When building I get:
0:25.89 c:\PROGRA2\MICROS2\2017\COMMUN1\VC\Tools\MSVC\14151.267\include\type_traits(847,55): error: incomplete type 'nsIMAPMessagePartID' used in type trait expression
0:25.89 _INLINE_VAR constexpr bool is_copy_constructible_v = __is_constructible(_Ty, add_lvalue_reference_t<const _Ty>);
0:25.89 ^
0:25.90 z:/Mozilla/TB/dist/include\nsTArray.h(257,38): note: in instantiation of variable template specialization 'std::is_copy_constructible_v<nsIMAPMessagePartID>' requested here
0:25.90 static constexpr bool Value = std::is_copy_constructible_v<E>;
0:25.90 ^
0:25.90 z:/Mozilla/TB/dist/include\nsTArray.h(277,42): note: in instantiation of static data member 'detail::SpecializableIsCopyConstructible<nsIMAPMessagePartID>::Value' requested here
0:25.90 SpecializableIsCopyConstructible<E>::Value;
0:25.90 ^
0:25.91 z:/Mozilla/TB/dist/include\nsTArray.h(284,38): note: in instantiation of variable template specialization 'detail::SpecializableIsCopyConstructibleValue<nsIMAPMessagePartID>' requested here
0:25.91 bool IsCopyConstructible = SpecializableIsCopyConstructibleValue<E>>
0:25.91 ^
0:25.91 z:/Mozilla/TB/dist/include\nsTArray.h(322,24): note: in instantiation of default argument for 'nsTArray_CopyEnabler<nsIMAPMessagePartID, nsTArray_Impl<nsIMAPMessagePartID, nsTArrayInfallibleAllocator> >' required here
0:25.92 : public ::detail::nsTArray_CopyEnabler<E, Derived> {
0:25.92 ^~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
0:25.93 z:/Mozilla/TB/dist/include\nsTArray.h(868,36): note: in instantiation of template class 'nsTArray_SafeElementAtHelper<nsIMAPMessagePartID, nsTArray_Impl<nsIMAPMessagePartID, nsTArrayInfallibleAllocator> >' requested here
0:25.95 struct nsTArray_TypedBase : public nsTArray_SafeElementAtHelper<E, Derived> {};
0:25.95 ^
0:25.96 z:/Mozilla/TB/dist/include\nsTArray.h(1006,14): note: in instantiation of template class 'nsTArray_TypedBase<nsIMAPMessagePartID, nsTArray_Impl<nsIMAPMessagePartID, nsTArrayInfallibleAllocator> >' requested here
0:25.97 public nsTArray_TypedBase<
0:25.98 ^
0:25.99 z:/Mozilla/TB/dist/include\nsTArray.h(2637,25): note: in instantiation of template class 'nsTArray_Impl<nsIMAPMessagePartID, nsTArrayInfallibleAllocator>' requested here
0:25.99 class nsTArray : public nsTArray_Impl<E, nsTArrayInfallibleAllocator> {
0:26.00 ^
0:26.00 z:/Mozilla/comm-central/comm/mailnews/imap/src/nsIMAPBodyShell.h(299,7): note: in instantiation of template class 'nsTArray<nsIMAPMessagePartID>' requested here
0:26.00 m_prefetchQueue; // Array of pipelined part prefetches.
0:26.00 ^
0:26.00 z:/Mozilla/comm-central/comm/mailnews/imap/src/nsIMAPBodyShell.h(220,7): note: forward declaration of 'nsIMAPMessagePartID'
0:26.01 class nsIMAPMessagePartID;
0:26.01 ^
0:27.49 1 error generated.

Seems to come from bug 1620632

Thanks for finding the cause. Looking at it now.

Only a slight rearrangement needed.

Pushed by mkmelin@iki.fi:
https://hg.mozilla.org/comm-central/rev/efb0a54d4ae2
fix bustage from bug 1620632: error: incomplete type 'nsIMAPMessagePartID' used in type trait expression. rs=bustage-fix