[compiler] add newtype for bound names in IR

[patrick-schultz]

This PR introduces a new type Name for representing bound variables in the IR, replacing String. For now, it is just an AnyVal wrapper around String, but in the future I would like to take advantage of the new type. For example, I'd like to:

• change equality of Name from string comparison to comparing object identity with eq. That way freshName becomes just new Name(), with stronger guarantees that the new name doesn't occur anywhere in the current IR, without needing to maintain global state as we do now.
• get rid of NormalizeNames, instead enforcing the global uniqueness of names as a basic invariant of the IR (typecheck could also check this invariant)
• keep a string in the Name, but no longer require it to be unique. Instead it's just a suggestion for how to show the name in printouts, adding a uniqueifying suffix as needed. With NormalizeNames gone, this would let us preserve meaningful variable names further in the lowering pipeline.
• possibly keep other state in the Name, for example to allow a more efficient implementation of environments, similar to the mark state on BaseIR

This is obviously a large change, but there are only a few conceptual pieces (appologies for not managing to separate these out):

• attempt to minimize the number of locations in which the Name constructor is called, to make future refactorings easier
• add freshName(), which just wraps genUID(), returning a Name
• convert IR construction to use the convenience methods in ir.package, which take scala lambdas to represent blocks with bound variables, instead of manually creating new variable names
• replace uses of the magic constant variable names (row, va, sa, g, global) with constants (TableIR.{rowName, globalName}, MatrixIR.{rowName, colName, entryName, globalName})
• the above changes modified the names we use for bound variables in many places. That shouldn't matter, but it cought a couple bugs where it did.
  • NormalizeNames optionally allows the IR to contain free variables. But it didn't do anything to ensure the newly generated variable names are distinct from any contained free variables. Thus it was possible to rename a bound variable to mistakenly capture a contained free variable. I've fixed that.
  • SimplifySuite compared simplified IR with the pre-constructed expected IR, carefully controlling the genUID global state to make simplify generate exactly the names expected. I've replaced that by just comparing with the expected IR using a alpha-equivalence comparison.

[patrick-schultz]

• [compiler] add newtype for bound names in IR #14547 👈
• main

This stack of pull requests is managed by Graphite. Learn more about stacking.

Join @patrick-schultz and the rest of your teammates on Graphite

[patrick-schultz]

cc @ehigham

[ehigham]

This is heroic.

[ehigham]

Think you can do this in one line :)

Suggested change

	case class Name(str: String) extends AnyVal {
	override def toString: String = str
	}
	case class Name(override val toString: String) extends AnyVal

[ehigham]

I like your idea of using object equality rather than string equality.
Keeping the string, however, might allow us to better keep track of where a name came from through lowering.

[patrick-schultz]

Absolutely! From the description:

keep a string in the Name, but no longer require it to be unique. Instead it's just a suggestion for how to show the name in printouts, adding a uniqueifying suffix as needed. With NormalizeNames gone, this would let us preserve meaningful variable names further in the lowering pipeline.

[patrick-schultz]

Think you can do this in one line :)

I could... but I'd rather not commit to this choice of toString permanently.