rust: Enable lto by default

This avoids us leaking all of the Rust symbols as public
into our shared library.

Came up in team chat since one person tried to copy just `/usr/bin/rpm-ostree`
from the dev container into a target and got a linker crash; but
really we want to avoid all the duplicated symbols entirely.

(And we should investigate cross-language LTO because that's the
 only way to get full savings)

configure.ac

@@ -236,19 +236,6 @@ AM_CONDITIONAL(RUST_DEBUG, [test "x$rust_debug_release" = "xdebug"])
 dnl Unconditional now.
 RPM_OSTREE_FEATURES="$RPM_OSTREE_FEATURES rust"
 
-dnl Only use this for package builds, as it will hackily edit Cargo.toml right now
-AC_ARG_ENABLE(lto,
-AC_HELP_STRING([--enable-lto],
-               [Build code with Link Time Optimization [default=no]]))
-dnl https://github.com/rust-lang/cargo/issues/4349
-dnl https://bugzilla.mozilla.org/show_bug.cgi?id=1386371
-if test "${enable_lto}" = yes; then
-    if ! grep -q '^lto.*=true' rust/Cargo.toml 2>/dev/null; then
-        echo '# Inserted by configure --enable-lto' >> rust/Cargo.toml
-        echo "lto = true" >> rust/Cargo.toml
-    fi
-fi
-
 AC_ARG_ENABLE(rojig,
   AC_HELP_STRING([--enable-rojig],
     [Support for shipping ostree commits via RPMs [default=no]]))
@@ -303,6 +290,6 @@ echo "
     rojig:                                   ${enable_rojig:-no}
     bubblewrap:                              $with_bubblewrap
     gtk-doc:                                 $enable_gtk_doc
-    rust:                                    $rust_debug_release (lto: ${enable_lto:-no})
+    rust:                                    $rust_debug_release
     cbindgen:                                ${cbindgen:-internal}
 "

rust/Cargo.toml

@@ -46,7 +46,6 @@ crate-type = ["staticlib"]
 panic = "abort"
 # We assume we're being delivered via e.g. RPM which supports split debuginfo
 debug = true
-# For true release builds, we do suggest you enable LTO via e.g.
-# env RUSTFLAGS='-C lto=true' as part of the outer build.  It's just
-# off by default because it's quite slow and the default is more oriented
-# towards local development.
+# We need this to avoid leaking symbols, see
+# https://internals.rust-lang.org/t/rust-staticlibs-and-optimizing-for-size/5746
+lto = true