Some new tests I added.

src/test/ui/borrowck/issue-54499-field-mutation-marks-mut-as-used.ast.stderr

@@ -0,0 +1,39 @@
+error[E0381]: use of possibly uninitialized variable: `t.0`
+  --> $DIR/issue-54499-field-mutation-marks-mut-as-used.rs:25:31
+   |
+LL |         println!("{:?} {:?}", t.0, t.1);
+   |                               ^^^ use of possibly uninitialized `t.0`
+
+error[E0381]: use of possibly uninitialized variable: `t.1`
+  --> $DIR/issue-54499-field-mutation-marks-mut-as-used.rs:25:36
+   |
+LL |         println!("{:?} {:?}", t.0, t.1);
+   |                                    ^^^ use of possibly uninitialized `t.1`
+
+error[E0381]: use of possibly uninitialized variable: `u.0`
+  --> $DIR/issue-54499-field-mutation-marks-mut-as-used.rs:35:31
+   |
+LL |         println!("{:?} {:?}", u.0, u.1);
+   |                               ^^^ use of possibly uninitialized `u.0`
+
+error[E0381]: use of possibly uninitialized variable: `u.1`
+  --> $DIR/issue-54499-field-mutation-marks-mut-as-used.rs:35:36
+   |
+LL |         println!("{:?} {:?}", u.0, u.1);
+   |                                    ^^^ use of possibly uninitialized `u.1`
+
+error[E0381]: use of possibly uninitialized variable: `v.x`
+  --> $DIR/issue-54499-field-mutation-marks-mut-as-used.rs:45:31
+   |
+LL |         println!("{:?} {:?}", v.x, v.y);
+   |                               ^^^ use of possibly uninitialized `v.x`
+
+error[E0381]: use of possibly uninitialized variable: `v.y`
+  --> $DIR/issue-54499-field-mutation-marks-mut-as-used.rs:45:36
+   |
+LL |         println!("{:?} {:?}", v.x, v.y);
+   |                                    ^^^ use of possibly uninitialized `v.y`
+
+error: aborting due to 6 previous errors
+
+For more information about this error, try `rustc --explain E0381`.

src/test/ui/borrowck/issue-54499-field-mutation-marks-mut-as-used.nll.stderr

@@ -0,0 +1,21 @@
+error[E0381]: assign to part of possibly uninitialized variable: `t`
+  --> $DIR/issue-54499-field-mutation-marks-mut-as-used.rs:22:9
+   |
+LL |         t.0 = S(1);
+   |         ^^^^^^^^^^ use of possibly uninitialized `t`
+
+error[E0381]: assign to part of possibly uninitialized variable: `u`
+  --> $DIR/issue-54499-field-mutation-marks-mut-as-used.rs:32:9
+   |
+LL |         u.0 = S(1);
+   |         ^^^^^^^^^^ use of possibly uninitialized `u`
+
+error[E0381]: assign to part of possibly uninitialized variable: `v`
+  --> $DIR/issue-54499-field-mutation-marks-mut-as-used.rs:42:9
+   |
+LL |         v.x = S(1);
+   |         ^^^^^^^^^^ use of possibly uninitialized `v`
+
+error: aborting due to 3 previous errors
+
+For more information about this error, try `rustc --explain E0381`.

