Bug 1430886 - Disable incremental rust in automation. r=froydnj

Work around excessive command-line length issues by
disabling incremental rust compilation, which is enabled
by default outside `cargo --release` starting with Rust 1.24.

Incremental rust builds shouldn't help much in automation,
where sccache provides the only continuity between build
environments. In the meantime, they add a lot of object
files to the link line.

See rust-lang/rust#47507 about addressing
the underlying issue upstream.

MozReview-Commit-ID: LRwUj3fhiaO

UltraBlame original commit: 261725e65af9b5d98ff593db3db08632bf019454

config/rules.mk

@@ -6292,6 +6292,34 @@ C
 lto
 endif
 endif
+#
+Disable
+incremental
+Rust
+compilation
+in
+automation
+builds
+where
+#
+the
+lack
+of
+environmental
+continuity
+makes
+it
+unhelpful
+.
+ifdef
+MOZ_AUTOMATION
+cargo_incremental
+:
+=
+CARGO_INCREMENTAL
+=
+0
+endif
 rustflags_override
 =
 RUSTFLAGS
@@ -6633,6 +6661,10 @@ topobjdir
 )
 \
 (
+cargo_incremental
+)
+\
+(
 2
 )
 \