feat: Update safe_math and move to libraries (#1803)

Fixes #1794 and address the wrong check in `mul`. 

Would prefer to add tests directly, but noir don't support failing tests
in noir yet, so there is really no good reason to do that currently. See
noir-lang/noir#1994

yarn-project/noir-contracts/src/contracts/lending_contract/Nargo.toml

@@ -6,3 +6,4 @@ type = "contract"
 
 [dependencies]
 aztec = { path = "../../../../noir-libs/noir-aztec" }
+safe_math = { path = "../../../../noir-libs/safe-math" }

yarn-project/noir-contracts/src/contracts/lending_contract/src/helpers.nr

@@ -1,8 +1,7 @@
-mod safe_math;
 mod interest_math;
-use crate::safe_math::SafeU120;
 use crate::interest_math::compute_multiplier;
 use dep::std::hash::pedersen;
+use dep::safe_math::SafeU120;
 
 
 // Utility used to easily get a "id" for a private user that sits in the same

yarn-project/noir-contracts/src/contracts/lending_contract/src/interest_math.nr

@@ -1,5 +1,4 @@
-mod safe_math;
-use crate::safe_math::SafeU120;
+use dep::safe_math::SafeU120;
 
 // Binomial approximation of exponential
 // using lower than decired precisions for everything due to u120 limit

yarn-project/noir-contracts/src/contracts/lending_contract/src/main.nr

@@ -1,5 +1,4 @@
 mod storage;
-mod safe_math;
 mod interest_math;
 mod helpers;
 mod interfaces;
@@ -23,7 +22,7 @@ contract Lending {
     };
     use dep::aztec::public_call_stack_item::PublicCallStackItem;
     use crate::storage::{Storage, Asset};
-    use crate::safe_math::SafeU120;
+    use dep::safe_math::SafeU120;
     use crate::interest_math::compute_multiplier;
     use crate::helpers::{covered_by_collateral, DebtReturn, debt_updates, debt_value, compute_identifier};
     use crate::interfaces::{Token, Lending, PriceFeed};

yarn-project/noir-libs/safe-math/Nargo.toml

@@ -0,0 +1,7 @@
+[package]
+name = "safe_math"
+authors = [""]
+compiler_version = "0.1"
+type = "lib"
+
+[dependencies]

yarn-project/noir-libs/safe-math/src/lib.nr

@@ -0,0 +1,3 @@
+mod safe_u120;
+
+use crate::safe_u120::SafeU120;

yarn-project/noir-contracts/src/contracts/lending_contract/src/safe_math.nr → yarn-project/noir-libs/safe-math/src/safe_u120.nr

@@ -1,4 +1,3 @@
-
 struct SafeU120 {
     value: u120,
 }
@@ -36,8 +35,9 @@ impl SafeU120 {
         b: SafeU120,
     ) -> SafeU120 {
         let c: u120 = self.value * b.value;
-        // checking for overflows. If both self.value and b are not 0, then c must be greater than or equal self.value.
-        assert((self.value == 0) | (b.value == 0) | (c >= self.value));
+        if b.value > 0 {
+            assert(c / b.value == self.value);
+        }
         SafeU120 {
             value: c
         }
@@ -53,8 +53,6 @@ impl SafeU120 {
         }
     }
 
-    // todo: Implement a version that avoids shadow-overflows
-    // (i.e., overflows by self*b where c would not overflow)
     fn mul_div(
         self: Self,
         b: SafeU120,
@@ -79,4 +77,6 @@ impl SafeU120 {
     }
 
     // todo: implement mul_div with 240 bit intermediate values.
-}
+}
+
+// Adding test in here is pretty useless as long as noir don't support failings tests.