Add more tests, add docs for is_mem_uninit field

compiler/rustc_lint/src/builtin.rs

@@ -2366,7 +2366,14 @@ impl<'tcx> LateLintPass<'tcx> for InvalidValue {
         #[derive(Debug, Copy, Clone, PartialEq)]
         enum InitKind {
             Zeroed,
-            Uninit { is_mem_uninit: bool },
+            /// `is_mem_uninit` is true *only* if this is a call to `mem::uninitialized()`, not if
+            /// this is a `MaybeUninit::uninit().assume_init()`.
+            ///
+            /// This lets us avoid duplicate errors being shown, for code that matches the
+            /// mem_uninitialized FCW.
+            Uninit {
+                is_mem_uninit: bool,
+            },
         }
 
         impl InitKind {

src/test/ui/intrinsics/mem-uninitialized-future-compat.rs

@@ -9,6 +9,19 @@ struct UninitStruct {
     b: char,
 }
 
+enum OneVariant {
+    Hello,
+}
+
+enum TwoVariant {
+    Hello,
+    Goodbye,
+}
+
+enum OneVariantWith<T> {
+    Hello(T),
+}
+
 unsafe fn unknown_type<T, const N: usize>() {
     std::mem::uninitialized::<T>();
     //~^ ERROR the type `T` is generic, and might not permit being left uninitialized
@@ -22,6 +35,12 @@ unsafe fn unknown_type<T, const N: usize>() {
     std::mem::uninitialized::<[UninitStruct; N]>();
     //~^ ERROR the type `[UninitStruct; N]` is generic, and might not permit being left uninitialized
 
+    std::mem::uninitialized::<Result<T, !>>();
+    //~^ ERROR the type `std::result::Result<T, !>` does not permit being left uninitialized
+
+    std::mem::uninitialized::<OneVariantWith<T>>();
+    //~^ ERROR the type `OneVariantWith<T>` is generic, and might not permit being left uninitialized
+
     std::mem::uninitialized::<[T; 0]>();
     std::mem::uninitialized::<[char; 0]>();
 }
@@ -58,10 +77,30 @@ fn main() {
         std::mem::uninitialized::<(u32, char)>();
         //~^ ERROR the type `(u32, char)` does not permit being left uninitialized
 
+        std::mem::uninitialized::<TwoVariant>();
+        //~^ ERROR the type `TwoVariant` does not permit being left uninitialized
+
+        std::mem::uninitialized::<Result<!, !>>();
+        //~^ ERROR the type `std::result::Result<!, !>` does not permit being left uninitialized
+
+        std::mem::uninitialized::<Result<!, u32>>();
+        //~^ ERROR the type `std::result::Result<!, u32>` does not permit being left uninitialized
+
+        std::mem::uninitialized::<Option<!>>();
+        //~^ ERROR the type `std::option::Option<!>` does not permit being left uninitialized
+
+        std::mem::uninitialized::<OneVariantWith<char>>();
+        //~^ ERROR the type `OneVariantWith<char>` does not permit being left uninitialized
+
+        std::mem::uninitialized::<OneVariantWith<!>>();
+        //~^ ERROR the type `OneVariantWith<!>` does not permit being left uninitialized
+
         std::mem::uninitialized::<MaybeUninit<Box<u32>>>();
         std::mem::uninitialized::<usize>();
         std::mem::uninitialized::<f32>();
         std::mem::uninitialized::<*const u8>();
         std::mem::uninitialized::<[u8; 64]>();
+        std::mem::uninitialized::<OneVariant>();
+        std::mem::uninitialized::<OneVariantWith<u32>>();
     }
 }