Make use of possibly uninitialized data a hard error

This is one of the behaviors we no longer allow in NLL. Since it can
lead to undefined behavior, I think it's definitely worth making it a
hard error without waiting to turn off migration mode (rust-lang#58781).

Closes rust-lang#60450.

My ulterior motive here is making it impossible to leave variables
partially initialized across a yield (see discussion at rust-lang#63035), so
tests are included for that.

src/librustc_mir/borrow_check/conflict_errors.rs

@@ -105,6 +105,9 @@ impl<'cx, 'tcx> MirBorrowckCtxt<'cx, 'tcx> {
                 format!("{} occurs due to use{}", desired_action.as_noun(), use_spans.describe()),
             );
 
+            // This error should not be downgraded to a warning,
+            // even in migrate mode.
+            self.disable_error_downgrading();
             err.buffer(&mut self.errors_buffer);
         } else {
             if let Some((reported_place, _)) = self.move_error_reported.get(&move_out_indices) {

src/librustc_mir/borrow_check/mod.rs

@@ -252,6 +252,7 @@ fn do_mir_borrowck<'a, 'tcx>(
         move_error_reported: BTreeMap::new(),
         uninitialized_error_reported: Default::default(),
         errors_buffer,
+        disable_error_downgrading: false,
         nonlexical_regioncx: regioncx,
         used_mut: Default::default(),
         used_mut_upvars: SmallVec::new(),
@@ -363,7 +364,7 @@ fn do_mir_borrowck<'a, 'tcx>(
     if !mbcx.errors_buffer.is_empty() {
         mbcx.errors_buffer.sort_by_key(|diag| diag.span.primary_span());
 
-        if tcx.migrate_borrowck() {
+        if !mbcx.disable_error_downgrading && tcx.migrate_borrowck() {
             // When borrowck=migrate, check if AST-borrowck would
             // error on the given code.
 
@@ -479,6 +480,9 @@ crate struct MirBorrowckCtxt<'cx, 'tcx> {
     uninitialized_error_reported: FxHashSet<PlaceRef<'cx, 'tcx>>,
     /// Errors to be reported buffer
     errors_buffer: Vec<Diagnostic>,
+    /// If there are no errors reported by the HIR borrow checker, we downgrade
+    /// all NLL errors to warnings. Setting this flag disables downgrading.
+    disable_error_downgrading: bool,
     /// This field keeps track of all the local variables that are declared mut and are mutated.
     /// Used for the warning issued by an unused mutable local variable.
     used_mut: FxHashSet<Local>,
@@ -919,6 +923,12 @@ impl InitializationRequiringAction {
 }
 
 impl<'cx, 'tcx> MirBorrowckCtxt<'cx, 'tcx> {
+    /// If there are no errors reported by the HIR borrow checker, we downgrade
+    /// all NLL errors to warnings. Calling this disables downgrading.
+    crate fn disable_error_downgrading(&mut self)  {
+        self.disable_error_downgrading = true;
+    }
+
     /// Checks an access to the given place to see if it is allowed. Examines the set of borrows
     /// that are in scope, as well as which paths have been initialized, to ensure that (a) the
     /// place is initialized and (b) it is not borrowed in some way that would prevent this

src/test/ui/async-await/partial-initialization-across-await.rs

@@ -0,0 +1,44 @@
+// Test that we don't allow awaiting from an async fn while a local is partially
+// initialized.
+
+// edition:2018
+
+#![feature(async_await)]
+
+struct S { x: i32, y: i32 }
+struct T(i32, i32);
+
+async fn noop() {}
+
+async fn test_tuple() {
+    let mut t: (i32, i32);
+    t.0 = 42;
+    //~^ ERROR assign to part of possibly uninitialized variable: `t` [E0381]
+    noop().await;
+    t.1 = 88;
+    let _ = t;
+}
+
+async fn test_tuple_struct() {
+    let mut t: T;
+    t.0 = 42;
+    //~^ ERROR assign to part of possibly uninitialized variable: `t` [E0381]
+    noop().await;
+    t.1 = 88;
+    let _ = t;
+}
+
+async fn test_struct() {
+    let mut t: S;
+    t.x = 42;
+    //~^ ERROR assign to part of possibly uninitialized variable: `t` [E0381]
+    noop().await;
+    t.y = 88;
+    let _ = t;
+}
+
+fn main() {
+    let _ = test_tuple();
+    let _ = test_tuple_struct();
+    let _ = test_struct();
+}

src/test/ui/async-await/partial-initialization-across-await.stderr

@@ -0,0 +1,21 @@
+error[E0381]: assign to part of possibly uninitialized variable: `t`
+  --> $DIR/partial-initialization-across-await.rs:15:5
+   |
+LL |     t.0 = 42;
+   |     ^^^^^^^^ use of possibly uninitialized `t`
+
+error[E0381]: assign to part of possibly uninitialized variable: `t`
+  --> $DIR/partial-initialization-across-await.rs:24:5
+   |
+LL |     t.0 = 42;
+   |     ^^^^^^^^ use of possibly uninitialized `t`
+
+error[E0381]: assign to part of possibly uninitialized variable: `t`
+  --> $DIR/partial-initialization-across-await.rs:33:5
+   |
+LL |     t.x = 42;
+   |     ^^^^^^^^ use of possibly uninitialized `t`
+
+error: aborting due to 3 previous errors
+
+For more information about this error, try `rustc --explain E0381`.

