Auto merge of #131193 - EFanZh:asserts-vec-len, r=<try>

Asserts the maximum value that can be returned from `Vec::len`

Currently, casting `Vec<i32>` to `Vec<u32>` takes O(1) time:

```rust
// See <https://godbolt.org/z/hxq3hnYKG> for assembly output.
pub fn cast(vec: Vec<i32>) -> Vec<u32> {
    vec.into_iter().map(|e| e as _).collect()
}
```

But the generated assembly is not the same as the identity function, which prevents us from casting `Vec<Vec<i32>>` to `Vec<Vec<u32>>` within O(1) time:

```rust
// See <https://godbolt.org/z/7n48bxd9f> for assembly output.
pub fn cast(vec: Vec<Vec<i32>>) -> Vec<Vec<u32>> {
    vec.into_iter()
        .map(|e| e.into_iter().map(|e| e as _).collect())
        .collect()
}
```

This change tries to fix the problem. You can see the comparison here: <https://godbolt.org/z/jdManrKvx>.

Author: bors

Diff for: library/alloc/src/vec/mod.rs

@@ -2630,6 +2630,14 @@ impl<T, A: Allocator> Vec<T, A> {
     #[stable(feature = "rust1", since = "1.0.0")]
     #[rustc_confusables("length", "size")]
     pub fn len(&self) -> usize {
+        if !T::IS_ZST {
+            // SAFETY: The maximum capacity of `Vec<T>` is `isize::MAX` bytes, so if `T` non-ZST,
+            // `Vec<T>` can have at most `isize::MAX as usize / mem::size_of::<T>()` elements.
+            unsafe {
+                core::hint::assert_unchecked(self.len <= isize::MAX as usize / mem::size_of::<T>());
+            }
+        }
+
         self.len
     }
 

Diff for: tests/codegen/vec-in-place.rs

@@ -37,6 +37,8 @@ pub struct Baz {
 #[no_mangle]
 pub fn vec_iterator_cast_primitive(vec: Vec<i8>) -> Vec<u8> {
     // CHECK-NOT: loop
+    // CHECK: call
+    // CHECK-SAME: void @llvm.assume(i1 %{{.+}})
     // CHECK-NOT: call
     vec.into_iter().map(|e| e as u8).collect()
 }
@@ -45,14 +47,35 @@ pub fn vec_iterator_cast_primitive(vec: Vec<i8>) -> Vec<u8> {
 #[no_mangle]
 pub fn vec_iterator_cast_wrapper(vec: Vec<u8>) -> Vec<Wrapper<u8>> {
     // CHECK-NOT: loop
+    // CHECK: call
+    // CHECK-SAME: void @llvm.assume(i1 %{{.+}})
     // CHECK-NOT: call
     vec.into_iter().map(|e| Wrapper(e)).collect()
 }
 
