Auto merge of #51570 - oli-obk:demotion, r=<try>

Do not promote constants that contain unpromotable code

r? @eddyb

cc @nikomatsakis

I'll add more tests, just getting this out for a crater run

Author: bors

src/librustc_mir/transform/qualify_consts.rs

@@ -566,12 +566,10 @@ impl<'a, 'tcx> Visitor<'tcx> for Qualifier<'a, 'tcx, 'tcx> {
 
                         ProjectionElem::Field(..) |
                         ProjectionElem::Index(_) => {
-                            if this.mode == Mode::Fn {
-                                let base_ty = proj.base.ty(this.mir, this.tcx).to_ty(this.tcx);
-                                if let Some(def) = base_ty.ty_adt_def() {
-                                    if def.is_union() {
-                                        this.not_const();
-                                    }
+                            let base_ty = proj.base.ty(this.mir, this.tcx).to_ty(this.tcx);
+                            if let Some(def) = base_ty.ty_adt_def() {
+                                if def.is_union() {
+                                    this.add(Qualif::NOT_PROMOTABLE);
                                 }
                             } else if this.qualif.intersects(Qualif::STATIC) {
                                 span_err!(this.tcx.sess, this.span, E0494,
@@ -615,6 +613,7 @@ impl<'a, 'tcx> Visitor<'tcx> for Qualifier<'a, 'tcx, 'tcx> {
                 } = constant.literal {
                     // Don't peek inside trait associated constants.
                     if self.tcx.trait_of_item(def_id).is_some() {
+                        self.add(Qualif::NOT_PROMOTABLE);
                         self.add_type(ty);
                     } else {
                         let (bits, _) = self.tcx.at(constant.span).mir_const_qualif(def_id);

src/librustc_passes/rvalue_promotion.rs

@@ -208,6 +208,11 @@ impl<'a, 'tcx> Visitor<'tcx> for CheckCrateVisitor<'a, 'tcx> {
 
         self.visit_body(body);
 
+        if self.promotable {
+            let body_hir_id = tcx.hir.node_to_hir_id(body_id.node_id);
+            assert!(!self.result.insert(body_hir_id.local_id));
+        }
+
         self.in_fn = outer_in_fn;
         self.tables = outer_tables;
         self.param_env = outer_param_env;
@@ -361,17 +366,14 @@ fn check_expr<'a, 'tcx>(v: &mut CheckCrateVisitor<'a, 'tcx>, e: &hir::Expr, node
 
                 Def::Const(did) |
                 Def::AssociatedConst(did) => {
-                    let promotable = if v.tcx.trait_of_item(did).is_some() {
+                    if v.tcx.trait_of_item(did).is_some() {
                         // Don't peek inside trait associated constants.
-                        false
+                        v.promotable = false;
                     } else {
-                        v.tcx.at(e.span).const_is_rvalue_promotable_to_static(did)
-                    };
-
-                    // Just in case the type is more specific than the definition,
-                    // e.g. impl associated const with type parameters, check it.
-                    // Also, trait associated consts are relaxed by this.
-                    v.promotable &= promotable || v.type_has_only_promotable_values(node_ty);
+                        // Just in case the type is more specific than the definition,
+                        // e.g. impl associated const with type parameters, check it.
+                        v.promotable &= v.tcx.at(e.span).const_is_rvalue_promotable_to_static(did);
+                    }
                 }
 
