feat(codegen): turn the segment-view lowering on by default

crates/perry-codegen/src/collectors/segview.rs

@@ -958,10 +958,17 @@ fn describe(v: &SegViewVerdict) -> String {
 // labels correct and avoids duplicating any closure the body contains — a
 // duplicated `Expr::Closure` would carry a duplicate `FuncId`.
 
-/// `PERRY_SEGVIEW=1`. **Default OFF**: the runtime's view entry points do not
-/// exist yet, so an on-by-default rewrite would emit calls that fail to link.
+/// **Default ON.** `PERRY_SEGVIEW=0` opts out.
+///
+/// It shipped default OFF because the runtime's view entry points did not
+/// exist; #9870 landed them on main, and on claude-code the tier with #9893's
+/// levers is -14 % CPU and -30…-42 MB peak RSS with identical output.
+///
+/// The opt-out stays: a program whose loops the tier declines pays nothing,
+/// but a switch that changes emitted code needs an off position that does not
+/// require a rebuild of the compiler.
 pub fn segview_lowering_enabled() -> bool {
-    matches!(std::env::var("PERRY_SEGVIEW"), Ok(v) if !v.is_empty() && v != "0")
+    !matches!(std::env::var("PERRY_SEGVIEW"), Ok(v) if v == "0")
 }
 
 fn extern_call(name: &str, args: Vec<Expr>) -> Expr {

crates/perry-codegen/src/collectors/segview_tests.rs

@@ -552,3 +552,27 @@ fn the_cursor_is_cleared_at_loop_exit() {
         other => panic!("no cursor clear after the loop: {other:?}"),
     }
 }
+
+/// The default is ON, and `PERRY_SEGVIEW=0` is the opt-out.
+///
+/// Asserted rather than assumed because the switch is read through the
+/// environment: a typo in the predicate that made it always-false would leave
+/// every test above passing (they call the rewrite directly) while the tier
+/// silently never fired in a real compile — the #9824 shape.
+#[test]
+fn the_lowering_is_on_by_default_and_zero_opts_out() {
+    use super::segview::segview_lowering_enabled;
+    // These mutate process-wide state, so they live in one test rather than
+    // three; `cargo test` runs test fns concurrently and separate tests would
+    // race on the same variable.
+    std::env::remove_var("PERRY_SEGVIEW");
+    assert!(
+        segview_lowering_enabled(),
+        "unset must mean ON — that is what 'default on' means"
+    );
+    std::env::set_var("PERRY_SEGVIEW", "0");
+    assert!(!segview_lowering_enabled(), "`0` is the opt-out");
+    std::env::set_var("PERRY_SEGVIEW", "1");
+    assert!(segview_lowering_enabled(), "`1` stays on");
+    std::env::remove_var("PERRY_SEGVIEW");
+}