src/test/ui/borrowck/issue-54499-field-mutation-marks-mut-as-used.rs

@@ -0,0 +1,49 @@
+// revisions: ast nll
+
+// Since we are testing nll migration explicitly as a separate
+// revision, don't worry about the --compare-mode=nll on this test.
+
+// ignore-compare-mode-nll
+
+//[ast]compile-flags: -Z borrowck=ast
+//[nll]compile-flags: -Z borrowck=migrate -Z two-phase-borrows
+
+#![warn(unused)]
+#[derive(Debug)]
+struct S(i32);
+
+type Tuple = (S, i32);
+struct Tpair(S, i32);
+struct Spair { x: S, y: i32 }
+
+fn main() {
+    {
+        let mut t: Tuple;
+        t.0 = S(1);
+        //[nll]~^ ERROR assign to part of possibly uninitialized variable: `t` [E0381]
+        t.1 = 2;
+        println!("{:?} {:?}", t.0, t.1);
+        //[ast]~^ ERROR use of possibly uninitialized variable: `t.0` [E0381]
+        //[ast]~| ERROR use of possibly uninitialized variable: `t.1` [E0381]
+    }
+
+    {
+        let mut u: Tpair;
+        u.0 = S(1);
+        //[nll]~^ ERROR assign to part of possibly uninitialized variable: `u` [E0381]
+        u.1 = 2;
+        println!("{:?} {:?}", u.0, u.1);
+        //[ast]~^ ERROR use of possibly uninitialized variable: `u.0` [E0381]
+        //[ast]~| ERROR use of possibly uninitialized variable: `u.1` [E0381]
+    }
+
+    {
+        let mut v: Spair;
+        v.x = S(1);
+        //[nll]~^ ERROR assign to part of possibly uninitialized variable: `v` [E0381]
+        v.y = 2;
+        println!("{:?} {:?}", v.x, v.y);
+        //[ast]~^ ERROR use of possibly uninitialized variable: `v.x` [E0381]
+        //[ast]~| ERROR use of possibly uninitialized variable: `v.y` [E0381]
+    }
+}

src/test/ui/borrowck/issue-54499-field-mutation-of-moved-out-with-mut.ast.stderr

@@ -0,0 +1,69 @@
+error[E0382]: use of moved value: `t.0`
+  --> $DIR/issue-54499-field-mutation-of-moved-out-with-mut.rs:25:31
+   |
+LL |         drop(t);
+   |              - value moved here
+...
+LL |         println!("{:?} {:?}", t.0, t.1);
+   |                               ^^^ value used here after move
+   |
+   = note: move occurs because `t` has type `(S, i32)`, which does not implement the `Copy` trait
+
+error[E0382]: use of moved value: `t.1`
+  --> $DIR/issue-54499-field-mutation-of-moved-out-with-mut.rs:25:36
+   |
+LL |         drop(t);
+   |              - value moved here
+...
+LL |         println!("{:?} {:?}", t.0, t.1);
+   |                                    ^^^ value used here after move
+   |
+   = note: move occurs because `t` has type `(S, i32)`, which does not implement the `Copy` trait
+
+error[E0382]: use of moved value: `u.0`
+  --> $DIR/issue-54499-field-mutation-of-moved-out-with-mut.rs:33:31
+   |
+LL |         drop(u);
+   |              - value moved here
+...
+LL |         println!("{:?} {:?}", u.0, u.1);
+   |                               ^^^ value used here after move
+   |
+   = note: move occurs because `u` has type `Tpair`, which does not implement the `Copy` trait
+
+error[E0382]: use of moved value: `u.1`
+  --> $DIR/issue-54499-field-mutation-of-moved-out-with-mut.rs:33:36
+   |
+LL |         drop(u);
+   |              - value moved here
+...
+LL |         println!("{:?} {:?}", u.0, u.1);
+   |                                    ^^^ value used here after move
+   |
+   = note: move occurs because `u` has type `Tpair`, which does not implement the `Copy` trait
+
+error[E0382]: use of moved value: `v.x`
+  --> $DIR/issue-54499-field-mutation-of-moved-out-with-mut.rs:41:31
+   |
+LL |         drop(v);
+   |              - value moved here
+...
+LL |         println!("{:?} {:?}", v.x, v.y);
+   |                               ^^^ value used here after move
+   |
+   = note: move occurs because `v` has type `Spair`, which does not implement the `Copy` trait
+
+error[E0382]: use of moved value: `v.y`
+  --> $DIR/issue-54499-field-mutation-of-moved-out-with-mut.rs:41:36
+   |
+LL |         drop(v);
+   |              - value moved here
+...
+LL |         println!("{:?} {:?}", v.x, v.y);
+   |                                    ^^^ value used here after move
+   |
+   = note: move occurs because `v` has type `Spair`, which does not implement the `Copy` trait
+
+error: aborting due to 6 previous errors
+
+For more information about this error, try `rustc --explain E0382`.

