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+### What it does
+Checks for maps with zero-sized value types anywhere in the code.
+
+### Why is this bad?
+Since there is only a single value for a zero-sized type, a map
+containing zero sized values is effectively a set. Using a set in that case improves
+readability and communicates intent more clearly.
+
+### Known problems
+* A zero-sized type cannot be recovered later if it contains private fields.
+* This lints the signature of public items
+
+### Example
+```
+fn unique_words(text: &str) -> HashMap<&str, ()> {
+    todo!();
+}
+```
+Use instead:
+```
+fn unique_words(text: &str) -> HashSet<&str> {
+    todo!();
+}
+```
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