--- a/build/moz.configure/toolchain.configure
+++ b/build/moz.configure/toolchain.configure
@@ -1293,16 +1293,64 @@ js_option('--enable-release',
 def developer_options(value):
     if not value:
         return True
 
 
 add_old_configure_assignment('DEVELOPER_OPTIONS', developer_options)
 set_config('DEVELOPER_OPTIONS', developer_options)
 
+# Rust compiler flags
+# ==============================================================
+
+@depends(moz_optimize, debug_rust, '--enable-debug-symbols')
+def rust_compiler_flags(moz_optimize, debug_rust, debug_symbols):
+    optimize = moz_optimize.optimize
+
+    # Cargo currently supports only two interesting profiles for building:
+    # development and release. Those map (roughly) to --enable-debug and
+    # --disable-debug in Gecko, respectively.
+    #
+    # But we'd also like to support an additional axis of control for
+    # optimization level. Since Cargo only supports 2 profiles, we're in
+    # a bit of a bind.
+    #
+    # Code here derives various compiler options given other configure options.
+    # The options defined here effectively override defaults specified in
+    # Cargo.toml files.
+
+    opt_level = None
+    debug_assertions = None
+    debug_info = None
+
+    if not optimize:
+        opt_level = '0'
+
+    # opt-level=0 implies -C debug-assertions, which may not be desired
+    # unless Rust debugging is enabled.
+    if opt_level == '0' and not debug_rust:
+        debug_assertions = False
+
+    if debug_symbols:
+        debug_info = '2'
+
+    flags = []
+
+    if opt_level is not None:
+        flags.append('-Copt-level=%s' % opt_level)
+    if debug_assertions is not None:
+        flags.append('-Cdebug-assertions=%s' %
+                     ('yes' if debug_assertions else 'no'))
+    if debug_info is not None:
+        flags.append('-Cdebuginfo=%s' % debug_info)
+
+    return flags
+
+set_config('MOZ_RUST_DEFAULT_FLAGS', rust_compiler_flags)
+
 # Linker detection
 # ==============================================================
 
 
 @depends(target)
 def is_linker_option_enabled(target):
     if target.kernel not in ('Darwin', 'WINNT', 'SunOS'):
         return True

--- a/config/rules.mk
+++ b/config/rules.mk
@@ -870,34 +870,17 @@ endif
 cargo_rustc_flags = $(CARGO_RUSTCFLAGS)
 ifndef DEVELOPER_OPTIONS
 ifndef MOZ_DEBUG_RUST
 # Enable link-time optimization for release builds.
 cargo_rustc_flags += -C lto
 endif
 endif
 
-# Cargo currently supports only two interesting profiles for building:
-# development and release.  Those map (roughly) to --enable-debug and
-# --disable-debug in Gecko, respectively, but there's another axis that we'd
-# like to support: --{disable,enable}-optimize.  Since that would be four
-# choices, and Cargo only supports two, we choose to enable various
-# optimization levels in our Cargo.toml files all the time, and override the
-# optimization level here, if necessary.  (The Cargo.toml files already
-# specify debug-assertions appropriately for --{disable,enable}-debug.)
-default_rustflags =
-ifndef MOZ_OPTIMIZE
-default_rustflags = -C opt-level=0
-# Unfortunately, -C opt-level=0 implies -C debug-assertions, so we need
-# to explicitly disable them when MOZ_DEBUG_RUST is not set.
-ifndef MOZ_DEBUG_RUST
-default_rustflags += -C debug-assertions=no
-endif
-endif
-rustflags_override = RUSTFLAGS='$(default_rustflags) $(RUSTFLAGS)'
+rustflags_override = RUSTFLAGS='$(MOZ_RUST_DEFAULT_FLAGS) $(RUSTFLAGS)'
 
 ifdef MOZ_MSVCBITS
 # If we are building a MozillaBuild shell, we want to clear out the
 # vcvars.bat environment variables for cargo builds. This is because
 # a 32-bit MozillaBuild shell on a 64-bit machine will try to use
 # the 32-bit compiler/linker for everything, while cargo/rustc wants
 # to use the 64-bit linker for build.rs scripts. This conflict results
 # in a build failure (see bug 1350001). So we clear out the environment
@@ -914,20 +897,16 @@ rust_unlock_unstable =
 ifdef MOZ_RUST_SIMD
 rust_unlock_unstable += RUSTC_BOOTSTRAP=1
 endif
 
 ifdef MOZ_USING_SCCACHE
 sccache_wrap := RUSTC_WRAPPER='$(CCACHE)'
 endif
 
-ifdef MOZ_DEBUG_SYMBOLS
-default_rustflags += -C debuginfo=2
-endif
-
 # We use the + prefix to pass down the jobserver fds to cargo, but we
 # don't use the prefix when make -n is used, so that cargo doesn't run
 # in that case)
 define RUN_CARGO
 $(if $(findstring n,$(filter-out --%, $(MAKEFLAGS))),,+)env $(environment_cleaner) $(rust_unlock_unstable) $(rustflags_override) $(sccache_wrap) \
 	CARGO_TARGET_DIR=$(CARGO_TARGET_DIR) \
 	RUSTC=$(RUSTC) \
 	MOZ_SRC=$(topsrcdir) \