Auto merge of #147900 - Zalathar:rollup-ril6jsi, r=Zalathar

Rollup of 3 pull requests

Successful merges:

 - #146167 (Deny-by-default never type lints)
 - #147382 (unused_must_use: Don't warn on `Result<(), Uninhabited>` or `ControlFlow<Uninhabited, ()>`)
 - #147821 (Do not GC the current active incremental session directory)

r? `@ghost`
`@rustbot` modify labels: rollup

Author: bors

compiler/rustc_incremental/src/persist/fs.rs

@@ -721,11 +721,37 @@ pub(crate) fn garbage_collect_session_directories(sess: &Session) -> io::Result<
         }
     }
 
+    let current_session_directory_name =
+        session_directory.file_name().expect("session directory is not `..`");
+
     // Now garbage collect the valid session directories.
     let deletion_candidates =
         lock_file_to_session_dir.items().filter_map(|(lock_file_name, directory_name)| {
             debug!("garbage_collect_session_directories() - inspecting: {}", directory_name);
 
+            if directory_name.as_str() == current_session_directory_name {
+                // Skipping our own directory is, unfortunately, important for correctness.
+                //
+                // To summarize #147821: we will try to lock directories before deciding they can be
+                // garbage collected, but the ability of `flock::Lock` to detect a lock held *by the
+                // same process* varies across file locking APIs. Then, if our own session directory
+                // has become old enough to be eligible for GC, we are beholden to platform-specific
+                // details about detecting the our own lock on the session directory.
+                //
+                // POSIX `fcntl(F_SETLK)`-style file locks are maintained across a process. On
+                // systems where this is the mechanism for `flock::Lock`, there is no way to
+                // discover if an `flock::Lock` has been created in the same process on the same
+                // file. Attempting to set a lock on the lockfile again will succeed, even if the
+                // lock was set by another thread, on another file descriptor. Then we would
+                // garbage collect our own live directory, unable to tell it was locked perhaps by
+                // this same thread.
+                //
+                // It's not clear that `flock::Lock` can be fixed for this in general, and our own
+                // incremental session directory is the only one which this process may own, so skip
+                // it here and avoid the problem. We know it's not garbage anyway: we're using it.
+                return None;
+            }
+
             let Ok(timestamp) = extract_timestamp_from_session_dir(directory_name) else {
                 debug!(
                     "found session-dir with malformed timestamp: {}",

compiler/rustc_lint/src/unused.rs

@@ -273,13 +273,13 @@ impl<'tcx> LateLintPass<'tcx> for UnusedResults {
             expr: &hir::Expr<'_>,
             span: Span,
         ) -> Option<MustUsePath> {
-            if ty.is_unit()
-                || !ty.is_inhabited_from(
-                    cx.tcx,
-                    cx.tcx.parent_module(expr.hir_id).to_def_id(),
-                    cx.typing_env(),
-                )
-            {
+            if ty.is_unit() {
+                return Some(MustUsePath::Suppressed);
+            }
+            let parent_mod_did = cx.tcx.parent_module(expr.hir_id).to_def_id();
+            let is_uninhabited =
+                |t: Ty<'tcx>| !t.is_inhabited_from(cx.tcx, parent_mod_did, cx.typing_env());
+            if is_uninhabited(ty) {
                 return Some(MustUsePath::Suppressed);
             }
 
@@ -293,6 +293,22 @@ impl<'tcx> LateLintPass<'tcx> for UnusedResults {
                     is_ty_must_use(cx, pinned_ty, expr, span)
                         .map(|inner| MustUsePath::Pinned(Box::new(inner)))
                 }
+                // Suppress warnings on `Result<(), Uninhabited>` (e.g. `Result<(), !>`).
+                ty::Adt(def, args)
+                    if cx.tcx.is_diagnostic_item(sym::Result, def.did())
+                        && args.type_at(0).is_unit()
+                        && is_uninhabited(args.type_at(1)) =>
+                {
+                    Some(MustUsePath::Suppressed)
+                }
+                // Suppress warnings on `ControlFlow<Uninhabited, ()>` (e.g. `ControlFlow<!, ()>`).
+                ty::Adt(def, args)
+                    if cx.tcx.is_diagnostic_item(sym::ControlFlow, def.did())
+                        && args.type_at(1).is_unit()
+                        && is_uninhabited(args.type_at(0)) =>
+                {
+                    Some(MustUsePath::Suppressed)
+                }
                 ty::Adt(def, _) => is_def_must_use(cx, def.did(), span),
                 ty::Alias(ty::Opaque | ty::Projection, ty::AliasTy { def_id: def, .. }) => {
                     elaborate(cx.tcx, cx.tcx.explicit_item_self_bounds(def).iter_identity_copied())

