Make must_not_suspend allow-by-default until edge cases are ironed out

compiler/rustc_lint_defs/src/builtin.rs

@@ -322,6 +322,7 @@ declare_lint! {
     ///
     /// ```rust
     /// #![feature(must_not_suspend)]
+    /// #![warn(must_not_suspend)]
     ///
     /// #[must_not_suspend]
     /// struct SyncThing {}
@@ -348,7 +349,7 @@ declare_lint! {
     /// `MutexGuard`'s)
     ///
     pub MUST_NOT_SUSPEND,
-    Warn,
+    Allow,
     "use of a `#[must_not_suspend]` value across a yield point",
 }
 

src/test/ui/lint/must_not_suspend/issue-89562.rs

@@ -0,0 +1,19 @@
+// edition:2018
+// run-pass
+
+use std::sync::Mutex;
+
+// Copied from the issue. Allow-by-default for now, so run-pass
+pub async fn foo() {
+    let foo = Mutex::new(1);
+    let lock = foo.lock().unwrap();
+
+    // Prevent mutex lock being held across `.await` point.
+    drop(lock);
+
+    bar().await;
+}
+
+async fn bar() {}
+
+fn main() {}

src/test/ui/lint/must_not_suspend/warn.rs

@@ -1,6 +1,7 @@
 // edition:2018
 // run-pass
 #![feature(must_not_suspend)]
+#![warn(must_not_suspend)]
 
 #[must_not_suspend = "You gotta use Umm's, ya know?"]
 struct Umm {

src/test/ui/lint/must_not_suspend/warn.stderr

@@ -1,19 +1,23 @@
 warning: `Umm` held across a suspend point, but should not be
-  --> $DIR/warn.rs:20:9
+  --> $DIR/warn.rs:21:9
    |
 LL |     let _guard = bar();
    |         ^^^^^^
 LL |     other().await;
    |     ------------- the value is held across this suspend point
    |
-   = note: `#[warn(must_not_suspend)]` on by default
+note: the lint level is defined here
+  --> $DIR/warn.rs:4:9
+   |
+LL | #![warn(must_not_suspend)]
+   |         ^^^^^^^^^^^^^^^^
 note: You gotta use Umm's, ya know?
-  --> $DIR/warn.rs:20:9
+  --> $DIR/warn.rs:21:9
    |
 LL |     let _guard = bar();
    |         ^^^^^^
 help: consider using a block (`{ ... }`) to shrink the value's scope, ending before the suspend point
-  --> $DIR/warn.rs:20:9
+  --> $DIR/warn.rs:21:9
    |
 LL |     let _guard = bar();
    |         ^^^^^^