Rope.Equals uses slow IEnumerable<char> iterator for StringRope, adding unnecessary overhead to large string switch compilation

[davidmikh]

Summary

Rope.Equals compares two ropes character-by-character using IEnumerable<char> iterators, which allocates a CharEnumerator and uses virtual dispatch per character. For StringRope (the common case wrapping a plain string), this is significantly slower than using string == directly.

This is on the hot path in DecisionDagBuilder.CheckConsistentDecision, which is called O(n²) times when building the decision DAG for large string switch statements. At 10,000 switch cases, Rope.Equals accounts for ~5.8% of total CPU time.

Benchmark

Using SwitchStatementBenchmarks.BindFile (compiles a C# file with an N-case string switch and emits IL) where Before represents the current implementation and After represents an implementation that just uses string == directly for StringRope:

SwitchCount	Before	After	Speedup	Alloc Before	Alloc After	Alloc Δ
100	62 ms	49 ms	-21%	2.7 MB	2.5 MB	-9%
1,000	972 ms	628 ms	-35%	115 MB	90 MB	-22%
5,000	12.6 s	9.9 s	-22%	2,488 MB	1,975 MB	-21%
10,000	44.7 s	38.8 s	-13%	10,261 MB	7,765 MB	-24%

Fix

Override Equals and GetHashCode in StringRope to use direct string operations instead of the IEnumerable<char> base implementation.