Never pattern in function arguments diverges

compiler/rustc_hir_typeck/src/check.rs

@@ -2,8 +2,7 @@ use std::cell::RefCell;
 
 use crate::coercion::CoerceMany;
 use crate::gather_locals::GatherLocalsVisitor;
-use crate::CoroutineTypes;
-use crate::FnCtxt;
+use crate::{CoroutineTypes, Diverges, FnCtxt};
 use rustc_hir as hir;
 use rustc_hir::def::DefKind;
 use rustc_hir::intravisit::Visitor;
@@ -76,6 +75,13 @@ pub(super) fn check_fn<'a, 'tcx>(
         let ty: Option<&hir::Ty<'_>> = try { inputs_hir?.get(idx)? };
         let ty_span = ty.map(|ty| ty.span);
         fcx.check_pat_top(param.pat, param_ty, ty_span, None, None);
+        if param.pat.is_never_pattern() {
+            fcx.diverges.set(Diverges::Always {
+                span: param.pat.span,
+                custom_note: Some("any code following a never pattern is unreachable"),
+            });
+            fcx.is_whole_body_and_arguments_diverge.set(true);
+        }
 
         // Check that argument is Sized.
         if !params_can_be_unsized {

compiler/rustc_hir_typeck/src/expr.rs

@@ -215,13 +215,17 @@ impl<'a, 'tcx> FnCtxt<'a, 'tcx> {
             _ => false,
         };
 
-        // Warn for expressions after diverging siblings.
-        if !is_try_block_generated_unit_expr {
-            self.warn_if_unreachable(expr.hir_id, expr.span, "expression");
-        }
+        let old_diverges = if self.is_whole_body_and_arguments_diverge.replace(false) {
+            self.diverges.get()
+        } else {
+            // Warn for expressions after diverging siblings.
+            if !is_try_block_generated_unit_expr {
+                self.warn_if_unreachable(expr.hir_id, expr.span, "expression");
+            }
 
-        // Hide the outer diverging and has_errors flags.
-        let old_diverges = self.diverges.replace(Diverges::Maybe);
+            // Hide the outer diverging flag.
+            self.diverges.replace(Diverges::Maybe)
+        };
 
         let ty = ensure_sufficient_stack(|| match &expr.kind {
             hir::ExprKind::Path(

compiler/rustc_hir_typeck/src/fn_ctxt/mod.rs

@@ -103,6 +103,10 @@ pub struct FnCtxt<'a, 'tcx> {
     /// the diverges flag is set to something other than `Maybe`.
     pub(super) diverges: Cell<Diverges>,
 
+    /// Whether the currently checked node is the whole body of the function, and one of the
+    /// function arguments is a never pattern. This makes the whole body of the function diverge.
+    pub(super) is_whole_body_and_arguments_diverge: Cell<bool>,
+
     pub(super) enclosing_breakables: RefCell<EnclosingBreakables<'tcx>>,
 
     pub(super) inh: &'a Inherited<'tcx>,
@@ -124,6 +128,7 @@ impl<'a, 'tcx> FnCtxt<'a, 'tcx> {
             ret_coercion_span: Cell::new(None),
             coroutine_types: None,
             diverges: Cell::new(Diverges::Maybe),
+            is_whole_body_and_arguments_diverge: Cell::new(false),
             enclosing_breakables: RefCell::new(EnclosingBreakables {
                 stack: Vec::new(),
                 by_id: Default::default(),

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

@@ -0,0 +1,35 @@
+#![feature(never_patterns)]
+#![allow(incomplete_features)]
+#![deny(unreachable_patterns)]
+#![deny(unreachable_code)]
+
+fn main() {}
+
+enum Void {}
+
+fn never_arg(!: Void) -> u32 {
+    println!();
+    //~^ ERROR unreachable statement
+}
+
+fn ref_never_arg(&!: &Void) -> u32 {
+    println!();
+    //~^ ERROR unreachable statement
+}
+
+//fn never_let() -> u32 {
+//    let ptr: *const Void = std::ptr::null();
+//    unsafe {
+//        let ! = *ptr;
+//    }
+//    println!();
+//}
+
+fn never_match() -> u32 {
+    let ptr: *const Void = std::ptr::null();
+    unsafe {
+        match *ptr { ! };
+    }
+    println!();
+    //~^ ERROR unreachable statement
+}

