Rollup merge of #120104 - Nadrieril:never-pat-diverges, r=compiler-errors

never_patterns: Count `!` bindings as diverging

A binding that is a never pattern is not reachable, hence counts as diverging code. This allows in particular `fn foo(!: Void) -> SomeType {}` to typecheck.

r? ``@compiler-errors``

4 files changed, 34 insertions, 6 deletions

diff --git a/compiler/rustc_hir_typeck/src/check.rs b/compiler/rustc_hir_typeck/src/check.rs
index c887368b2a2..4d9f5b831c1 100644
--- a/compiler/rustc_hir_typeck/src/check.rs
+++ b/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 +75,12 @@ 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.function_diverges_because_of_empty_arguments.set(Diverges::Always {
+                span: param.pat.span,
+                custom_note: Some("any code following a never pattern is unreachable"),
+            });
+        }
 
         // Check that argument is Sized.
         if !params_can_be_unsized {
@@ -105,6 +110,7 @@ pub(super) fn check_fn<'a, 'tcx>(
         hir::FnRetTy::Return(ty) => ty.span,
     };
     fcx.require_type_is_sized(declared_ret_ty, return_or_body_span, traits::SizedReturnType);
+    fcx.is_whole_body.set(true);
     fcx.check_return_expr(body.value, false);
 
     // Finalize the return check by taking the LUB of the return types
diff --git a/compiler/rustc_hir_typeck/src/expr.rs b/compiler/rustc_hir_typeck/src/expr.rs
index f48d712cd60..3bc259c17b7 100644
--- a/compiler/rustc_hir_typeck/src/expr.rs
+++ b/compiler/rustc_hir_typeck/src/expr.rs
@@ -208,10 +208,10 @@ impl<'a, 'tcx> FnCtxt<'a, 'tcx> {
         // without the final expr (e.g. `try { return; }`). We don't want to generate an
         // unreachable_code lint for it since warnings for autogenerated code are confusing.
         let is_try_block_generated_unit_expr = match expr.kind {
-            ExprKind::Call(_, args) if expr.span.is_desugaring(DesugaringKind::TryBlock) => {
-                args.len() == 1 && args[0].span.is_desugaring(DesugaringKind::TryBlock)
+            ExprKind::Call(_, [arg]) => {
+                expr.span.is_desugaring(DesugaringKind::TryBlock)
+                    && arg.span.is_desugaring(DesugaringKind::TryBlock)
             }
-
             _ => false,
         };
 
@@ -220,9 +220,16 @@ impl<'a, 'tcx> FnCtxt<'a, 'tcx> {
             self.warn_if_unreachable(expr.hir_id, expr.span, "expression");
         }
 
-        // Hide the outer diverging and has_errors flags.
+        // Whether a past expression diverges doesn't affect typechecking of this expression, so we
+        // reset `diverges` while checking `expr`.
         let old_diverges = self.diverges.replace(Diverges::Maybe);
 
+        if self.is_whole_body.replace(false) {
+            // If this expression is the whole body and the function diverges because of its
+            // arguments, we check this here to ensure the body is considered to diverge.
+            self.diverges.set(self.function_diverges_because_of_empty_arguments.get())
+        };
+
         let ty = ensure_sufficient_stack(|| match &expr.kind {
             hir::ExprKind::Path(
                 qpath @ (hir::QPath::Resolved(..) | hir::QPath::TypeRelative(..)),
diff --git a/compiler/rustc_hir_typeck/src/fn_ctxt/checks.rs b/compiler/rustc_hir_typeck/src/fn_ctxt/checks.rs
index ddb4224b60d..136ed1a709e 100644
--- a/compiler/rustc_hir_typeck/src/fn_ctxt/checks.rs
+++ b/compiler/rustc_hir_typeck/src/fn_ctxt/checks.rs
@@ -1471,6 +1471,12 @@ impl<'a, 'tcx> FnCtxt<'a, 'tcx> {
     /// Type check a `let` statement.
     pub fn check_decl_local(&self, local: &'tcx hir::Local<'tcx>) {
         self.check_decl(local.into());
+        if local.pat.is_never_pattern() {
+            self.diverges.set(Diverges::Always {
+                span: local.pat.span,
+                custom_note: Some("any code following a never pattern is unreachable"),
+            });
+        }
     }
 
     pub fn check_stmt(&self, stmt: &'tcx hir::Stmt<'tcx>) {
diff --git a/compiler/rustc_hir_typeck/src/fn_ctxt/mod.rs b/compiler/rustc_hir_typeck/src/fn_ctxt/mod.rs
index e6c2091d85a..f65e9b698ab 100644
--- a/compiler/rustc_hir_typeck/src/fn_ctxt/mod.rs
+++ b/compiler/rustc_hir_typeck/src/fn_ctxt/mod.rs
@@ -103,6 +103,13 @@ pub struct FnCtxt<'a, 'tcx> {
     /// the diverges flag is set to something other than `Maybe`.
     pub(super) diverges: Cell<Diverges>,
 
+    /// If one of the function arguments is a never pattern, this counts as diverging code. This
+    /// affect typechecking of the function body.
+    pub(super) function_diverges_because_of_empty_arguments: Cell<Diverges>,
+
+    /// Whether the currently checked node is the whole body of the function.
+    pub(super) is_whole_body: Cell<bool>,
+
     pub(super) enclosing_breakables: RefCell<EnclosingBreakables<'tcx>>,
 
     pub(super) inh: &'a Inherited<'tcx>,
@@ -124,6 +131,8 @@ impl<'a, 'tcx> FnCtxt<'a, 'tcx> {
             ret_coercion_span: Cell::new(None),
             coroutine_types: None,
             diverges: Cell::new(Diverges::Maybe),
+            function_diverges_because_of_empty_arguments: Cell::new(Diverges::Maybe),
+            is_whole_body: Cell::new(false),
             enclosing_breakables: RefCell::new(EnclosingBreakables {
                 stack: Vec::new(),
                 by_id: Default::default(),