Proposed Syntax

{ <field>: { $_internalSchemaUniqueItems: true } }

The match expression takes a single value: the boolean true. It matches an array whose elements are all unique, according to our definition of equality.

With this, we could implement JSON Schema's "uniqueItems" validation keyword by compiling it to the MatchExpression

{$or: [{<field>: {$not: {$type: "array"}}}, {<field>: {$uniqueItems: true}}]}