Remove note about iteration count in coerce

and replace it with a simple note suggesting
returning a value.

The type mismatch error was never due to
how many times the loop iterates. It is more
because of the peculiar structure of what the for
loop desugars to. So the note talking about
iteration count didn't make sense

Author: gurry

compiler/rustc_hir_typeck/src/coercion.rs

@@ -37,11 +37,9 @@
 
 use crate::errors::SuggestBoxingForReturnImplTrait;
 use crate::FnCtxt;
-use rustc_errors::{codes::*, struct_span_code_err, Applicability, Diag, MultiSpan};
+use rustc_errors::{codes::*, struct_span_code_err, Applicability, Diag};
 use rustc_hir as hir;
 use rustc_hir::def_id::{DefId, LocalDefId};
-use rustc_hir::intravisit::{self, Visitor};
-use rustc_hir::Expr;
 use rustc_hir_analysis::hir_ty_lowering::HirTyLowerer;
 use rustc_infer::infer::type_variable::TypeVariableOrigin;
 use rustc_infer::infer::{Coercion, DefineOpaqueTypes, InferOk, InferResult};
@@ -95,22 +93,6 @@ impl<'a, 'tcx> Deref for Coerce<'a, 'tcx> {
 
 type CoerceResult<'tcx> = InferResult<'tcx, (Vec<Adjustment<'tcx>>, Ty<'tcx>)>;
 
-struct CollectRetsVisitor<'tcx> {
-    ret_exprs: Vec<&'tcx hir::Expr<'tcx>>,
-}
-
-impl<'tcx> Visitor<'tcx> for CollectRetsVisitor<'tcx> {
-    fn visit_expr(&mut self, expr: &'tcx Expr<'tcx>) {
-        match expr.kind {
-            hir::ExprKind::Ret(_) => self.ret_exprs.push(expr),
-            // `return` in closures does not return from the outer function
-            hir::ExprKind::Closure(_) => return,
-            _ => {}
-        }
-        intravisit::walk_expr(self, expr);
-    }
-}
-
 /// Coercing a mutable reference to an immutable works, while
 /// coercing `&T` to `&mut T` should be forbidden.
 fn coerce_mutbls<'tcx>(
@@ -1601,7 +1583,6 @@ impl<'tcx, 'exprs, E: AsCoercionSite> CoerceMany<'tcx, 'exprs, E> {
 
                 let mut err;
                 let mut unsized_return = false;
-                let mut visitor = CollectRetsVisitor { ret_exprs: vec![] };
                 match *cause.code() {
                     ObligationCauseCode::ReturnNoExpression => {
                         err = struct_span_code_err!(
@@ -1636,11 +1617,6 @@ impl<'tcx, 'exprs, E: AsCoercionSite> CoerceMany<'tcx, 'exprs, E> {
                         if !fcx.tcx.features().unsized_locals {
                             unsized_return = self.is_return_ty_definitely_unsized(fcx);
                         }
-                        if let Some(expression) = expression
-                            && let hir::ExprKind::Loop(loop_blk, ..) = expression.kind
-                        {
-                            intravisit::walk_block(&mut visitor, loop_blk);
-                        }
                     }
                     ObligationCauseCode::ReturnValue(id) => {
                         err = self.report_return_mismatched_types(
@@ -1741,6 +1717,22 @@ impl<'tcx, 'exprs, E: AsCoercionSite> CoerceMany<'tcx, 'exprs, E> {
                 augment_error(&mut err);
 
                 if let Some(expr) = expression {
+                    if let hir::ExprKind::Loop(
+                        _,
+                        _,
+                        loop_src @ (hir::LoopSource::While | hir::LoopSource::ForLoop),
+                        _,
+                    ) = expr.kind
+                    {
+                        let loop_type = if loop_src == hir::LoopSource::While {
+                            "`while` loops"
+                        } else {
+                            "`for` loops"
+                        };
+
+                        err.note(format!("{loop_type} evaluate to unit type `()`"));
+                    }
+
                     fcx.emit_coerce_suggestions(
                         &mut err,
                         expr,
@@ -1749,15 +1741,6 @@ impl<'tcx, 'exprs, E: AsCoercionSite> CoerceMany<'tcx, 'exprs, E> {
                         None,
                         Some(coercion_error),
                     );
-                    if visitor.ret_exprs.len() > 0 {
-                        self.note_unreachable_loop_return(
-                            &mut err,
-                            fcx.tcx,
-                            &expr,
-                            &visitor.ret_exprs,
-                            expected,
-                        );
-                    }
                 }
 
                 let reported = err.emit_unless(unsized_return);
@@ -1831,110 +1814,6 @@ impl<'tcx, 'exprs, E: AsCoercionSite> CoerceMany<'tcx, 'exprs, E> {
         );
     }
 
