Auto merge of rust-lang#15168 - HKalbasi:mir, r=HKalbasi

Fix realloc problem in allocating smaller amounts

Author: bors

crates/hir-ty/src/consteval/tests/intrinsics.rs

@@ -183,6 +183,7 @@ fn allocator() {
             *ptr = 23;
             *ptr2 = 32;
             let ptr = __rust_realloc(ptr, 4, 1, 8);
+            let ptr = __rust_realloc(ptr, 8, 1, 3);
             let ptr2 = ((ptr as usize) + 1) as *mut u8;
             *ptr + *ptr2
         };

crates/hir-ty/src/mir/eval/shim.rs

@@ -144,14 +144,18 @@ impl Evaluator<'_> {
                 let [ptr, old_size, align, new_size] = args else {
                     return Err(MirEvalError::TypeError("rustc_allocator args are not provided"));
                 };
-                let ptr = Address::from_bytes(ptr.get(self)?)?;
                 let old_size = from_bytes!(usize, old_size.get(self)?);
                 let new_size = from_bytes!(usize, new_size.get(self)?);
-                let align = from_bytes!(usize, align.get(self)?);
-                let result = self.heap_allocate(new_size, align);
-                Interval { addr: result, size: old_size }
-                    .write_from_interval(self, Interval { addr: ptr, size: old_size })?;
-                destination.write_from_bytes(self, &result.to_bytes())?;
+                if old_size >= new_size {
+                    destination.write_from_interval(self, ptr.interval)?;
+                } else {
+                    let ptr = Address::from_bytes(ptr.get(self)?)?;
+                    let align = from_bytes!(usize, align.get(self)?);
+                    let result = self.heap_allocate(new_size, align);
+                    Interval { addr: result, size: old_size }
+                        .write_from_interval(self, Interval { addr: ptr, size: old_size })?;
+                    destination.write_from_bytes(self, &result.to_bytes())?;
+                }
             }
             _ => not_supported!("unknown alloc function"),
         }