src/test/ui/borrowck/disallow-possibly-uninitialized.rs

@@ -0,0 +1,22 @@
+// Test that we don't allow partial initialization.
+// This may be relaxed in the future (see #54987).
+
+fn main() {
+    let mut t: (u64, u64);
+    t.0 = 1;
+    //~^ ERROR assign to part of possibly uninitialized variable: `t` [E0381]
+    t.1 = 1;
+
+    let mut t: (u64, u64);
+    t.1 = 1;
+    //~^ ERROR assign to part of possibly uninitialized variable: `t` [E0381]
+    t.0 = 1;
+
+    let mut t: (u64, u64);
+    t.0 = 1;
+    //~^ ERROR assign to part of possibly uninitialized variable: `t` [E0381]
+
+    let mut t: (u64,);
+    t.0 = 1;
+    //~^ ERROR assign to part of possibly uninitialized variable: `t` [E0381]
+}

src/test/ui/borrowck/disallow-possibly-uninitialized.stderr

@@ -0,0 +1,27 @@
+error[E0381]: assign to part of possibly uninitialized variable: `t`
+  --> $DIR/disallow-possibly-uninitialized.rs:6:5
+   |
+LL |     t.0 = 1;
+   |     ^^^^^^^ use of possibly uninitialized `t`
+
+error[E0381]: assign to part of possibly uninitialized variable: `t`
+  --> $DIR/disallow-possibly-uninitialized.rs:11:5
+   |
+LL |     t.1 = 1;
+   |     ^^^^^^^ use of possibly uninitialized `t`
+
+error[E0381]: assign to part of possibly uninitialized variable: `t`
+  --> $DIR/disallow-possibly-uninitialized.rs:16:5
+   |
+LL |     t.0 = 1;
+   |     ^^^^^^^ use of possibly uninitialized `t`
+
+error[E0381]: assign to part of possibly uninitialized variable: `t`
+  --> $DIR/disallow-possibly-uninitialized.rs:20:5
+   |
+LL |     t.0 = 1;
+   |     ^^^^^^^ use of possibly uninitialized `t`
+
+error: aborting due to 4 previous errors
+
+For more information about this error, try `rustc --explain E0381`.

src/test/ui/consts/const_let_refutable.rs

@@ -2,10 +2,6 @@ fn main() {}
 
 const fn slice([a, b]: &[i32]) -> i32 { //~ ERROR refutable pattern in function argument
     a + b //~ ERROR can only call other `const fn` within a `const fn`
-    //~^ WARN use of possibly uninitialized variable: `a`
-    //~| WARN this error has been downgraded to a warning for backwards compatibility
-    //~| WARN this represents potential undefined behavior in your code and this warning will
-    //~| WARN use of possibly uninitialized variable: `b`
-    //~| WARN this error has been downgraded to a warning for backwards compatibility
-    //~| WARN this represents potential undefined behavior in your code and this warning will
+    //~^ ERROR use of possibly uninitialized variable: `a`
+    //~| ERROR use of possibly uninitialized variable: `b`
 }

src/test/ui/consts/const_let_refutable.stderr

@@ -13,27 +13,19 @@ LL |     a + b
    = note: for more information, see issue https://github.com/rust-lang/rust/issues/57563
    = help: add `#![feature(const_fn)]` to the crate attributes to enable
 
-warning[E0381]: use of possibly uninitialized variable: `a`
+error[E0381]: use of possibly uninitialized variable: `a`
   --> $DIR/const_let_refutable.rs:4:5
    |
 LL |     a + b
    |     ^ use of possibly uninitialized `a`
-   |
-   = warning: this error has been downgraded to a warning for backwards compatibility with previous releases
-   = warning: this represents potential undefined behavior in your code and this warning will become a hard error in the future
-   = note: for more information, try `rustc --explain E0729`
 
-warning[E0381]: use of possibly uninitialized variable: `b`
+error[E0381]: use of possibly uninitialized variable: `b`
   --> $DIR/const_let_refutable.rs:4:9
    |
 LL |     a + b
    |         ^ use of possibly uninitialized `b`
-   |
-   = warning: this error has been downgraded to a warning for backwards compatibility with previous releases
-   = warning: this represents potential undefined behavior in your code and this warning will become a hard error in the future
-   = note: for more information, try `rustc --explain E0729`
 