-    fn note_unreachable_loop_return(
-        &self,
-        err: &mut Diag<'_>,
-        tcx: TyCtxt<'tcx>,
-        expr: &hir::Expr<'tcx>,
-        ret_exprs: &Vec<&'tcx hir::Expr<'tcx>>,
-        ty: Ty<'tcx>,
-    ) {
-        let hir::ExprKind::Loop(_, _, _, loop_span) = expr.kind else {
-            return;
-        };
-        let mut span: MultiSpan = vec![loop_span].into();
-        span.push_span_label(loop_span, "this might have zero elements to iterate on");
-        const MAXITER: usize = 3;
-        let iter = ret_exprs.iter().take(MAXITER);
-        for ret_expr in iter {
-            span.push_span_label(
-                ret_expr.span,
-                "if the loop doesn't execute, this value would never get returned",
-            );
-        }
-        err.span_note(
-            span,
-            "the function expects a value to always be returned, but loops might run zero times",
-        );
-        if MAXITER < ret_exprs.len() {
-            err.note(format!(
-                "if the loop doesn't execute, {} other values would never get returned",
-                ret_exprs.len() - MAXITER
-            ));
-        }
-        let hir = tcx.hir();
-        let item = hir.get_parent_item(expr.hir_id);
-        let ret_msg = "return a value for the case when the loop has zero elements to iterate on";
-        let ret_ty_msg =
-            "otherwise consider changing the return type to account for that possibility";
-        let node = tcx.hir_node(item.into());
-        if let Some(body_id) = node.body_id()
-            && let Some(sig) = node.fn_sig()
-            && let hir::ExprKind::Block(block, _) = hir.body(body_id).value.kind
-            && !ty.is_never()
-        {
-            let indentation = if let None = block.expr
-                && let [.., last] = &block.stmts
-            {
-                tcx.sess.source_map().indentation_before(last.span).unwrap_or_else(String::new)
-            } else if let Some(expr) = block.expr {
-                tcx.sess.source_map().indentation_before(expr.span).unwrap_or_else(String::new)
-            } else {
-                String::new()
-            };
-            if let None = block.expr
-                && let [.., last] = &block.stmts
-            {
-                err.span_suggestion_verbose(
-                    last.span.shrink_to_hi(),
-                    ret_msg,
-                    format!("\n{indentation}/* `{ty}` value */"),
-                    Applicability::MaybeIncorrect,
-                );
-            } else if let Some(expr) = block.expr {
-                err.span_suggestion_verbose(
-                    expr.span.shrink_to_hi(),
-                    ret_msg,
-                    format!("\n{indentation}/* `{ty}` value */"),
-                    Applicability::MaybeIncorrect,
-                );
-            }
-            let mut sugg = match sig.decl.output {
-                hir::FnRetTy::DefaultReturn(span) => {
-                    vec![(span, " -> Option<()>".to_string())]
-                }
-                hir::FnRetTy::Return(ty) => {
-                    vec![
-                        (ty.span.shrink_to_lo(), "Option<".to_string()),
-                        (ty.span.shrink_to_hi(), ">".to_string()),
-                    ]
-                }
-            };
-            for ret_expr in ret_exprs {
-                match ret_expr.kind {
-                    hir::ExprKind::Ret(Some(expr)) => {
-                        sugg.push((expr.span.shrink_to_lo(), "Some(".to_string()));
-                        sugg.push((expr.span.shrink_to_hi(), ")".to_string()));
-                    }
-                    hir::ExprKind::Ret(None) => {
-                        sugg.push((ret_expr.span.shrink_to_hi(), " Some(())".to_string()));
-                    }
-                    _ => {}
-                }
-            }
-            if let None = block.expr
-                && let [.., last] = &block.stmts
-            {
-                sugg.push((last.span.shrink_to_hi(), format!("\n{indentation}None")));
-            } else if let Some(expr) = block.expr {
-                sugg.push((expr.span.shrink_to_hi(), format!("\n{indentation}None")));
-            }
-            err.multipart_suggestion(ret_ty_msg, sugg, Applicability::MaybeIncorrect);
-        } else {
-            err.help(format!("{ret_msg}, {ret_ty_msg}"));
-        }
-    }
-
     fn report_return_mismatched_types<'a>(
         &self,
         cause: &ObligationCause<'tcx>,

