Report let bindings and statements as unstable

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

@@ -0,0 +1,30 @@
+// 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.
+
+#![feature(const_let)]
+
+fn main() {}
+
+struct FakeNeedsDrop;
+
+impl Drop for FakeNeedsDrop {
+    fn drop(&mut self) {}
+}
+
+// ok
+const X: FakeNeedsDrop = { let x = FakeNeedsDrop; x };
+
+// error
+const Y: FakeNeedsDrop = { let mut x = FakeNeedsDrop; x = FakeNeedsDrop; x };
+//~^ ERROR constant contains unimplemented expression type
+
+// error
+const Z: () = { let mut x = None; x = Some(FakeNeedsDrop); };
+//~^ ERROR constant contains unimplemented expression type

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

@@ -0,0 +1,15 @@
+error[E0019]: constant contains unimplemented expression type
+  --> $DIR/const_let.rs:25:55
+   |
+LL | const Y: FakeNeedsDrop = { let mut x = FakeNeedsDrop; x = FakeNeedsDrop; x };
+   |                                                       ^
+
+error[E0019]: constant contains unimplemented expression type
+  --> $DIR/const_let.rs:29:35
+   |
+LL | const Z: () = { let mut x = None; x = Some(FakeNeedsDrop); };
+   |                                   ^
+
+error: aborting due to 2 previous errors
+
+For more information about this error, try `rustc --explain E0019`.

src/test/ui/const-fn-error.rs

@@ -14,7 +14,8 @@ const X : usize = 2;
 
 const fn f(x: usize) -> usize {
     let mut sum = 0;
-    //~^ ERROR E0016
+    //~^ let bindings in constant functions are unstable
+    //~| statements in constant functions are unstable
     for i in 0..x {
         //~^ ERROR E0015
         //~| ERROR E0019

src/test/ui/const-fn-error.stderr

@@ -1,23 +1,33 @@
-error[E0016]: blocks in constant functions are limited to items and tail expressions
+error[E0658]: let bindings in constant functions are unstable (see issue #48821)
   --> $DIR/const-fn-error.rs:16:19
    |
 LL |     let mut sum = 0;
    |                   ^
+   |
+   = help: add #![feature(const_let)] to the crate attributes to enable
+
+error[E0658]: statements in constant functions are unstable (see issue #48821)
+  --> $DIR/const-fn-error.rs:16:19
+   |
+LL |     let mut sum = 0;
+   |                   ^
+   |
+   = help: add #![feature(const_let)] to the crate attributes to enable
 
 error[E0015]: calls in constant functions are limited to constant functions, tuple structs and tuple variants
-  --> $DIR/const-fn-error.rs:18:14
+  --> $DIR/const-fn-error.rs:19:14
    |
 LL |     for i in 0..x {
    |              ^^^^
 
 error[E0019]: constant function contains unimplemented expression type
-  --> $DIR/const-fn-error.rs:18:14
+  --> $DIR/const-fn-error.rs:19:14
    |
 LL |     for i in 0..x {
    |              ^^^^
 
 error[E0080]: constant evaluation error
-  --> $DIR/const-fn-error.rs:18:14
+  --> $DIR/const-fn-error.rs:19:14
    |
 LL |     for i in 0..x {
    |              ^^^^ calling non-const fn `<I as std::iter::IntoIterator><std::ops::Range<usize>>::into_iter`
@@ -26,12 +36,12 @@ LL |     let a : [i32; f(X)];
    |                   ---- inside call to `f`
    |
 note: for constant expression here
-  --> $DIR/const-fn-error.rs:29:13
+  --> $DIR/const-fn-error.rs:30:13
    |
 LL |     let a : [i32; f(X)];
    |             ^^^^^^^^^^^
 
-error: aborting due to 4 previous errors
+error: aborting due to 5 previous errors
 
-Some errors occurred: E0015, E0016, E0019, E0080.
+Some errors occurred: E0015, E0019, E0080, E0658.
 For more information about an error, try `rustc --explain E0015`.

src/test/ui/feature-gate-const_let.rs

@@ -13,7 +13,9 @@
 #![feature(const_fn)]
 
 const fn foo() -> usize {
-    let x = 42; //~ ERROR blocks in constant functions are limited to items and tail expressions
+    let x = 42;
+    //~^ ERROR statements in constant functions are unstable
+    //~| ERROR: let bindings in constant functions are unstable
     42
 }
 

src/test/ui/feature-gate-const_let.stderr

@@ -1,9 +1,19 @@
-error[E0016]: blocks in constant functions are limited to items and tail expressions
+error[E0658]: let bindings in constant functions are unstable (see issue #48821)
   --> $DIR/feature-gate-const_let.rs:16:13
    |
-LL |     let x = 42; //~ ERROR blocks in constant functions are limited to items and tail expressions
+LL |     let x = 42;
    |             ^^
+   |
+   = help: add #![feature(const_let)] to the crate attributes to enable
 
-error: aborting due to previous error
+error[E0658]: statements in constant functions are unstable (see issue #48821)
+  --> $DIR/feature-gate-const_let.rs:16:13
+   |
+LL |     let x = 42;
+   |             ^^
+   |
+   = help: add #![feature(const_let)] to the crate attributes to enable
 
-For more information about this error, try `rustc --explain E0016`.
+error: aborting due to 2 previous errors
+
+For more information about this error, try `rustc --explain E0658`.