It looks like escapeFrom always has the same value as _localScopeDepth.

In my opinion, escapeFrom is a confusing name for this parameter, as it implies that the safe-context of BoundExpression expr is already known. But, usually that safe-context is actually determined by calling a helper like GetParameterValEscape(...) and passing in expr, as the BoundKind of expr is switched on/narrowed down.

Instead, it seems like escapeFrom is used when the expression is thought to hold a reference to the current block.

It seems like things would be more clear if escapeFrom were either renamed to localScopeDepth a la GetValEscape(), or, if the parameter were deleted and _localScopeDepth were used instead.

Note that theoretically it seems like we could implement our ref safety checks by patterns like GetValEscape(source).IsConvertibleTo(GetValEscape(dest)). The reason we have CheckValEscape methods is, that when the conversion of the source safe-context fails, we can give an error which is appropriate for the particular expression.