Aiur: support non-tail match on a non-variable scrutinee#435
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The match compiler hoists a non-variable match scrutinee into a fresh
let (MatchCompiler.switch), so `let x = match foo(bar) {..}` simplified
to `let x = (let w = foo(bar); match w {..})`. That buried the match one
let deep, where Lower's non-tail-match detector — which only fires when a
match is the immediate letVar/letWild RHS — couldn't see it, and lowering
threw "Non-tail match in arbitrary position (not supported)".
Fix in the Simple stage: mkLetFloating floats leading lets out of a
let-RHS (`let x = (let w = e; rest); b` -> `let w = e; let x = rest; b`),
restoring the invariant that a non-tail match sits directly in its
letVar/letWild RHS. The hoisted w's are fresh match-compiler locals, so
widening their scope cannot capture anything.
Tests: ntm_match_on_call exercises a function-call scrutinee, folded into
the non_tail_match aggregate in both the aiur and aiur-cross suites.
gabriel-barrett
approved these changes
Jun 4, 2026
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The match compiler hoists a non-variable match scrutinee into a fresh let (MatchCompiler.switch), so
let x = match foo(bar) {..}simplified tolet x = (let w = foo(bar); match w {..}). That buried the match one let deep, where Lower's non-tail-match detector — which only fires when a match is the immediate letVar/letWild RHS — couldn't see it, and lowering threw "Non-tail match in arbitrary position (not supported)".Fix in the Simple stage: mkLetFloating floats leading lets out of a let-RHS (
let x = (let w = e; rest); b->let w = e; let x = rest; b), restoring the invariant that a non-tail match sits directly in its letVar/letWild RHS. The hoisted w's are fresh match-compiler locals, so widening their scope cannot capture anything.Tests: ntm_match_on_call exercises a function-call scrutinee, folded into the non_tail_match aggregate in both the aiur and aiur-cross suites.