Rollup merge of #83519 - oli-obk:assign_shrink_your_normal_code, r=pnkfelix

Implement a lint that highlights all moves larger than a configured limit

Tracking issue: #83518
[MCP 420](https://github.com/rust-lang/compiler-team/issues/420) still ~blazing~ in progress

r? ```@pnkfelix```

The main open issue I see with this minimal impl of the feature is that the lint is immediately "stable" (so it can be named on stable), even if it is never executed on stable. I don't think we have the concept of unstable lint names or hiding lint names without an active feature gate, so that would be a bigger change.

src/test/ui/async-await/large_moves.rs

@@ -0,0 +1,24 @@
+#![deny(large_assignments)]
+#![feature(large_assignments)]
+#![move_size_limit = "1000"]
+// build-fail
+// only-x86_64
+
+// edition:2018
+
+fn main() {
+    let x = async { //~ ERROR large_assignments
+        let y = [0; 9999];
+        dbg!(y);
+        thing(&y).await;
+        dbg!(y);
+    };
+    let z = (x, 42); //~ ERROR large_assignments
+    //~^ ERROR large_assignments
+    let a = z.0; //~ ERROR large_assignments
+    let b = z.1;
+}
+
+async fn thing(y: &[u8]) {
+    dbg!(y);
+}

src/test/ui/async-await/large_moves.stderr

@@ -0,0 +1,38 @@
+error: moving 10024 bytes
+  --> $DIR/large_moves.rs:10:13
+   |
+LL |       let x = async {
+   |  _____________^
+LL | |         let y = [0; 9999];
+LL | |         dbg!(y);
+LL | |         thing(&y).await;
+LL | |         dbg!(y);
+LL | |     };
+   | |_____^ value moved from here
+   |
+note: the lint level is defined here
+  --> $DIR/large_moves.rs:1:9
+   |
+LL | #![deny(large_assignments)]
+   |         ^^^^^^^^^^^^^^^^^
+
+error: moving 10024 bytes
+  --> $DIR/large_moves.rs:16:14
+   |
+LL |     let z = (x, 42);
+   |              ^ value moved from here
+
+error: moving 10024 bytes
+  --> $DIR/large_moves.rs:16:13
+   |
+LL |     let z = (x, 42);
+   |             ^^^^^^^ value moved from here
+
+error: moving 10024 bytes
+  --> $DIR/large_moves.rs:18:13
+   |
+LL |     let a = z.0;
+   |             ^^^ value moved from here
+
+error: aborting due to 4 previous errors
+

src/test/ui/feature-gates/feature-gate-large-assignments.rs

@@ -0,0 +1,5 @@
+// check that `move_size_limit is feature-gated
+
+#![move_size_limit = "42"] //~ ERROR the `#[move_size_limit]` attribute is an experimental feature
+
+fn main() {}

src/test/ui/feature-gates/feature-gate-large-assignments.stderr

@@ -0,0 +1,12 @@
+error[E0658]: the `#[move_size_limit]` attribute is an experimental feature
+  --> $DIR/feature-gate-large-assignments.rs:3:1
+   |
+LL | #![move_size_limit = "42"]
+   | ^^^^^^^^^^^^^^^^^^^^^^^^^^
+   |
+   = note: see issue #83518 <https://github.com/rust-lang/rust/issues/83518> for more information
+   = help: add `#![feature(large_assignments)]` to the crate attributes to enable
+
+error: aborting due to previous error
+
+For more information about this error, try `rustc --explain E0658`.