stdlib: Fix temp alloc size in try_as_str_array. (#5753)

Allocation must be based on the type size with padding.

In addition to the usual testing, I've also tested against
https://github.com/FuelLabs/fuel-vm/tree/feature/0.43.3-memory-grow.

Author: vaivaswatha

docs/book/src/reference/compiler_intrinsics.md

@@ -28,7 +28,7 @@ ___
 __size_of_str_array<T>() -> u64
 ```
 
-**Description:** Return the size of type `T` in bytes. This intrinsic differs from `__size_of` in the case of "string arrays" where the actual length in bytes of the string is returned without padding the byte size to the next word alignment. When `T` is not a string `0` is returned.
+**Description:** Return the size of type `T` in bytes. This intrinsic differs from `__size_of` in the case of "string arrays" where the actual length in bytes of the string is returned without padding the byte size to the next word alignment. When `T` is not a "string array" `0` is returned.
 
 **Constraints:** None.
 

sway-lib-std/src/primitive_conversions/str.sw

@@ -6,17 +6,23 @@ impl str {
     pub fn try_as_str_array<S>(self) -> Option<S> {
         __assert_is_str_array::<S>();
         let str_size = __size_of_str_array::<S>();
+        let tmp_alloc_size = __size_of::<S>();
         let source = self.as_ptr();
 
         if self.len() == str_size {
-            let s: S = asm(str_size: str_size, source: source, dest) {
+            let s: S = asm(
+                str_size: str_size,
+                tmp_alloc_size: tmp_alloc_size,
+                source: source,
+                dest,
+            ) {
                 move dest sp;
-                cfe str_size;
+                cfe tmp_alloc_size;
                 mcp dest source str_size;
                 dest: S
             };
-            asm(str_size: str_size) {
-                cfs str_size;
+            asm(tmp_alloc_size: tmp_alloc_size) {
+                cfs tmp_alloc_size;
             }
             Some(s)
         } else {
@@ -32,6 +38,8 @@ fn str_slice_to_str_array() {
 
     let a = "abcd";
     let b: str[4] = a.try_as_str_array().unwrap();
+    assert(__size_of_str_array::<str[4]>() == a.len() && __size_of_val(b) == 8);
+
     let c = from_str_array(b);
 
     assert(a == c);