                 _ => {

src/test/ui/const-eval/issue-44578.rs

@@ -20,7 +20,8 @@ enum Bar<A, B> {
 }
 
 impl<A: Foo, B: Foo> Foo for Bar<A, B> {
-    const AMT: usize = [A::AMT][(A::AMT > B::AMT) as usize];
+    const AMT: usize = [A::AMT][(A::AMT > B::AMT) as usize]; //~ ERROR E0080
+    //~^ ERROR E0080
 }
 
 impl Foo for u8 {
@@ -33,6 +34,4 @@ impl Foo for u16 {
 
 fn main() {
     println!("{}", <Bar<u16, u8> as Foo>::AMT);
-    //~^ ERROR erroneous constant used
-    //~| ERROR E0080
 }

src/test/ui/const-eval/issue-44578.stderr

@@ -1,17 +1,20 @@
-error[E0080]: referenced constant
-  --> $DIR/issue-44578.rs:35:20
+error[E0080]: constant evaluation error
+  --> $DIR/issue-44578.rs:23:24
    |
-LL |     const AMT: usize = [A::AMT][(A::AMT > B::AMT) as usize];
-   |                        ------------------------------------ index out of bounds: the len is 1 but the index is 1
-...
-LL |     println!("{}", <Bar<u16, u8> as Foo>::AMT);
-   |                    ^^^^^^^^^^^^^^^^^^^^^^^^^^
+LL |     const AMT: usize = [A::AMT][(A::AMT > B::AMT) as usize]; //~ ERROR E0080
+   |                        ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^ index out of bounds: the len is 1 but the index is 1
 
-error[E0080]: erroneous constant used
-  --> $DIR/issue-44578.rs:35:20
+error[E0080]: constant evaluation error
+  --> $DIR/issue-44578.rs:23:24
    |
-LL |     println!("{}", <Bar<u16, u8> as Foo>::AMT);
-   |                    ^^^^^^^^^^^^^^^^^^^^^^^^^^ referenced constant has errors
+LL |     const AMT: usize = [A::AMT][(A::AMT > B::AMT) as usize]; //~ ERROR E0080
+   |                        ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^ index out of bounds: the len is 1 but the index is 1
+   |
+note: for constant here
+  --> $DIR/issue-44578.rs:14:5
+   |
+LL |     const AMT: usize;
+   |     ^^^^^^^^^^^^^^^^^
 
 error: aborting due to 2 previous errors
 

src/test/ui/const-eval/unpromotable_constant.rs

@@ -0,0 +1,26 @@
+// Copyright 2017 The Rust Project Developers. See the COPYRIGHT
+// file at the top-level directory of this distribution and at
+// http://rust-lang.org/COPYRIGHT.
+//
+// Licensed under the Apache License, Version 2.0 <LICENSE-APACHE or
+// http://www.apache.org/licenses/LICENSE-2.0> or the MIT license
+// <LICENSE-MIT or http://opensource.org/licenses/MIT>, at your
+// option. This file may not be copied, modified, or distributed
+// except according to those terms.
+
+#![allow(const_err)]
+
+union Bad {
+    usize: usize,
+    ptr: &'static u32,
+}
+
+const FOO: usize = unsafe {
+    Bad { ptr: &1 }.usize
+};
+
+
+fn main() {
+    let x: &'static usize = &FOO; //~ ERROR does not live long enough
+    let y: &'static usize = &(FOO % 42); //~ ERROR does not live long enough
+}

src/test/ui/const-eval/unpromotable_constant.stderr

@@ -0,0 +1,24 @@
+error[E0597]: borrowed value does not live long enough
+  --> $DIR/unpromotable_constant.rs:24:30
+   |
+LL |     let x: &'static usize = &FOO; //~ ERROR does not live long enough
+   |                              ^^^ temporary value does not live long enough
+LL |     let y: &'static usize = &(FOO % 42); //~ ERROR does not live long enough
+LL | }
+   | - temporary value only lives until here
+   |
+   = note: borrowed value must be valid for the static lifetime...
+
+error[E0597]: borrowed value does not live long enough
+  --> $DIR/unpromotable_constant.rs:25:30
+   |
+LL |     let y: &'static usize = &(FOO % 42); //~ ERROR does not live long enough
+   |                              ^^^^^^^^^^ temporary value does not live long enough
+LL | }
+   | - temporary value only lives until here
+   |
+   = note: borrowed value must be valid for the static lifetime...
+
+error: aborting due to 2 previous errors
+
+For more information about this error, try `rustc --explain E0597`.