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Currently predicates are written with positional placeholders `{N}` and rely on
`formatv` as the engine to do substitution. The problem with this approach is that
the definitions of those positional placeholders are not consistent; they are
entirely up to the defining predicate of question. For example, `{0}` in various
attribute constraints is used to mean the attribute, while it is used to main the
builder for certain attribute transformations. This can become very confusing.
This CL introduces `tgfmt` as a new mechanism to better support for predicate and
rewrite rule specification. Instead of entirely relying on positional placeholders,
`tgfmt` support both positional and special placeholders. The former is used for
DAG operands. The latter, including $_builder, $_op, $_self, are used as special
"hooks" to entities in the context. With this, the predicate and rewrite rules
specification can be more consistent is more readable.
--
PiperOrigin-RevId: 243249671
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