Error on @trace if assignments to untraced mutable struct fields - #3271
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Pangoraw merged 1 commit intoSep 14, 2026
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An untraced location in a captured mutable argument has nothing an if result can be written back into, so a branch assigning to it was silently dropped. Detect the discarded assignment during if_condition lowering and error, pointing at promote_to_traced. Applies to plain numbers and enums alike. Co-Authored-By: Chris Rackauckas <accounts@chrisrackauckas.com> Co-Authored-By: Devin <158243242+devin-ai-integration[bot]@users.noreply.github.com> Agent-Harness: Devin CLI Agent-Model: SWE-2 High Agent-Session: local CLI session (no shareable URL)
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This looks correct to me, thanks!
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Summary
Split out of #3232 (comment), where this change was flagged as independent of the enum tracing work.
When a
@trace ifbranch assigns a new value to a field of a captured mutable struct whose slot is untraced (e.g. a plainIntorFloat64field), theifresult has no location to be written back into. Previously the assignment was silently discarded;if_conditionnow checks such:resargpaths and raises an error explaining that the field must be traced before theif, e.g. viaReactantCore.promote_to_traced.Caveat: the check only inspects collected result paths, so it is not a general detector of unsupported mutations — a leaf the tracing machinery never tracks produces no path and remains undetectable (this is noted in a comment on
check_untraced_branch_state).The tutorial's conditional-control-flow section now documents the requirement on mutable struct fields.
Test plan
core/control_flowpasses locally (Linux CPU, PJRT): 150/150, including the new "if: assignment to an untraced struct field" testset which asserts the error fires for plainIntandFloat64fields and that an untouched untraced field still works."untraced location"with this change and silently dropped the assignment without it.🤖 Generated with Devin (harness: Devin CLI, model: SWE-2 High) — local CLI session, no shareable URL.