feat: inject unreachable! after break-less repeat#13506
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…-code warnings When a `repeat` body has no `break`, the loop does not terminate normally and the `ForIn.forIn` result type is fixed to `PUnit`. Using such a `repeat` in a do block that expects a non-`Unit` result type used to require a trailing `return`/`unreachable!` from the user. To make matters worse, that trailing element was flagged as dead by the `ControlInfo`-based warning, yet removing it broke the do block's type, so the warning was not actionable and had to be suppressed. This PR teaches `elabDoRepeat` to append its own `unreachable!` after the `for _ in Loop.mk do ...` expansion when the body cannot `break`. The `unreachable!` gives the continuation a polymorphic value, so the enclosing do block's result type is inferred from context and the user no longer needs to write any trailing filler. The `ControlInfo` for break-less `repeat` now reports `noFallthrough` (and `numRegularExits := 0`) honestly, so dead-code warnings fire on any user-written element after the loop — and because the loop supplies its own `unreachable!`, those warnings are actionable: the user can simply delete the flagged element. Co-authored-by: Rob23oba <robin.arnez@web.de>
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This PR makes the new `do` elaborator (leanprover#12459) the default by flipping `backward.do.legacy` to `false`. Legacy behavior remains available via `set_option backward.do.legacy true`. New surface syntax that is now accepted (and may shadow user-defined macros of the same shape): * `while let pat := e do …`, `while let pat ← e do …`, and `while h : cond do …` via a unified condition syntax (leanprover#13534). * `let ⟨a, b⟩ : T ← act` propagates the type annotation to the monadic action (leanprover#12866). * `letConfig` options (`(eq := h)`, `+nondep`, `+usedOnly`, `+zeta`) on do `let`/`have` (leanprover#13250, leanprover#13255). They are rejected on `let mut`, and `+postponeValue`/`+generalize` are rejected outright. * Anonymous dependent `if _ : p then … else …` now elaborates inside `do` blocks (leanprover#13192). Breaking changes: * `do` notation now requires a `Pure` instance, not just `Bind`. * The arms of `do match` are non-dependent by default; write `do match (dependent := true)` to recover the legacy term-match expansion. The patterns are still dependently refined (leanprover#12673), which subsumes many former dependent uses. * `try`/`catch` no longer accepts a body whose result type matches the surrounding expected type only via coercion (leanprover#13849). The same expression has never been accepted outside `do`. * Unreachable code now triggers a warning instead of an error. Code written after a `break`-less `repeat` is now reported as unreachable (leanprover#13506). * The syntax for `let pat := rhs | otherwise` and similar now scope over the `doSeq` that follows. Furthermore, `otherwise` and the sequence that follows are now `doSeqIndented` in order not to steal syntax from record syntax.
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This PR appends
unreachable!to the expansion ofbreak-lessrepeatwhen the expected result type does not unify withPUnit. The continuation then has a polymorphic value, so the enclosing do block's result type is inferred without a user-written filler, andControlInfofor break-lessrepeatcan reportnoFallthroughhonestly — dead-code warnings on subsequent elements are now actionable.