Fix parsing negative hexadecimal integers

Rust's from_str_radix() method doesn't support negative hexadecimal
integer values such as "-0xA". This commit fixes this by special-casing
such inputs.

Changelog: fixed

compiler/src/hir.rs

@@ -1617,8 +1617,15 @@ impl<'a> LowerToHir<'a> {
             input = input.replace('_', "");
         }
 
-        let result = if let Some(slice) = input.strip_prefix("0x") {
-            i64::from_str_radix(slice, 16)
+        let hex_prefix = if input.starts_with("-") { "-0x" } else { "0x" };
+        let result = if let Some(slice) = input.strip_prefix(hex_prefix) {
+            i64::from_str_radix(slice, 16).map(|v| {
+                if input.starts_with("-") {
+                    0_i64.wrapping_sub(v)
+                } else {
+                    v
+                }
+            })
         } else {
             i64::from_str(&input)
         };

libstd/test/std/test_int.inko

@@ -34,6 +34,9 @@ fn pub tests(t: mut Tests) {
     t.equal(Int.from_base16(' 11'), Option.None)
     t.equal(Int.from_base16('11 '), Option.None)
     t.equal(Int.from_base16('zz'), Option.None)
+    t.equal(
+      Int.from_base16('-0x4a3f043013b2c4d1'), Option.Some(-5349999486874862801)
+    )
   }
 
   t.test('Int.to_base2') fn (t) {

vm/src/builtin_functions/string.rs

@@ -94,7 +94,19 @@ pub(crate) fn string_to_int(
         string
     };
 
-    let res = i64::from_str_radix(slice, radix as u32)
+    // Rust doesn't handle parsing strings like "-0x4a3f043013b2c4d1" out of the
+    // box.
+    let parsed = if radix == 16 {
+        if let Some(tail) = string.strip_prefix("-0x") {
+            i64::from_str_radix(tail, 16).map(|v| 0_i64.wrapping_sub(v))
+        } else {
+            i64::from_str_radix(slice, 16)
+        }
+    } else {
+        i64::from_str_radix(slice, radix as u32)
+    };
+
+    let res = parsed
         .map(|val| Int::alloc(state.permanent_space.int_class(), val))
         .unwrap_or_else(|_| Pointer::undefined_singleton());