-error: aborting due to 2 previous errors
+error: aborting due to 4 previous errors
 
 Some errors have detailed explanations: E0005, E0381, E0723.
 For more information about an error, try `rustc --explain E0005`.

src/test/ui/empty/empty-never-array.rs

@@ -10,9 +10,7 @@ fn transmute<T, U>(t: T) -> U {
     let Helper::U(u) = Helper::T(t, []);
     //~^ ERROR refutable pattern in local binding: `T(_, _)` not covered
     u
-    //~^ WARN use of possibly uninitialized variable: `u`
-    //~| WARN this error has been downgraded to a warning for backwards compatibility
-    //~| WARN this represents potential undefined behavior in your code and this warning will
+    //~^ ERROR use of possibly uninitialized variable: `u`
 }
 
 fn main() {

src/test/ui/empty/empty-never-array.stderr

@@ -11,17 +11,13 @@ LL | | }
 LL |       let Helper::U(u) = Helper::T(t, []);
    |           ^^^^^^^^^^^^ pattern `T(_, _)` not covered
 
-warning[E0381]: use of possibly uninitialized variable: `u`
+error[E0381]: use of possibly uninitialized variable: `u`
   --> $DIR/empty-never-array.rs:12:5
    |
 LL |     u
    |     ^ use of possibly uninitialized `u`
-   |
-   = warning: this error has been downgraded to a warning for backwards compatibility with previous releases
-   = warning: this represents potential undefined behavior in your code and this warning will become a hard error in the future
-   = note: for more information, try `rustc --explain E0729`
 
-error: aborting due to previous error
+error: aborting due to 2 previous errors
 
 Some errors have detailed explanations: E0005, E0381.
 For more information about an error, try `rustc --explain E0005`.

src/test/ui/generator/partial-initialization-across-yield.rs

@@ -0,0 +1,46 @@
+// Test that we don't allow yielding from a generator while a local is partially
+// initialized.
+
+#![feature(generators)]
+
+struct S { x: i32, y: i32 }
+struct T(i32, i32);
+
+fn test_tuple() {
+    let _ = || {
+        let mut t: (i32, i32);
+        t.0 = 42;
+        //~^ ERROR assign to part of possibly uninitialized variable: `t` [E0381]
+        yield;
+        t.1 = 88;
+        let _ = t;
+    };
+}
+
+fn test_tuple_struct() {
+    let _ = || {
+        let mut t: T;
+        t.0 = 42;
+        //~^ ERROR assign to part of possibly uninitialized variable: `t` [E0381]
+        yield;
+        t.1 = 88;
+        let _ = t;
+    };
+}
+
+fn test_struct() {
+    let _ = || {
+        let mut t: S;
+        t.x = 42;
+        //~^ ERROR assign to part of possibly uninitialized variable: `t` [E0381]
+        yield;
+        t.y = 88;
+        let _ = t;
+    };
+}
+
+fn main() {
+    test_tuple();
+    test_tuple_struct();
+    test_struct();
+}

src/test/ui/generator/partial-initialization-across-yield.stderr

@@ -0,0 +1,21 @@
+error[E0381]: assign to part of possibly uninitialized variable: `t`
+  --> $DIR/partial-initialization-across-yield.rs:12:9
+   |
+LL |         t.0 = 42;
+   |         ^^^^^^^^ use of possibly uninitialized `t`
+
+error[E0381]: assign to part of possibly uninitialized variable: `t`
+  --> $DIR/partial-initialization-across-yield.rs:23:9
+   |
+LL |         t.0 = 42;
+   |         ^^^^^^^^ use of possibly uninitialized `t`
+
+error[E0381]: assign to part of possibly uninitialized variable: `t`
+  --> $DIR/partial-initialization-across-yield.rs:34:9
+   |
+LL |         t.x = 42;
+   |         ^^^^^^^^ use of possibly uninitialized `t`
+
+error: aborting due to 3 previous errors
+
+For more information about this error, try `rustc --explain E0381`.

src/test/ui/issues/issue-15381.rs

@@ -4,8 +4,6 @@ fn main() {
     for &[x,y,z] in values.chunks(3).filter(|&xs| xs.len() == 3) {
         //~^ ERROR refutable pattern in `for` loop binding: `&[]` not covered
         println!("y={}", y);
-        //~^ WARN borrow of possibly uninitialized variable: `y`
-        //~| WARN this error has been downgraded to a warning for backwards compatibility
-        //~| WARN this represents potential undefined behavior in your code and this warning will
+        //~^ ERROR borrow of possibly uninitialized variable: `y`
     }
 }