Introduce lint DivergeReason.

Author: cjgillot

compiler/rustc_hir_typeck/src/diverges.rs

@@ -14,6 +14,14 @@ pub enum Diverges {
     /// others require a CFG to determine them.
     Maybe,
 
+    /// This expression is uninhabited, we want to
+    /// emit a diagnostic but not pollute type checking.
+    UninhabitedExpr(HirId, Span),
+
+    /// Same as `UninhabitedExpr` but with reachability
+    /// warning already emitted.
+    Warned,
+
     /// Definitely known to diverge and therefore
     /// not reach the next sibling or its parent.
     Always(DivergeReason, Span),
@@ -55,16 +63,18 @@ impl ops::BitOrAssign for Diverges {
 impl Diverges {
     pub(super) fn is_always(self) -> bool {
         match self {
-            Self::Maybe => false,
+            Self::Maybe | Diverges::UninhabitedExpr(..) | Diverges::Warned => false,
             Self::Always(..) | Self::WarnedAlways => true,
         }
     }
 
     fn ordinal(&self) -> u8 {
         match self {
             Self::Maybe => 0,
-            Self::Always { .. } => 1,
-            Self::WarnedAlways => 2,
+            Self::UninhabitedExpr(..) => 1,
+            Self::Warned => 2,
+            Self::Always { .. } => 3,
+            Self::WarnedAlways => 4,
         }
     }
 }

compiler/rustc_hir_typeck/src/expr.rs

@@ -238,20 +238,21 @@ impl<'a, 'tcx> FnCtxt<'a, 'tcx> {
             ExprKind::Field(..)
                 if matches!(
                     self.diverges.get(),
-                    Diverges::Always(DivergeReason::UninhabitedExpr(_), _)
+                    Diverges::UninhabitedExpr(_, _)
                 ) && self.ty_is_uninhabited(ty) => {}
             _ => self.warn_if_unreachable(expr.hir_id, expr.span, "expression"),
         }
 
-        if !self.diverges.get().is_always() {
+        let cur_diverges = self.diverges.get();
+        if !cur_diverges.is_always() {
             if ty.is_never() {
                 // Any expression that produces a value of type `!` must have diverged.
-                self.diverges.set(Diverges::Always(DivergeReason::Other, expr.span));
+                self.diverges.set(cur_diverges | Diverges::Always(DivergeReason::Other, expr.span));
             } else if self.ty_is_uninhabited(ty) {
                 // This expression produces a value of uninhabited type.
                 // This means it has diverged somehow.
                 self.diverges
-                    .set(Diverges::Always(DivergeReason::UninhabitedExpr(expr.hir_id), expr.span));
+                    .set(cur_diverges | Diverges::UninhabitedExpr(expr.hir_id, expr.span));
             }
         }
 

compiler/rustc_hir_typeck/src/fn_ctxt/_impl.rs

@@ -51,8 +51,12 @@ impl<'a, 'tcx> FnCtxt<'a, 'tcx> {
     /// Produces warning on the given node, if the current point in the
     /// function is unreachable, and there hasn't been another warning.
     pub(crate) fn warn_if_unreachable(&self, id: HirId, span: Span, kind: &str) {
-        let Diverges::Always(reason, orig_span) = self.diverges.get() else {
-            return;
+        let (reason, orig_span) = match self.diverges.get() {
+            Diverges::UninhabitedExpr(hir_id, orig_span) => {
+                (DivergeReason::UninhabitedExpr(hir_id), orig_span)
+            }
+            Diverges::Always(reason, orig_span) => (reason, orig_span),
+            Diverges::Maybe | Diverges::Warned | Diverges::WarnedAlways => return,
         };
 
         match span.desugaring_kind() {
@@ -74,9 +78,6 @@ impl<'a, 'tcx> FnCtxt<'a, 'tcx> {
             _ => {}
         }
 
-        // Don't warn twice.
-        self.diverges.set(Diverges::WarnedAlways);
-
         if matches!(reason, DivergeReason::UninhabitedExpr(_)) {
             if let Some(impl_of) = self.tcx.impl_of_method(self.body_id.to_def_id()) {
                 if self.tcx.has_attr(impl_of, sym::automatically_derived) {
@@ -87,6 +88,13 @@ impl<'a, 'tcx> FnCtxt<'a, 'tcx> {
             }
         }
 
+        // Don't warn twice.
+        self.diverges.set(match self.diverges.get() {
+            Diverges::UninhabitedExpr(..) => Diverges::Warned,
+            Diverges::Always(..) => Diverges::WarnedAlways,
+            Diverges::Maybe | Diverges::Warned | Diverges::WarnedAlways => bug!(),
+        });
+
         debug!("warn_if_unreachable: id={:?} span={:?} kind={}", id, span, kind);
 
         let msg = format!("unreachable {kind}");

tests/ui/consts/let-irrefutable-pattern-ice-120337.rs

@@ -1,8 +1,9 @@
+//@ check-pass
 #![feature(never_type)]
 #[derive(Copy, Clone)]
 pub enum E { A(!), }
 pub union U { u: (), e: E, }
-pub const C: () = { //~ ERROR evaluation of constant value failed
+pub const C: () = {
     let E::A(ref a) = unsafe { &(&U { u: () }).e};
 };
 

tests/ui/consts/let-irrefutable-pattern-ice-120337.stderr

@@ -2,7 +2,7 @@ error: unreachable statement
   --> $DIR/match-no-arms-unreachable-after.rs:8:5
    |
 LL |     match v { }
-   |           - this expression has type `Void`, which is uninhabited
+   |     ----------- any code following this expression is unreachable
 LL |     let x = 2;
    |     ^^^^^^^^^^ unreachable statement
    |

tests/ui/match/match-no-arms-unreachable-after.stderr

@@ -33,4 +33,5 @@ fn never_match() -> u32 {
         //~^ ERROR unreachable arm
     }
     println!();
+    //~^ ERROR unreachable statement
 }

tests/ui/rfcs/rfc-0000-never_patterns/diverge-causes-unreachable-code.rs

@@ -42,5 +42,16 @@ LL |         match *ptr { ! };
    |               |
    |               this expression has type `Void`, which is uninhabited
 
