add some Constraints statistics reporting

compiler/can/src/constraint.rs

@@ -1,7 +1,7 @@
 use crate::expected::{Expected, PExpected};
 use roc_collections::soa::{EitherIndex, Index, Slice};
 use roc_module::ident::TagName;
-use roc_module::symbol::Symbol;
+use roc_module::symbol::{ModuleId, Symbol};
 use roc_region::all::{Loc, Region};
 use roc_types::subs::Variable;
 use roc_types::types::{Category, PatternCategory, Type};
@@ -140,6 +140,26 @@ impl Constraints {
         }
     }
 
+    pub fn statistics(&self, module_id: ModuleId) -> Result<String, std::fmt::Error> {
+        use std::fmt::Write;
+
+        let mut buf = String::new();
+
+        writeln!(buf, "Constraints statistics for module {:?}:", module_id)?;
+
+        writeln!(buf, "   constraints length: {}:", self.constraints.len())?;
+        writeln!(buf, "   types length: {}:", self.types.len())?;
+        writeln!(
+            buf,
+            "   let_constraints length: {}:",
+            self.let_constraints.len()
+        )?;
+        writeln!(buf, "   expectations length: {}:", self.expectations.len())?;
+        writeln!(buf, "   categories length: {}:", self.categories.len())?;
+
+        Ok(buf)
+    }
+
     #[inline(always)]
     const fn push_type_variable(var: Variable) -> EitherIndex<Type, Variable> {
         // that's right, we use the variable's integer value as the index