Auto merge of #52712 - oli-obk:const_eval_cleanups, r=RalfJung

Reintroduce `Undef` and properly check constant value sizes

r? @RalfJung

cc @eddyb

basically all kinds of silent failures that never occurred are assertions now

src/test/compile-fail/const-err4.rs

@@ -16,7 +16,7 @@ union Foo {
 
 enum Bar {
     Boo = [unsafe { Foo { b: () }.a }; 4][3],
-    //~^ ERROR constant evaluation of enum discriminant resulted in non-integer
+    //~^ ERROR could not evaluate enum discriminant
 }
 
 fn main() {

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

@@ -0,0 +1,16 @@
+// Copyright 2018 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.
+
+// compile-pass
+
+const PARSE_BOOL: Option<&'static str> = None;
+static FOO: (Option<&str>, u32) = (PARSE_BOOL, 42);
+
+fn main() {}

src/test/ui/const-eval/union-const-eval-field.rs

@@ -34,7 +34,7 @@ const fn read_field2() -> Field2 {
 }
 
 const fn read_field3() -> Field3 {
-    const FIELD3: Field3 = unsafe { UNION.field3 }; //~ ERROR exhibits undefined behavior
+    const FIELD3: Field3 = unsafe { UNION.field3 }; //~ ERROR cannot be used
     FIELD3
 }
 

src/test/ui/const-eval/union-const-eval-field.stderr

@@ -1,11 +1,10 @@
-error[E0080]: this constant likely exhibits undefined behavior
+error: this constant cannot be used
   --> $DIR/union-const-eval-field.rs:37:5
    |
-LL |     const FIELD3: Field3 = unsafe { UNION.field3 }; //~ ERROR exhibits undefined behavior
-   |     ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^ type validation failed: encountered undefined bytes
+LL |     const FIELD3: Field3 = unsafe { UNION.field3 }; //~ ERROR cannot be used
+   |     ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^ attempted to read undefined bytes
    |
-   = note: The rules on what exactly is undefined behavior aren't clear, so this check might be overzealous. Please open an issue on the rust compiler repository if you believe it should not be considered undefined behavior
+   = note: #[deny(const_err)] on by default
 
 error: aborting due to previous error
 
-For more information about this error, try `rustc --explain E0080`.

src/test/ui/const-eval/union-ice.rs

@@ -20,9 +20,9 @@ union DummyUnion {
 
 const UNION: DummyUnion = DummyUnion { field1: 1065353216 };
 
-const FIELD3: Field3 = unsafe { UNION.field3 }; //~ ERROR this constant likely exhibits undefined
+const FIELD3: Field3 = unsafe { UNION.field3 }; //~ ERROR this constant cannot be used
 
-const FIELD_PATH: Struct = Struct { //~ ERROR this constant likely exhibits undefined behavior
+const FIELD_PATH: Struct = Struct { //~ ERROR this constant cannot be used
     a: 42,
     b: unsafe { UNION.field3 },
 };

src/test/ui/const-eval/union-ice.stderr

@@ -1,21 +1,19 @@
-error[E0080]: this constant likely exhibits undefined behavior
+error: this constant cannot be used
   --> $DIR/union-ice.rs:23:1
    |
-LL | const FIELD3: Field3 = unsafe { UNION.field3 }; //~ ERROR this constant likely exhibits undefined
-   | ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^ type validation failed: encountered undefined bytes
+LL | const FIELD3: Field3 = unsafe { UNION.field3 }; //~ ERROR this constant cannot be used
+   | ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^ attempted to read undefined bytes
    |
-   = note: The rules on what exactly is undefined behavior aren't clear, so this check might be overzealous. Please open an issue on the rust compiler repository if you believe it should not be considered undefined behavior
+   = note: #[deny(const_err)] on by default
 
-error[E0080]: this constant likely exhibits undefined behavior
+error: this constant cannot be used
   --> $DIR/union-ice.rs:25:1
    |
-LL | / const FIELD_PATH: Struct = Struct { //~ ERROR this constant likely exhibits undefined behavior
+LL | / const FIELD_PATH: Struct = Struct { //~ ERROR this constant cannot be used
 LL | |     a: 42,
 LL | |     b: unsafe { UNION.field3 },
 LL | | };
-   | |__^ type validation failed: encountered undefined bytes at .b
-   |
-   = note: The rules on what exactly is undefined behavior aren't clear, so this check might be overzealous. Please open an issue on the rust compiler repository if you believe it should not be considered undefined behavior
+   | |__^ attempted to read undefined bytes
 
 error[E0080]: this constant likely exhibits undefined behavior
   --> $DIR/union-ice.rs:35:1