Auto merge of #88865 - guswynn:must_not_suspend, r=oli-obk

Implement `#[must_not_suspend]`

implements #83310

Some notes on the impl:

1. The code that searches for the attribute on the ADT is basically copied from the `must_use` lint. It's not shared, as the logic did diverge
2. The RFC does specify that the attribute can be placed on fn's (and fn-like objects), like `must_use`. I think this is a direct copy from the `must_use` reference definition. This implementation does NOT support this, as I felt that ADT's (+ `impl Trait` + `dyn Trait`) cover the usecase's people actually want on the RFC, and adding an imp for the fn call case would be significantly harder. The `must_use` impl can do a single check at fn call stmt time, but `must_not_suspend` would need to answer the question: "for some value X with type T, find any fn call that COULD have produced this value". That would require significant changes to `generator_interior.rs`, and I would need mentorship on that. `@eholk` and I are discussing it.
3. `@estebank` do you know a way I can make the user-provided `reason` note pop out? right now it seems quite hidden

Also, I am not sure if we should run perf on this

r? `@nikomatsakis`

compiler/rustc_lint_defs/src/builtin.rs

@@ -314,6 +314,44 @@ declare_lint! {
     "imports that are never used"
 }
 
+declare_lint! {
+    /// The `must_not_suspend` lint guards against values that shouldn't be held across suspend points
+    /// (`.await`)
+    ///
+    /// ### Example
+    ///
+    /// ```rust
+    /// #![feature(must_not_suspend)]
+    ///
+    /// #[must_not_suspend]
+    /// struct SyncThing {}
+    ///
+    /// async fn yield_now() {}
+    ///
+    /// pub async fn uhoh() {
+    ///     let guard = SyncThing {};
+    ///     yield_now().await;
+    /// }
+    /// ```
+    ///
+    /// {{produces}}
+    ///
+    /// ### Explanation
+    ///
+    /// The `must_not_suspend` lint detects values that are marked with the `#[must_not_suspend]`
+    /// attribute being held across suspend points. A "suspend" point is usually a `.await` in an async
+    /// function.
+    ///
+    /// This attribute can be used to mark values that are semantically incorrect across suspends
+    /// (like certain types of timers), values that have async alternatives, and values that
+    /// regularly cause problems with the `Send`-ness of async fn's returned futures (like
+    /// `MutexGuard`'s)
+    ///
+    pub MUST_NOT_SUSPEND,
+    Warn,
+    "use of a `#[must_not_suspend]` value across a yield point",
+}
+
 declare_lint! {
     /// The `unused_extern_crates` lint guards against `extern crate` items
     /// that are never used.
@@ -2993,6 +3031,7 @@ declare_lint_pass! {
         CENUM_IMPL_DROP_CAST,
         CONST_EVALUATABLE_UNCHECKED,
         INEFFECTIVE_UNSTABLE_TRAIT_IMPL,
+        MUST_NOT_SUSPEND,
         UNINHABITED_STATIC,
         FUNCTION_ITEM_REFERENCES,
         USELESS_DEPRECATED,