Regressed by https://github.com/rust-lang/rust/pull/60445

$ rustc -vV
rustc 1.38.0-nightly
binary: rustc
commit-hash: 481068a70767
commit-date: 2019-07-05
host: x86_64-unknown-freebsd
release: 1.38.0-nightly
LLVM version: 8.0

$ echo "ac_add_options --enable-warnings-as-errors" >>.mozconfig
$ ./mach build
[...]
error: use of deprecated item 'std::mem::uninitialized': use `mem::MaybeUninit` instead
   --> gfx/wr/webrender/src/prim_store/gradient.rs:665:77
    |
665 |         let mut entries: [GradientDataEntry; GRADIENT_DATA_SIZE] = unsafe { mem::uninitialized() };
    |                                                                             ^^^^^^^^^^^^^^^^^^
    |
    = note: `-D deprecated` implied by `-D warnings`
error: use of deprecated item 'std::mem::uninitialized': use `mem::MaybeUninit` instead
    --> gfx/wr/webrender/src/renderer.rs:1467:32
     |
1467 |             data.push(unsafe { mem::uninitialized() });
     |                                ^^^^^^^^^^^^^^^^^^
error: use of deprecated item 'std::mem::uninitialized': use `mem::MaybeUninit` instead
    --> gfx/wr/webrender/src/renderer.rs:1475:36
     |
1475 |                 data.push(unsafe { mem::uninitialized() });
     |                                    ^^^^^^^^^^^^^^^^^^
error: use of deprecated item 'std::mem::uninitialized': use `mem::MaybeUninit` instead
   --> gfx/wr/webrender/src/platform/unix/font.rs:546:42
    |
546 |         let mut cbox: FT_BBox = unsafe { mem::uninitialized() };
    |                                          ^^^^^^^^^^^^^^^^^^
error: use of deprecated item 'std::mem::uninitialized': use `mem::MaybeUninit` instead
   --> gfx/wr/webrender/src/platform/unix/font.rs:729:37
    |
729 |             let mut cbox: FT_BBox = mem::uninitialized();
    |                                     ^^^^^^^^^^^^^^^^^^
error: aborting due to 5 previous errors
error: Could not compile `webrender`.

Trying to use MaybeUninit on non-nightly rust I get told not to use an unstable library feature. So this will probably have to wait to be fixed.

Looks like Simon has patches for this. According to https://bugzilla.mozilla.org/show_bug.cgi?id=1563623#c0 1.36 stable has support for MaybeUninit, so maybe you need to bump your stable version?

(In reply to Kartikaya Gupta (email:kats@mozilla.com) (away 17-Jul-2019 to Feb-2020) from comment #2)

Looks like Simon has patches for this. According to
https://bugzilla.mozilla.org/show_bug.cgi?id=1563623#c0 1.36 stable has
support for MaybeUninit, so maybe you need to bump your stable version?

*** This bug has been marked as a duplicate of bug 1564873 ***

I'm on 1.35, I did a mach bootstrap immediately before. So either 1.35 is still above the minimum required or there is a bug somewhere.

(In reply to Timothy Nikkel (:tnikkel) from comment #3)

I'm on 1.35, I did a mach bootstrap immediately before. So either 1.35 is
still above the minimum required or there is a bug somewhere.

Bug 1564873 is dependent on bug 1565193 which is to bump rust requirements from 1.35 to 1.36, so that explains it.