Auto merge of #44275 - eddyb:deferred-ctfe, r=nikomatsakis

Evaluate fixed-length array length expressions lazily.

This is in preparation for polymorphic array lengths (aka `[T; T::A]`) and const generics.
We need deferred const-evaluation to break cycles when array types show up in positions which require knowing the array type to typeck the array length, e.g. the array type is in a `where` clause.

The final step - actually passing bounds in scope to array length expressions from the parent - is not done because it still produces cycles when *normalizing* `ParamEnv`s, and @nikomatsakis' in-progress lazy normalization work is needed to deal with that uniformly.

However, the changes here are still useful to unlock work on const generics, which @EpicatSupercell manifested interest in, and I might be mentoring them for that, but we need this baseline first.

r? @nikomatsakis cc @oli-obk

src/test/compile-fail/const-block-non-item-statement-3.rs

@@ -0,0 +1,15 @@
+// Copyright 2014 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.
+
+type Array = [u32; {  let x = 2; 5 }];
+//~^ ERROR: blocks in constants are limited to items and tail expressions
+//~^^ ERROR: blocks in constants are limited to items and tail expressions
+
+pub fn main() {}

src/test/compile-fail/const-block-non-item-statement.rs

@@ -14,8 +14,4 @@ enum Foo {
     //~^^ ERROR: blocks in constants are limited to items and tail expressions
 }
 
-type Array = [u32; {  let x = 2; 5 }];
-//~^ ERROR: blocks in constants are limited to items and tail expressions
-//~^^ ERROR: blocks in constants are limited to items and tail expressions
-
 pub fn main() {}

src/test/compile-fail/const-fn-error.rs

@@ -23,5 +23,5 @@ const fn f(x: usize) -> usize {
 
 #[allow(unused_variables)]
 fn main() {
-    let a : [i32; f(X)]; //~ NOTE for array length here
+    let a : [i32; f(X)]; //~ NOTE for constant expression here
 }

src/test/compile-fail/const-len-underflow-separate-spans.rs

@@ -20,5 +20,5 @@ const LEN: usize = ONE - TWO;
 
 fn main() {
     let a: [i8; LEN] = unimplemented!();
-    //~^ NOTE for array length here
+    //~^ NOTE for constant expression here
 }

src/test/compile-fail/const-size_of-cycle.rs

@@ -8,11 +8,12 @@
 // option. This file may not be copied, modified, or distributed
 // except according to those terms.
 
+// error-pattern: unsupported cyclic reference between types/traits detected
+
 #![feature(const_fn)]
 
 struct Foo {
     bytes: [u8; std::mem::size_of::<Foo>()]
-    //~^ ERROR unsupported cyclic reference between types/traits detected
 }
 
 fn main() {}

src/test/compile-fail/issue-22933-2.rs

@@ -15,7 +15,4 @@ enum Delicious {
     //~^ ERROR no associated item named `PIE` found for type `Delicious`
 }
 
-const FOO: [u32; u8::MIN as usize] = [];
-//~^ ERROR no associated item named `MIN` found for type `u8`
-
 fn main() {}

src/test/compile-fail/issue-22933-3.rs

@@ -0,0 +1,14 @@
+// Copyright 2016 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.
+
+const FOO: [u32; u8::MIN as usize] = [];
+//~^ ERROR no associated item named `MIN` found for type `u8`
+
+fn main() {}

src/test/compile-fail/issue-39559.rs

@@ -21,8 +21,9 @@ impl Dim for Dim3 {
 }
 
 pub struct Vector<T, D: Dim> {
-    entries: [T; D::dim()]
+    entries: [T; D::dim()],
     //~^ ERROR no function or associated item named `dim` found for type `D` in the current scope
+    _dummy: D,
 }
 
 fn main() {}

src/test/ui/issue-38875/issue_38875.stderr

@@ -4,7 +4,7 @@ error[E0080]: constant evaluation error
 11 | pub const FOO: usize = *&0;
    |                        ^^^ unimplemented constant expression: deref operation
    |
-note: for repeat count here
+note: for constant expression here
   --> $DIR/issue_38875.rs:16:22
    |
 16 |     let test_x = [0; issue_38875_b::FOO];