Feature gate and make must_not_suspend allow-by-default

This lint is not yet ready for stable use, primarily due to false positives in edge
cases; we want to test it out more before stabilizing.

compiler/rustc_lint/src/lib.rs

@@ -300,7 +300,6 @@ fn register_builtins(store: &mut LintStore, no_interleave_lints: bool) {
         UNUSED_LABELS,
         UNUSED_PARENS,
         UNUSED_BRACES,
-        MUST_NOT_SUSPEND,
         REDUNDANT_SEMICOLONS
     );
 

compiler/rustc_lint_defs/src/builtin.rs

@@ -323,6 +323,7 @@ declare_lint! {
     ///
     /// ```rust
     /// #![feature(must_not_suspend)]
+    /// #![warn(must_not_suspend)]
     ///
     /// #[must_not_suspend]
     /// struct SyncThing {}
@@ -349,8 +350,9 @@ declare_lint! {
     /// `MutexGuard`'s)
     ///
     pub MUST_NOT_SUSPEND,
-    Warn,
+    Allow,
     "use of a `#[must_not_suspend]` value across a yield point",
+    @feature_gate = rustc_span::symbol::sym::must_not_suspend;
 }
 
 declare_lint! {

src/test/ui/async-await/issue-64130-non-send-future-diags.rs

@@ -1,4 +1,6 @@
 // edition:2018
+#![feature(must_not_suspend)]
+#![allow(must_not_suspend)]
 
 // This tests the basic example case for the async-await-specific error.
 

src/test/ui/async-await/issue-64130-non-send-future-diags.stderr

@@ -1,12 +1,12 @@
 error: future cannot be sent between threads safely
-  --> $DIR/issue-64130-non-send-future-diags.rs:21:13
+  --> $DIR/issue-64130-non-send-future-diags.rs:23:13
    |
 LL |     is_send(foo());
    |             ^^^^^ future returned by `foo` is not `Send`
    |
    = help: within `impl Future`, the trait `Send` is not implemented for `MutexGuard<'_, u32>`
 note: future is not `Send` as this value is used across an await
-  --> $DIR/issue-64130-non-send-future-diags.rs:15:5
+  --> $DIR/issue-64130-non-send-future-diags.rs:17:5
    |
 LL |     let g = x.lock().unwrap();
    |         - has type `MutexGuard<'_, u32>` which is not `Send`
@@ -15,7 +15,7 @@ LL |     baz().await;
 LL | }
    | - `g` is later dropped here
 note: required by a bound in `is_send`
-  --> $DIR/issue-64130-non-send-future-diags.rs:7:15
+  --> $DIR/issue-64130-non-send-future-diags.rs:9:15
    |
 LL | fn is_send<T: Send>(t: T) { }
    |               ^^^^ required by this bound in `is_send`

src/test/ui/async-await/issue-71137.rs

@@ -1,4 +1,6 @@
 // edition:2018
+#![feature(must_not_suspend)]
+#![allow(must_not_suspend)]
 
 use std::future::Future;
 use std::sync::Mutex;

src/test/ui/async-await/issue-71137.stderr

@@ -1,12 +1,12 @@
 error: future cannot be sent between threads safely
-  --> $DIR/issue-71137.rs:20:14
+  --> $DIR/issue-71137.rs:22:14
    |
 LL |   fake_spawn(wrong_mutex());
    |              ^^^^^^^^^^^^^ future returned by `wrong_mutex` is not `Send`
    |
    = help: within `impl Future`, the trait `Send` is not implemented for `MutexGuard<'_, i32>`
 note: future is not `Send` as this value is used across an await
-  --> $DIR/issue-71137.rs:12:5
+  --> $DIR/issue-71137.rs:14:5
    |
 LL |     let mut guard = m.lock().unwrap();
    |         --------- has type `MutexGuard<'_, i32>` which is not `Send`
@@ -16,7 +16,7 @@ LL |     *guard += 1;
 LL |   }
    |   - `mut guard` is later dropped here
 note: required by a bound in `fake_spawn`
-  --> $DIR/issue-71137.rs:6:27
+  --> $DIR/issue-71137.rs:8:27
    |
 LL | fn fake_spawn<F: Future + Send + 'static>(f: F) { }
    |                           ^^^^ required by this bound in `fake_spawn`

