optimize sanity check path printing

During the sanity check, we keep track of the path we are below in a `Vec`.  We
avoid cloning that `Vec` unless we hit a pointer indirection.  The `String`
representation is only computed when validation actually fails.

src/test/ui/consts/const-eval/double_check2.stderr

@@ -5,7 +5,7 @@ LL | / static FOO: (&Foo, &Bar) = unsafe {( //~ undefined behavior
 LL | |     Union { usize: &BAR }.foo,
 LL | |     Union { usize: &BAR }.bar,
 LL | | )};
-   | |___^ type validation failed: encountered 5 at (*.1).TAG, but expected something in the range 42..=99
+   | |___^ type validation failed: encountered 5 at .1.<deref>.<enum-tag>, but expected something in the range 42..=99
    |
    = 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
 

src/test/ui/consts/const-eval/ub-enum-ptr.rs

@@ -1,27 +0,0 @@
-// 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.
-
-#[repr(usize)]
-#[derive(Copy, Clone)]
-enum Enum {
-    A = 0,
-}
-
-union Foo {
-    a: &'static u8,
-    b: Enum,
-}
-
-// A pointer is guaranteed non-null
-const BAD_ENUM: Enum = unsafe { Foo { a: &1 }.b};
-//~^ ERROR this constant likely exhibits undefined behavior
-
-fn main() {
-}

src/test/ui/consts/const-eval/ub-enum-ptr.stderr

@@ -1,11 +0,0 @@
-error[E0080]: this constant likely exhibits undefined behavior
-  --> $DIR/ub-enum-ptr.rs:23:1
-   |
-LL | const BAD_ENUM: Enum = unsafe { Foo { a: &1 }.b};
-   | ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^ type validation failed: encountered pointer at .TAG, but expected something in the range 0..=0
-   |
-   = 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
-
-error: aborting due to previous error
-
-For more information about this error, try `rustc --explain E0080`.

src/test/ui/consts/const-eval/ub-enum.rs

@@ -0,0 +1,49 @@
+// 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.
+
+#[repr(usize)]
+#[derive(Copy, Clone)]
+enum Enum {
+    A = 0,
+}
+union TransmuteEnum {
+    a: &'static u8,
+    b: Enum,
+}
+
+// A pointer is guaranteed non-null
+const BAD_ENUM: Enum = unsafe { TransmuteEnum { a: &1 }.b };
+//~^ ERROR this constant likely exhibits undefined behavior
+
+// Invalid enum discriminant
+#[repr(usize)]
+#[derive(Copy, Clone)]
+enum Enum2 {
+    A = 2,
+}
+union TransmuteEnum2 {
+    a: usize,
+    b: Enum2,
+}
+const BAD_ENUM2 : Enum2 = unsafe { TransmuteEnum2 { a: 0 }.b };
+//~^ ERROR this constant likely exhibits undefined behavior
+
+// Invalid enum field content (mostly to test printing of apths for enum tuple
+// variants and tuples).
+union TransmuteChar {
+    a: u32,
+    b: char,
+}
+// Need to create something which does not clash with enum layout optimizations.
+const BAD_ENUM_CHAR : Option<(char, char)> = Some(('x', unsafe { TransmuteChar { a: !0 }.b }));
+//~^ ERROR this constant likely exhibits undefined behavior
+
+fn main() {
+}

src/test/ui/consts/const-eval/ub-enum.stderr

@@ -0,0 +1,27 @@
+error[E0080]: this constant likely exhibits undefined behavior
+  --> $DIR/ub-enum.rs:22:1
+   |
+LL | const BAD_ENUM: Enum = unsafe { TransmuteEnum { a: &1 }.b };
+   | ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^ type validation failed: encountered pointer at .<enum-tag>, but expected something in the range 0..=0
+   |
+   = 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
+
+error[E0080]: this constant likely exhibits undefined behavior
+  --> $DIR/ub-enum.rs:35:1
+   |
+LL | const BAD_ENUM2 : Enum2 = unsafe { TransmuteEnum2 { a: 0 }.b };
+   | ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^ type validation failed: encountered 0 at .<enum-tag>, but expected something in the range 2..=2
+   |
+   = 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
+
+error[E0080]: this constant likely exhibits undefined behavior
+  --> $DIR/ub-enum.rs:45:1
+   |
+LL | const BAD_ENUM_CHAR : Option<(char, char)> = Some(('x', unsafe { TransmuteChar { a: !0 }.b }));
+   | ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^ type validation failed: encountered character at .Some.0.1, but expected a valid unicode codepoint
+   |
+   = 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
+
+error: aborting due to 3 previous errors
+
+For more information about this error, try `rustc --explain E0080`.

src/test/ui/union-ub-fat-ptr.stderr

@@ -58,7 +58,7 @@ error[E0080]: this constant likely exhibits undefined behavior
   --> $DIR/union-ub-fat-ptr.rs:102:1
    |
 LL | const G: &Trait = &unsafe { BoolTransmute { val: 3 }.bl };
-   | ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^ type validation failed: encountered 3 at .data_ptr, but expected something in the range 0..=1
+   | ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^ type validation failed: encountered 3 at .<deref>, but expected something in the range 0..=1
    |
    = 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
 
@@ -66,7 +66,7 @@ error[E0080]: this constant likely exhibits undefined behavior
   --> $DIR/union-ub-fat-ptr.rs:106:1
    |
 LL | const H: &[bool] = &[unsafe { BoolTransmute { val: 3 }.bl }];
-   | ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^ type validation failed: encountered 3 at .data_ptr[0], but expected something in the range 0..=1
+   | ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^ type validation failed: encountered 3 at .<deref>[0], but expected something in the range 0..=1
    |
    = 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