Avoid creating small tables with a capacity of 1

src/raw/mod.rs

@@ -177,16 +177,22 @@ impl Iterator for ProbeSeq {
 // Workaround for emscripten bug emscripten-core/emscripten-fastcomp#258
 #[cfg_attr(target_os = "emscripten", inline(never))]
 fn capacity_to_buckets(cap: usize) -> Option<usize> {
-    let adjusted_cap = if cap < 8 {
-        // Need at least 1 free bucket on small tables
-        cap + 1
-    } else {
-        // Otherwise require 1/8 buckets to be empty (87.5% load)
-        //
-        // Be careful when modifying this, calculate_layout relies on the
-        // overflow check here.
-        cap.checked_mul(8)? / 7
-    };
+    debug_assert_ne!(cap, 0);
+
+    // For small tables we require at least 1 empty bucket so that lookups are
+    // guaranteed to terminate if an element doesn't exist in the table.
+    if cap < 8 {
+        // We don't bother with a table size of 2 buckets since that can only
+        // hold a single element. Instead we skip directly to a 4 bucket table
+        // which can hold 3 elements.
+        return Some(if cap < 4 { 4 } else { 8 });
+    }
+
+    // Otherwise require 1/8 buckets to be empty (87.5% load)
+    //
+    // Be careful when modifying this, calculate_layout relies on the
+    // overflow check here.
+    let adjusted_cap = cap.checked_mul(8)? / 7;
 
     // Any overflows will have been caught by the checked_mul. Also, any
     // rounding errors from the division above will be cleaned up by