ty_fixed_length_vec's length const-expr not eval'ed consistently (vs other const-exprs)

[pnkfelix]

From: https://gist.github.com/pnkfelix/7198893

Part of #5551 metabug.

First, an illustration that field-dereference can be used in some const expressions. This compiles and runs:

struct foo { x: int, y: int }
static A : foo = foo { x: 3, y: 4 };
static B : int = A.x;

fn main() {
println!("{:?}", B);

But, even though the above compiles and runs, a use of B as the length of a fixed length vec fails to compile.

struct foo { x: int, y: int }
static A : foo = foo { x: 3, y: 4 };

#[cfg(field_proj)]
static B : int = A.x;

#[cfg(inline_val)]
static B : int = 3;

fn main() {
let v : [int, ..B] = [5, 6, 7];
println!("{:?}", v);
}

% ./x86_64-apple-darwin/stage0/bin/rustc --cfg inline_val /tmp/f.rs && /tmp/f
warning: no debug symbols in executable (-arch x86_64)
[5, 6, 7]
% ./x86_64-apple-darwin/stage0/bin/rustc --cfg field_proj /tmp/f.rs && /tmp/f
/tmp/f.rs:11:12: 11:22 error: expected constant expr for vector length: Unsupported constant expr
/tmp/f.rs:11 let v : [int, ..B] = [5, 6, 7];
^~~~~~~~~~
task '<unnamed>' failed at 'explicit failure', /Users/rustbuild/src/rust-buildbot/slave/snap3-mac/build/src/libsyntax/diagnostic.rs:75
task '<unnamed>' failed at 'explicit failure', /Users/rustbuild/src/rust-buildbot/slave/snap3-mac/build/src/librustc/rustc.rs:395
% 

[pnkfelix]

See also some relevant discussion from #rust-internals: https://botbot.me/mozilla/rust-internals/msg/7305244/

[steveklabnik]

Triage: updated code:

struct foo { x: i32, y: i32 }
static A : foo = foo { x: 3, y: 4 };

#[cfg(field_proj)]
static B : i32 = A.x;

#[cfg(inline_val)]
static B : i32 = 3;

fn main() {
    let v : [i32; B] = [5, 6, 7];
    println!("{:?}", v);
}

updated error:

hello.rs:11:19: 11:20 error: unresolved name `B`
hello.rs:11     let v : [i32; B] = [5, 6, 7];
                              ^

[pnkfelix]

@steveklabnik were you passing in the necessary --cfg flags to expose the distinct cases here?

Plus I think this bug dates from before we had the static/const distinction; one needs to use const now.

So I think the appropriate update looks more like this:

struct foo { x: usize, y: usize }
const A : foo = foo { x: 3, y: 4 };

#[cfg(field_proj)]
const B : usize = A.x;

#[cfg(inline_val)]
const B : usize = 3;

fn main() {
    let v : [i32; B] = [5, 6, 7];
    println!("{:?}", v);
}

which, huzzah, works now for both the --cfg field_proj and --cfg inline_val configurations! So I'm going to close this bug.