make sure we use the ID of the resolved memory, not the lazy one!
                     let id = raw_const.alloc_id;
                     let allocation = tcx.alloc_map.lock().unwrap_memory(id);
+
+                    M::before_access_static(allocation)?;
                     Cow::Borrowed(allocation)
                 }
             }

src/librustc_mir/interpret/place.rs

@@ -601,8 +601,6 @@ where
             }
 
             StaticKind::Static => {
-                M::before_eval_static(self, place_static)?;
-
                 let ty = place_static.ty;
                 assert!(!ty.needs_subst());
                 let layout = self.layout_of(ty)?;

src/librustc_mir/transform/const_prop.rs

@@ -8,7 +8,7 @@ use rustc::hir::def::DefKind;
 use rustc::hir::def_id::DefId;
 use rustc::mir::{
     AggregateKind, Constant, Location, Place, PlaceBase, Body, Operand, Rvalue,
-    Local, NullOp, UnOp, StatementKind, Statement, LocalKind, Static,
+    Local, NullOp, UnOp, StatementKind, Statement, LocalKind,
     TerminatorKind, Terminator,  ClearCrossCrate, SourceInfo, BinOp,
     SourceScope, SourceScopeLocalData, LocalDecl, BasicBlock,
 };
@@ -17,6 +17,7 @@ use rustc::mir::visit::{
 };
 use rustc::mir::interpret::{Scalar, InterpResult, PanicInfo};
 use rustc::ty::{self, Instance, ParamEnv, Ty, TyCtxt};
+use syntax::ast::Mutability;
 use syntax_pos::{Span, DUMMY_SP};
 use rustc::ty::subst::InternalSubsts;
 use rustc_data_structures::fx::FxHashMap;
@@ -229,11 +230,16 @@ impl<'mir, 'tcx> interpret::Machine<'mir, 'tcx> for ConstPropMachine {
         l.access()
     }
 
-    fn before_eval_static(
-        _ecx: &InterpCx<'mir, 'tcx, Self>,
-        _place_static: &Static<'tcx>,
+    fn before_access_static(
+        allocation: &Allocation<Self::PointerTag, Self::AllocExtra>,
     ) -> InterpResult<'tcx> {
-        throw_unsup_format!("can't eval statics in ConstProp");
+        // if the static allocation is mutable or if it has relocations (it may be legal to mutate
+        // the memory behind that in the future), then we can't const prop it
+        if allocation.mutability == Mutability::Mutable || allocation.relocations().len() > 0 {
+            throw_unsup_format!("can't eval mutable statics in ConstProp");
+        }
+
+        Ok(())
     }
 
     fn before_terminator(_ecx: &mut InterpCx<'mir, 'tcx, Self>) -> InterpResult<'tcx> {

src/test/mir-opt/const_prop/read_immutable_static.rs

@@ -0,0 +1,27 @@
+static FOO: u8 = 2;
+
+fn main() {
+    let x = FOO + FOO;
+}
+
+// END RUST SOURCE
+// START rustc.main.ConstProp.before.mir
+//  bb0: {
+//      ...
+//      _2 = (FOO: u8);
+//      ...
+//      _3 = (FOO: u8);
+//      _1 = Add(move _2, move _3);
+//      ...
+//  }
+// END rustc.main.ConstProp.before.mir
+// START rustc.main.ConstProp.after.mir
+//  bb0: {
+//      ...
+//      _2 = const 2u8;
+//      ...
+//      _3 = const 2u8;
+//      _1 = Add(move _2, move _3);
+//      ...
+//  }
+// END rustc.main.ConstProp.after.mir