src/test/ui/borrowck/issue-54499-field-mutation-of-moved-out-with-mut.nll.stderr

@@ -0,0 +1,33 @@
+error[E0382]: assign to part of moved value: `t`
+  --> $DIR/issue-54499-field-mutation-of-moved-out-with-mut.rs:23:9
+   |
+LL |         drop(t);
+   |              - value moved here
+LL |         t.0 = S(1);
+   |         ^^^^^^^^^^ value partially assigned here after move
+   |
+   = note: move occurs because `t` has type `(S, i32)`, which does not implement the `Copy` trait
+
+error[E0382]: assign to part of moved value: `u`
+  --> $DIR/issue-54499-field-mutation-of-moved-out-with-mut.rs:31:9
+   |
+LL |         drop(u);
+   |              - value moved here
+LL |         u.0 = S(1);
+   |         ^^^^^^^^^^ value partially assigned here after move
+   |
+   = note: move occurs because `u` has type `Tpair`, which does not implement the `Copy` trait
+
+error[E0382]: assign to part of moved value: `v`
+  --> $DIR/issue-54499-field-mutation-of-moved-out-with-mut.rs:39:9
+   |
+LL |         drop(v);
+   |              - value moved here
+LL |         v.x = S(1);
+   |         ^^^^^^^^^^ value partially assigned here after move
+   |
+   = note: move occurs because `v` has type `Spair`, which does not implement the `Copy` trait
+
+error: aborting due to 3 previous errors
+
+For more information about this error, try `rustc --explain E0382`.

src/test/ui/borrowck/issue-54499-field-mutation-of-moved-out-with-mut.rs

@@ -0,0 +1,43 @@
+// revisions: ast nll
+
+// Since we are testing nll migration explicitly as a separate
+// revision, don't worry about the --compare-mode=nll on this test.
+
+// ignore-compare-mode-nll
+
+//[ast]compile-flags: -Z borrowck=ast
+//[nll]compile-flags: -Z borrowck=migrate -Z two-phase-borrows
+
+#![warn(unused)]
+#[derive(Debug)]
+struct S(i32);
+
+type Tuple = (S, i32);
+struct Tpair(S, i32);
+struct Spair { x: S, y: i32 }
+
+fn main() {
+    {
+        let mut t: Tuple = (S(0), 0);
+        drop(t);
+        t.0 = S(1);
+        t.1 = 2;
+        println!("{:?} {:?}", t.0, t.1);
+    }
+
+    {
+        let mut u: Tpair = Tpair(S(0), 0);
+        drop(u);
+        u.0 = S(1);
+        u.1 = 2;
+        println!("{:?} {:?}", u.0, u.1);
+    }
+
+    {
+        let mut v: Spair = Spair { x: S(0), y: 0 };
+        drop(v);
+        v.x = S(1);
+        v.y = 2;
+        println!("{:?} {:?}", v.x, v.y);
+    }
+}

