The MOZ_MUST_USE macro is defined as clang's and gcc's nonstandard attribute((warn_unused_result)). Now that we compile as C++17 by default (bug 1560664), we can replace MOZ_MUST_USE with C++17's standard [[nodiscard]] attribute.

Also move MOZ_MUST_USE before function declarations' specifiers and return type. While clang and gcc's attribute((warn_unused_result)) can appear before, between, or after function specifiers and return types, the [[nodiscard]] attribute must precede the function specifiers.

Also move MOZ_MUST_USE before function declarations' specifiers and return type. While clang and gcc's attribute((warn_unused_result)) can appear before, between, or after function specifiers and return types, the [[nodiscard]] attribute must precede the function specifiers.

Backed out changeset 5e89020502f7 (bug 1625834) for causing build bustages

Backout revision https://hg.mozilla.org/integration/autoland/rev/5a5b189e0fc90e9f8bbe44a0a7126ad62ec94da6

Failed push https://treeherder.mozilla.org/#/jobs?repo=autoland&resultStatus=testfailed%2Cbusted%2Cexception&revision=5e89020502f79ced17433215db8a0c74264549e2

Failure logs https://treeherder.mozilla.org/logviewer.html#/jobs?job_id=295686641&repo=autoland&lineNumber=28183
https://treeherder.mozilla.org/logviewer.html#?job_id=295686634&repo=autoland

Chris can you please take a look?

Also move MOZ_MUST_USE before function declarations' specifiers and return type. While clang and gcc's attribute((warn_unused_result)) can appear before, between, or after function specifiers and return types, the [[nodiscard]] attribute must precede the function specifiers.

https://hg.mozilla.org/mozilla-central/rev/8ed83142ac7f