+// CHECK-LABEL: @vec_iterator_cast_signed
+#[no_mangle]
+pub fn vec_iterator_cast_signed(vec: Vec<i32>) -> Vec<u32> {
+    // CHECK-NOT: and i{{[0-9]+}} %{{.*}}, {{[0-9]+}}
+    vec.into_iter().map(|e| u32::from_ne_bytes(e.to_ne_bytes())).collect()
+}
+
+// CHECK-LABEL: @vec_iterator_cast_signed_nested
+#[no_mangle]
+pub fn vec_iterator_cast_signed_nested(vec: Vec<Vec<i32>>) -> Vec<Vec<u32>> {
+    // CHECK-NOT: br i1 %{{.*}}, label %{{.*}}, label %{{.*}}
+    // CHECK-NOT: %{{.*}} = udiv
+    vec.into_iter()
+        .map(|e| e.into_iter().map(|e| u32::from_ne_bytes(e.to_ne_bytes())).collect())
+        .collect()
+}
+
 // CHECK-LABEL: @vec_iterator_cast_unwrap
 #[no_mangle]
 pub fn vec_iterator_cast_unwrap(vec: Vec<Wrapper<u8>>) -> Vec<u8> {
     // CHECK-NOT: loop
+    // CHECK: call
+    // CHECK-SAME: void @llvm.assume(i1 %{{.+}})
     // CHECK-NOT: call
     vec.into_iter().map(|e| e.0).collect()
 }
@@ -61,6 +84,8 @@ pub fn vec_iterator_cast_unwrap(vec: Vec<Wrapper<u8>>) -> Vec<u8> {
 #[no_mangle]
 pub fn vec_iterator_cast_aggregate(vec: Vec<[u64; 4]>) -> Vec<Foo> {
     // CHECK-NOT: loop
+    // CHECK: call
+    // CHECK-SAME: void @llvm.assume(i1 %{{.+}})
     // CHECK-NOT: call
     vec.into_iter().map(|e| unsafe { std::mem::transmute(e) }).collect()
 }
@@ -69,6 +94,8 @@ pub fn vec_iterator_cast_aggregate(vec: Vec<[u64; 4]>) -> Vec<Foo> {
 #[no_mangle]
 pub fn vec_iterator_cast_deaggregate_tra(vec: Vec<Bar>) -> Vec<[u64; 4]> {
     // CHECK-NOT: loop
+    // CHECK: call
+    // CHECK-SAME: void @llvm.assume(i1 %{{.+}})
     // CHECK-NOT: call
 
     // Safety: For the purpose of this test we assume that Bar layout matches [u64; 4].
@@ -82,6 +109,8 @@ pub fn vec_iterator_cast_deaggregate_tra(vec: Vec<Bar>) -> Vec<[u64; 4]> {
 #[no_mangle]
 pub fn vec_iterator_cast_deaggregate_fold(vec: Vec<Baz>) -> Vec<[u64; 4]> {
     // CHECK-NOT: loop
+    // CHECK: call
+    // CHECK-SAME: void @llvm.assume(i1 %{{.+}})
     // CHECK-NOT: call
 
     // Safety: For the purpose of this test we assume that Bar layout matches [u64; 4].
@@ -95,6 +124,8 @@ pub fn vec_iterator_cast_deaggregate_fold(vec: Vec<Baz>) -> Vec<[u64; 4]> {
 #[no_mangle]
 pub fn vec_iterator_cast_unwrap_drop(vec: Vec<Wrapper<String>>) -> Vec<String> {
     // CHECK-NOT: br i1 %{{.*}}, label %{{.*}}, label %{{.*}}
+    // CHECK: call
+    // CHECK-SAME: void @llvm.assume(i1 %{{.+}})
     // CHECK-NOT: call
     // CHECK-NOT: %{{.*}} = mul
     // CHECK-NOT: %{{.*}} = udiv
@@ -106,9 +137,12 @@ pub fn vec_iterator_cast_unwrap_drop(vec: Vec<Wrapper<String>>) -> Vec<String> {
 #[no_mangle]
 pub fn vec_iterator_cast_wrap_drop(vec: Vec<String>) -> Vec<Wrapper<String>> {
     // CHECK-NOT: br i1 %{{.*}}, label %{{.*}}, label %{{.*}}
+    // CHECK: call
+    // CHECK-SAME: void @llvm.assume(i1 %{{.+}})
     // CHECK-NOT: call
     // CHECK-NOT: %{{.*}} = mul
     // CHECK-NOT: %{{.*}} = udiv
+    // CHECK: ret void
 
     vec.into_iter().map(Wrapper).collect()
 }

Diff for: tests/codegen/vec_pop_push_noop.rs

@@ -9,7 +9,6 @@ pub fn noop(v: &mut Vec<u8>) {
     // CHECK-NOT: call
     // CHECK: tail call void @llvm.assume
     // CHECK-NOT: grow_one
-    // CHECK-NOT: call
     // CHECK: ret
     if let Some(x) = v.pop() {
         v.push(x)