src/test/ui/borrowck/issue-54499-field-mutation-of-moved-out.ast.stderr

@@ -0,0 +1,124 @@
+error[E0594]: cannot assign to field `t.0` of immutable binding
+  --> $DIR/issue-54499-field-mutation-of-moved-out.rs:23:9
+   |
+LL |         let t: Tuple = (S(0), 0);
+   |             - help: make this binding mutable: `mut t`
+LL |         drop(t);
+LL |         t.0 = S(1);
+   |         ^^^^^^^^^^ cannot mutably borrow field of immutable binding
+
+error[E0594]: cannot assign to field `t.1` of immutable binding
+  --> $DIR/issue-54499-field-mutation-of-moved-out.rs:27:9
+   |
+LL |         let t: Tuple = (S(0), 0);
+   |             - help: make this binding mutable: `mut t`
+...
+LL |         t.1 = 2;
+   |         ^^^^^^^ cannot mutably borrow field of immutable binding
+
+error[E0594]: cannot assign to field `u.0` of immutable binding
+  --> $DIR/issue-54499-field-mutation-of-moved-out.rs:38:9
+   |
+LL |         let u: Tpair = Tpair(S(0), 0);
+   |             - help: make this binding mutable: `mut u`
+LL |         drop(u);
+LL |         u.0 = S(1);
+   |         ^^^^^^^^^^ cannot mutably borrow field of immutable binding
+
+error[E0594]: cannot assign to field `u.1` of immutable binding
+  --> $DIR/issue-54499-field-mutation-of-moved-out.rs:42:9
+   |
+LL |         let u: Tpair = Tpair(S(0), 0);
+   |             - help: make this binding mutable: `mut u`
+...
+LL |         u.1 = 2;
+   |         ^^^^^^^ cannot mutably borrow field of immutable binding
+
+error[E0594]: cannot assign to field `v.x` of immutable binding
+  --> $DIR/issue-54499-field-mutation-of-moved-out.rs:53:9
+   |
+LL |         let v: Spair = Spair { x: S(0), y: 0 };
+   |             - help: make this binding mutable: `mut v`
+LL |         drop(v);
+LL |         v.x = S(1);
+   |         ^^^^^^^^^^ cannot mutably borrow field of immutable binding
+
+error[E0594]: cannot assign to field `v.y` of immutable binding
+  --> $DIR/issue-54499-field-mutation-of-moved-out.rs:57:9
+   |
+LL |         let v: Spair = Spair { x: S(0), y: 0 };
+   |             - help: make this binding mutable: `mut v`
+...
+LL |         v.y = 2;
+   |         ^^^^^^^ cannot mutably borrow field of immutable binding
+
+error[E0382]: use of moved value: `t.0`
+  --> $DIR/issue-54499-field-mutation-of-moved-out.rs:30:31
+   |
+LL |         drop(t);
+   |              - value moved here
+...
+LL |         println!("{:?} {:?}", t.0, t.1);
+   |                               ^^^ value used here after move
+   |
+   = note: move occurs because `t` has type `(S, i32)`, which does not implement the `Copy` trait
+
+error[E0382]: use of moved value: `t.1`
+  --> $DIR/issue-54499-field-mutation-of-moved-out.rs:30:36
+   |
+LL |         drop(t);
+   |              - value moved here
+...
+LL |         println!("{:?} {:?}", t.0, t.1);
+   |                                    ^^^ value used here after move
+   |
+   = note: move occurs because `t` has type `(S, i32)`, which does not implement the `Copy` trait
+
+error[E0382]: use of moved value: `u.0`
+  --> $DIR/issue-54499-field-mutation-of-moved-out.rs:45:31
+   |
+LL |         drop(u);
+   |              - value moved here
+...
+LL |         println!("{:?} {:?}", u.0, u.1);
+   |                               ^^^ value used here after move
+   |
+   = note: move occurs because `u` has type `Tpair`, which does not implement the `Copy` trait
+
+error[E0382]: use of moved value: `u.1`
+  --> $DIR/issue-54499-field-mutation-of-moved-out.rs:45:36
+   |
+LL |         drop(u);
+   |              - value moved here
+...
+LL |         println!("{:?} {:?}", u.0, u.1);
+   |                                    ^^^ value used here after move
+   |
+   = note: move occurs because `u` has type `Tpair`, which does not implement the `Copy` trait
+
+error[E0382]: use of moved value: `v.x`
+  --> $DIR/issue-54499-field-mutation-of-moved-out.rs:60:31
+   |
+LL |         drop(v);
+   |              - value moved here
+...
+LL |         println!("{:?} {:?}", v.x, v.y);
+   |                               ^^^ value used here after move
+   |
+   = note: move occurs because `v` has type `Spair`, which does not implement the `Copy` trait
+
+error[E0382]: use of moved value: `v.y`
+  --> $DIR/issue-54499-field-mutation-of-moved-out.rs:60:36
+   |
+LL |         drop(v);
+   |              - value moved here
+...
+LL |         println!("{:?} {:?}", v.x, v.y);
+   |                                    ^^^ value used here after move
+   |
+   = note: move occurs because `v` has type `Spair`, which does not implement the `Copy` trait
+
+error: aborting due to 12 previous errors
+
+Some errors occurred: E0382, E0594.
+For more information about an error, try `rustc --explain E0382`.