bindings: put lto behind a flag

The first hypothesis in the last commit was correct; it was compiler verion mismatch.

Unfortunately, though things now work locally, we can't enable LTO by default since we can't assume that all users will have the right version of clang/lld (or even be using clang/lld at all).

.cargo/config

@@ -1,3 +1,5 @@
 [build]
 rustdocflags = ["--cfg", "docs"]
-rustflags = ["-Clinker-plugin-lto", "-Clinker=clang",  "-Clink-arg=-fuse-ld=lld"]
+
+# Uncomment this when using the `lto` feature.
+# rustflags = ["-Clinker-plugin-lto", "-Clinker=clang",  "-Clink-arg=-fuse-ld=lld"]

Cargo.toml

@@ -51,6 +51,7 @@ syn = { version = "1.0", optional = true, default-features = false, features = [
 [profile.release]
 opt-level = 3
 lto = true
+codegen-units = 1
 
 
 [features]
@@ -59,6 +60,7 @@ grader = ["frontend"]
 frontend = []
 
 generate-fresh = ["bindgen", "proc-macro2", "quote", "syn", "grader", "frontend"]
+lto = []
 
 
 [package.metadata.docs.rs]

build.rs

@@ -495,9 +495,11 @@ fn main() -> Result<()> {
         build.flag("/EHsc");
     }
 
+    if cfg!(feature = "lto") {
+        build.flag_if_supported("-flto=thin");
+    }
+
     build
-        // .flag_if_supported("-flto")
-        // .flag_if_supported("-flto=thin")
         .flag_if_supported("-std=c++14")
         .flag_if_supported("-Wno-format-security")
         .cargo_metadata(true)