compiler/rustc_lint_defs/src/builtin.rs

@@ -4065,7 +4065,6 @@ declare_lint! {
     /// ### Example
     ///
     /// ```rust,compile_fail
-    /// #![deny(never_type_fallback_flowing_into_unsafe)]
     /// fn main() {
     ///     if true {
     ///         // return has type `!` which, is some cases, causes never type fallback
@@ -4100,7 +4099,7 @@ declare_lint! {
     /// [`!`]: https://doc.rust-lang.org/core/primitive.never.html
     /// [`()`]: https://doc.rust-lang.org/core/primitive.unit.html
     pub NEVER_TYPE_FALLBACK_FLOWING_INTO_UNSAFE,
-    Warn,
+    Deny,
     "never type fallback affecting unsafe function calls",
     @future_incompatible = FutureIncompatibleInfo {
         reason: FutureIncompatibilityReason::EditionAndFutureReleaseSemanticsChange(Edition::Edition2024),
@@ -4122,7 +4121,7 @@ declare_lint! {
     /// ### Example
     ///
     /// ```rust,compile_fail,edition2021
-    /// #![deny(dependency_on_unit_never_type_fallback)]
+    /// # #![deny(dependency_on_unit_never_type_fallback)]
     /// fn main() {
     ///     if true {
     ///         // return has type `!` which, is some cases, causes never type fallback
@@ -4155,7 +4154,7 @@ declare_lint! {
     ///
     /// See [Tracking Issue for making `!` fall back to `!`](https://github.com/rust-lang/rust/issues/123748).
     pub DEPENDENCY_ON_UNIT_NEVER_TYPE_FALLBACK,
-    Warn,
+    Deny,
     "never type fallback affecting unsafe function calls",
     @future_incompatible = FutureIncompatibleInfo {
         reason: FutureIncompatibilityReason::EditionAndFutureReleaseError(Edition::Edition2024),

tests/ui/editions/never-type-fallback-breaking.e2021.fixed

@@ -3,9 +3,7 @@
 //@[e2021] edition: 2021
 //@[e2024] edition: 2024
 //
-//@[e2021] run-pass
 //@[e2021] run-rustfix
-//@[e2024] check-fail
 
 fn main() {
     m();
@@ -16,8 +14,8 @@ fn main() {
 }
 
 fn m() {
-    //[e2021]~^ WARN this function depends on never type fallback being `()`
-    //[e2021]~| WARN this was previously accepted by the compiler but is being phased out; it will become a hard error in Rust 2024 and in a future release in all editions!
+    //[e2021]~^ error: this function depends on never type fallback being `()`
+    //[e2021]~| warn: this was previously accepted by the compiler but is being phased out; it will become a hard error in Rust 2024 and in a future release in all editions!
     let x: () = match true {
         true => Default::default(),
         //[e2024]~^ error: the trait bound `!: Default` is not satisfied
@@ -28,8 +26,8 @@ fn m() {
 }
 
 fn q() -> Option<()> {
-    //[e2021]~^ WARN this function depends on never type fallback being `()`
-    //[e2021]~| WARN this was previously accepted by the compiler but is being phased out; it will become a hard error in Rust 2024 and in a future release in all editions!
+    //[e2021]~^ error: this function depends on never type fallback being `()`
+    //[e2021]~| warn: this was previously accepted by the compiler but is being phased out; it will become a hard error in Rust 2024 and in a future release in all editions!
     fn deserialize<T: Default>() -> Option<T> {
         Some(T::default())
     }
@@ -45,8 +43,8 @@ fn help<'a: 'a, T: Into<()>, U>(_: U) -> Result<T, ()> {
     Err(())
 }
 fn meow() -> Result<(), ()> {
-    //[e2021]~^ WARN this function depends on never type fallback being `()`
-    //[e2021]~| WARN this was previously accepted by the compiler but is being phased out; it will become a hard error in Rust 2024 and in a future release in all editions!
+    //[e2021]~^ error: this function depends on never type fallback being `()`
+    //[e2021]~| warn: this was previously accepted by the compiler but is being phased out; it will become a hard error in Rust 2024 and in a future release in all editions!
     help::<(), _>(1)?;
     //[e2024]~^ error: the trait bound `(): From<!>` is not satisfied
     Ok(())
@@ -57,8 +55,8 @@ pub fn takes_apit<T>(_y: impl Fn() -> T) -> Result<T, ()> {
 }
 
 pub fn fallback_return() -> Result<(), ()> {
-    //[e2021]~^ WARN this function depends on never type fallback being `()`
-    //[e2021]~| WARN this was previously accepted by the compiler but is being phased out; it will become a hard error in Rust 2024 and in a future release in all editions!
+    //[e2021]~^ error: this function depends on never type fallback being `()`
+    //[e2021]~| warn: this was previously accepted by the compiler but is being phased out; it will become a hard error in Rust 2024 and in a future release in all editions!
     takes_apit::<()>(|| Default::default())?;
     //[e2024]~^ error: the trait bound `!: Default` is not satisfied
     Ok(())
@@ -71,8 +69,8 @@ fn mk<T>() -> Result<T, ()> {
 fn takes_apit2(_x: impl Default) {}
 
 fn fully_apit() -> Result<(), ()> {
-    //[e2021]~^ WARN this function depends on never type fallback being `()`
-    //[e2021]~| WARN this was previously accepted by the compiler but is being phased out; it will become a hard error in Rust 2024 and in a future release in all editions!
+    //[e2021]~^ error: this function depends on never type fallback being `()`
+    //[e2021]~| warn: this was previously accepted by the compiler but is being phased out; it will become a hard error in Rust 2024 and in a future release in all editions!
     takes_apit2(mk::<()>()?);
     //[e2024]~^ error: the trait bound `!: Default` is not satisfied
     Ok(())