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

@@ -0,0 +1,14 @@
+// edition:2018
+#![deny(must_not_suspend)]  //~ ERROR the `must_not_suspend`
+//~| ERROR the `must_not_suspend`
+//~| ERROR the `must_not_suspend`
+
+async fn other() {}
+
+pub async fn uhoh(m: std::sync::Mutex<()>) {
+    let _guard = m.lock().unwrap(); //~ ERROR `MutexGuard` held across
+    other().await;
+}
+
+fn main() {
+}

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

@@ -0,0 +1,54 @@
+error[E0658]: the `must_not_suspend` lint is unstable
+  --> $DIR/gated.rs:2:1
+   |
+LL | #![deny(must_not_suspend)]
+   | ^^^^^^^^^^^^^^^^^^^^^^^^^^
+   |
+   = note: see issue #83310 <https://github.com/rust-lang/rust/issues/83310> for more information
+   = help: add `#![feature(must_not_suspend)]` to the crate attributes to enable
+
+error[E0658]: the `must_not_suspend` lint is unstable
+  --> $DIR/gated.rs:2:1
+   |
+LL | #![deny(must_not_suspend)]
+   | ^^^^^^^^^^^^^^^^^^^^^^^^^^
+   |
+   = note: see issue #83310 <https://github.com/rust-lang/rust/issues/83310> for more information
+   = help: add `#![feature(must_not_suspend)]` to the crate attributes to enable
+
+error[E0658]: the `must_not_suspend` lint is unstable
+  --> $DIR/gated.rs:2:1
+   |
+LL | #![deny(must_not_suspend)]
+   | ^^^^^^^^^^^^^^^^^^^^^^^^^^
+   |
+   = note: see issue #83310 <https://github.com/rust-lang/rust/issues/83310> for more information
+   = help: add `#![feature(must_not_suspend)]` to the crate attributes to enable
+
+error: `MutexGuard` held across a suspend point, but should not be
+  --> $DIR/gated.rs:9:9
+   |
+LL |     let _guard = m.lock().unwrap();
+   |         ^^^^^^
+LL |     other().await;
+   |     ------------- the value is held across this suspend point
+   |
+note: the lint level is defined here
+  --> $DIR/gated.rs:2:9
+   |
+LL | #![deny(must_not_suspend)]
+   |         ^^^^^^^^^^^^^^^^
+note: holding a MutexGuard across suspend points can cause deadlocks, delays, and cause Futures to not implement `Send`
+  --> $DIR/gated.rs:9:9
+   |
+LL |     let _guard = m.lock().unwrap();
+   |         ^^^^^^
+help: consider using a block (`{ ... }`) to shrink the value's scope, ending before the suspend point
+  --> $DIR/gated.rs:9:9
+   |
+LL |     let _guard = m.lock().unwrap();
+   |         ^^^^^^
+
+error: aborting due to 4 previous errors
+
+For more information about this error, try `rustc --explain E0658`.

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/mutex.rs

@@ -1,4 +1,5 @@
 // edition:2018
+#![feature(must_not_suspend)]
 #![deny(must_not_suspend)]
 
 async fn other() {}

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

@@ -1,5 +1,5 @@
 error: `MutexGuard` held across a suspend point, but should not be
-  --> $DIR/mutex.rs:7:9
+  --> $DIR/mutex.rs:8:9
    |
 LL |     let _guard = m.lock().unwrap();
    |         ^^^^^^
@@ -7,17 +7,17 @@ LL |     other().await;
    |     ------------- the value is held across this suspend point
    |
 note: the lint level is defined here
-  --> $DIR/mutex.rs:2:9
+  --> $DIR/mutex.rs:3:9
    |
 LL | #![deny(must_not_suspend)]
    |         ^^^^^^^^^^^^^^^^
 note: holding a MutexGuard across suspend points can cause deadlocks, delays, and cause Futures to not implement `Send`
-  --> $DIR/mutex.rs:7:9
+  --> $DIR/mutex.rs:8:9
    |
 LL |     let _guard = m.lock().unwrap();
    |         ^^^^^^
 help: consider using a block (`{ ... }`) to shrink the value's scope, ending before the suspend point
-  --> $DIR/mutex.rs:7:9
+  --> $DIR/mutex.rs:8:9
    |
 LL |     let _guard = m.lock().unwrap();
    |         ^^^^^^

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();
    |         ^^^^^^