Auto merge of #51052 - nikomatsakis:obsolete-arrow, r=petrochenkov

restore emplacement syntax (obsolete)

Fix https://github.com/rust-lang/rust/issues/50832

r? @petrochenkov

diff --cc src/librustc/hir/lowering.rs
Simple merge

diff --cc src/libsyntax/ast.rs
Simple merge

diff --cc src/libsyntax/feature_gate.rs
Simple merge