-error: aborting due to 4 previous errors
+error: unreachable statement
+  --> $DIR/diverge-causes-unreachable-code.rs:34:5
+   |
+LL |         match *ptr { ! };
+   |         ---------------- any code following this `match` expression is unreachable, as all arms diverge
+LL |     }
+LL |     println!();
+   |     ^^^^^^^^^^ unreachable statement
+   |
+   = note: this error originates in the macro `$crate::print` which comes from the expansion of the macro `println` (in Nightly builds, run with -Z macro-backtrace for more info)
+
+error: aborting due to 5 previous errors
 

tests/ui/rfcs/rfc-0000-never_patterns/diverge-causes-unreachable-code.stderr

@@ -15,6 +15,7 @@ fn wild_void(_: Void) -> u32 {}
 fn wild_let() -> u32 {
     let ptr: *const Void = std::ptr::null();
     unsafe {
+        //~^ ERROR: mismatched types
         let _ = *ptr;
     }
 }
@@ -34,8 +35,8 @@ fn binding_void(_x: Void) -> u32 {}
 fn binding_let() -> u32 {
     let ptr: *const Void = std::ptr::null();
     unsafe {
+        //~^ ERROR: mismatched types
         let _x = *ptr;
-        //~^ ERROR: cannot move
     }
 }
 

tests/ui/rfcs/rfc-0000-never_patterns/diverges-not.rs

@@ -7,47 +7,49 @@ LL | fn wild_void(_: Void) -> u32 {}
    |    implicitly returns `()` as its body has no tail or `return` expression
 
 error[E0308]: mismatched types
-  --> $DIR/diverges-not.rs:26:18
+  --> $DIR/diverges-not.rs:17:5
+   |
+LL | /     unsafe {
+LL | |
+LL | |         let _ = *ptr;
+LL | |     }
+   | |_____^ expected `u32`, found `()`
+
+error[E0308]: mismatched types
+  --> $DIR/diverges-not.rs:27:18
    |
 LL |             _ => {}
    |                  ^^ expected `u32`, found `()`
 
 error[E0308]: mismatched types
-  --> $DIR/diverges-not.rs:31:30
+  --> $DIR/diverges-not.rs:32:30
    |
 LL | fn binding_void(_x: Void) -> u32 {}
    |    ------------              ^^^ expected `u32`, found `()`
    |    |
    |    implicitly returns `()` as its body has no tail or `return` expression
 
 error[E0308]: mismatched types
-  --> $DIR/diverges-not.rs:46:19
+  --> $DIR/diverges-not.rs:37:5
+   |
+LL | /     unsafe {
+LL | |
+LL | |         let _x = *ptr;
+LL | |     }
+   | |_____^ expected `u32`, found `()`
+
+error[E0308]: mismatched types
+  --> $DIR/diverges-not.rs:47:19
    |
 LL |             _x => {}
    |                   ^^ expected `u32`, found `()`
 
 error[E0308]: mismatched types
-  --> $DIR/diverges-not.rs:53:37
+  --> $DIR/diverges-not.rs:54:37
    |
 LL |     if let true = true && let ! = x {}
    |                                     ^^ expected `u32`, found `()`
 
-error[E0507]: cannot move out of `*ptr` which is behind a raw pointer
-  --> $DIR/diverges-not.rs:37:18
-   |
-LL |         let _x = *ptr;
-   |                  ^^^^ move occurs because `*ptr` has type `Void`, which does not implement the `Copy` trait
-   |
-note: if `Void` implemented `Clone`, you could clone the value
-  --> $DIR/diverges-not.rs:8:1
-   |
-LL | enum Void {}
-   | ^^^^^^^^^ consider implementing `Clone` for this type
-...
-LL |         let _x = *ptr;
-   |                  ---- you could clone this value
-
-error: aborting due to 6 previous errors
+error: aborting due to 7 previous errors
 
-Some errors have detailed explanations: E0308, E0507.
-For more information about an error, try `rustc --explain E0308`.
+For more information about this error, try `rustc --explain E0308`.

tests/ui/rfcs/rfc-0000-never_patterns/diverges-not.stderr

@@ -6,6 +6,22 @@ LL | fn loop_break_break() -> i32 {
    |    |
    |    implicitly returns `()` as its body has no tail or `return` expression
 
+error[E0308]: mismatched types
+  --> $DIR/break-diverging-value.rs:33:25
+   |
+LL | fn loop_break_void() -> i32 {
+   |    ---------------      ^^^ expected `i32`, found `()`
+   |    |
+   |    implicitly returns `()` as its body has no tail or `return` expression
+
+error[E0308]: mismatched types
+  --> $DIR/break-diverging-value.rs:43:34
+   |
+LL | fn loop_break_indirect_void() -> i32 {
+   |    ------------------------      ^^^ expected `i32`, found `()`
+   |    |
+   |    implicitly returns `()` as its body has no tail or `return` expression
+
 error[E0308]: mismatched types
   --> $DIR/break-diverging-value.rs:55:33
    |
@@ -14,6 +30,6 @@ LL | fn loop_break_private_void() -> i32 {
    |    |
    |    implicitly returns `()` as its body has no tail or `return` expression
 
-error: aborting due to 2 previous errors
+error: aborting due to 4 previous errors
 
 For more information about this error, try `rustc --explain E0308`.