checked cast can never succeed or has no exact implementation

Why this fires¶

value as? T promises an exact, runtime-checked conversion. The diagnostic fires when the source and destination have no value-preserving checked path.

let value = "42" as? int // error[TE836]: parsing is not casting

Fix it¶

Use parse for text, try_into for domain values, or choose a numeric cast policy when information may be discarded.

let value = "42".parse::<int>()?

See also¶

• TE835 — invalid total cast.
• TE837 — lossy numeric cast.