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

@@ -0,0 +1,38 @@
+error: unreachable statement
+  --> $DIR/diverge-causes-unreachable-code.rs:11:5
+   |
+LL | fn never_arg(!: Void) -> u32 {
+   |              - any code following a never pattern is unreachable
+LL |     println!();
+   |     ^^^^^^^^^^ unreachable statement
+   |
+note: the lint level is defined here
+  --> $DIR/diverge-causes-unreachable-code.rs:4:9
+   |
+LL | #![deny(unreachable_code)]
+   |         ^^^^^^^^^^^^^^^^
+   = note: this error originates in the macro `$crate::print` which comes from the expansion of the macro `println` (in Nightly builds, run with -Z macro-backtrace for more info)
+
+error: unreachable statement
+  --> $DIR/diverge-causes-unreachable-code.rs:16:5
+   |
+LL | fn ref_never_arg(&!: &Void) -> u32 {
+   |                  -- any code following a never pattern is unreachable
+LL |     println!();
+   |     ^^^^^^^^^^ unreachable statement
+   |
+   = note: this error originates in the macro `$crate::print` which comes from the expansion of the macro `println` (in Nightly builds, run with -Z macro-backtrace for more info)
+
+error: unreachable statement
+  --> $DIR/diverge-causes-unreachable-code.rs:33:5
+   |
+LL |         match *ptr { ! };
+   |         ---------------- any code following this `match` expression is unreachable, as all arms diverge
+LL |     }
+LL |     println!();
+   |     ^^^^^^^^^^ unreachable statement
+   |
+   = note: this error originates in the macro `$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 3 previous errors
+

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

@@ -0,0 +1,56 @@
+#![feature(never_patterns)]
+#![feature(let_chains)]
+#![allow(incomplete_features)]
+#![deny(unreachable_patterns)]
+
+fn main() {}
+
+enum Void {}
+
+// Contrast with `./diverges.rs`: merely having an empty type around isn't enough to diverge.
+
+fn wild_void(_: Void) -> u32 {}
+//~^ ERROR: mismatched types
+
+fn wild_let() -> u32 {
+    let ptr: *const Void = std::ptr::null();
+    unsafe {
+        //~^ ERROR: mismatched types
+        let _ = *ptr;
+    }
+}
+
+fn wild_match() -> u32 {
+    let ptr: *const Void = std::ptr::null();
+    unsafe {
+        match *ptr {
+            _ => {} //~ ERROR: mismatched types
+        }
+    }
+}
+
+fn binding_void(_x: Void) -> u32 {}
+//~^ ERROR: mismatched types
+
+fn binding_let() -> u32 {
+    let ptr: *const Void = std::ptr::null();
+    unsafe {
+        //~^ ERROR: mismatched types
+        let _x = *ptr;
+    }
+}
+
+fn binding_match() -> u32 {
+    let ptr: *const Void = std::ptr::null();
+    unsafe {
+        match *ptr {
+            _x => {} //~ ERROR: mismatched types
+        }
+    }
+}
+
+// Don't confuse this with a `let !` statement.
+fn let_chain(x: Void) -> u32 {
+    if let true = true && let ! = x {}
+    //~^ ERROR: mismatched types
+}

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

@@ -0,0 +1,55 @@
+error[E0308]: mismatched types
+  --> $DIR/diverges-not.rs:12:26
+   |
+LL | fn wild_void(_: Void) -> u32 {}
+   |    ---------             ^^^ expected `u32`, found `()`
+   |    |
+   |    implicitly returns `()` as its body has no tail or `return` expression
+
+error[E0308]: mismatched types
+  --> $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: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: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:54:37
+   |
+LL |     if let true = true && let ! = x {}
+   |                                     ^^ expected `u32`, found `()`
+
+error: aborting due to 7 previous errors
+
+For more information about this error, try `rustc --explain E0308`.

tests/ui/rfcs/rfc-0000-never_patterns/diverges.rs

@@ -0,0 +1,29 @@
+// check-pass
+#![feature(never_patterns)]
+#![allow(incomplete_features)]
+#![deny(unreachable_patterns)]
+
+fn main() {}
+
+enum Void {}
+
+// A never pattern alone diverges.
+
+fn never_arg(!: Void) -> u32 {}
+
+fn ref_never_arg(&!: &Void) -> u32 {}
+
+// fn never_let() -> u32 {
+//     let ptr: *const Void = std::ptr::null();
+//     unsafe {
+//         let ! = *ptr;
+//     }
+// }
+
+fn never_match() -> u32 {
+    let ptr: *const Void = std::ptr::null();
+    unsafe {
+        match *ptr { ! };
+    }
+    println!(); // Ensures this typechecks because of divergence.
+}