compiler/rustc_hir_typeck/src/demand.rs

@@ -57,7 +57,8 @@ impl<'a, 'tcx> FnCtxt<'a, 'tcx> {
             || self.suggest_into(err, expr, expr_ty, expected)
             || self.suggest_floating_point_literal(err, expr, expected)
             || self.suggest_null_ptr_for_literal_zero_given_to_ptr_arg(err, expr, expected)
-            || self.suggest_coercing_result_via_try_operator(err, expr, expected, expr_ty);
+            || self.suggest_coercing_result_via_try_operator(err, expr, expected, expr_ty)
+            || self.suggest_returning_value_after_loop(err, expr, expected);
 
         if !suggested {
             self.note_source_of_type_mismatch_constraint(

compiler/rustc_hir_typeck/src/fn_ctxt/suggestions.rs

@@ -18,8 +18,8 @@ use rustc_hir::def::Res;
 use rustc_hir::def::{CtorKind, CtorOf, DefKind};
 use rustc_hir::lang_items::LangItem;
 use rustc_hir::{
-    CoroutineDesugaring, CoroutineKind, CoroutineSource, Expr, ExprKind, GenericBound, HirId, Node,
-    Path, QPath, Stmt, StmtKind, TyKind, WherePredicate,
+    Arm, CoroutineDesugaring, CoroutineKind, CoroutineSource, Expr, ExprKind, GenericBound, HirId,
+    Node, Path, QPath, Stmt, StmtKind, TyKind, WherePredicate,
 };
 use rustc_hir_analysis::collect::suggest_impl_trait;
 use rustc_hir_analysis::hir_ty_lowering::HirTyLowerer;
@@ -1942,6 +1942,91 @@ impl<'a, 'tcx> FnCtxt<'a, 'tcx> {
         false
     }
 
+    // If the expr is a while or for loop and is the tail expr of its
+    // enclosing body suggest returning a value right after it
+    pub fn suggest_returning_value_after_loop(
+        &self,
+        err: &mut Diag<'_>,
+        expr: &hir::Expr<'tcx>,
+        expected: Ty<'tcx>,
+    ) -> bool {
+        let hir = self.tcx.hir();
+        let enclosing_scope =
+            hir.get_enclosing_scope(expr.hir_id).map(|hir_id| self.tcx.hir_node(hir_id));
+
+        // Get tail expr of the enclosing block or body
+        let tail_expr = if let Some(Node::Block(hir::Block { expr, .. })) = enclosing_scope
+            && expr.is_some()
+        {
+            *expr
+        } else {
+            let body_def_id = hir.enclosing_body_owner(expr.hir_id);
+            let body_id = hir.body_owned_by(body_def_id);
+            let body = hir.body(body_id);
+
+            // Get tail expr of the body
+            match body.value.kind {
+                // Regular function body etc.
+                hir::ExprKind::Block(block, _) => block.expr,
+                // Anon const body (there's no block in this case)
+                hir::ExprKind::DropTemps(expr) => Some(expr),
+                _ => None,
+            }
+        };
+
+        let Some(tail_expr) = tail_expr else {
+            return false; // Body doesn't have a tail expr we can compare with
+        };
+
+        // Get the loop expr within the tail expr
+        let loop_expr_in_tail = match expr.kind {
+            hir::ExprKind::Loop(_, _, hir::LoopSource::While, _) => tail_expr,
+            hir::ExprKind::Loop(_, _, hir::LoopSource::ForLoop, _) => {
+                match tail_expr.peel_drop_temps() {
+                    Expr { kind: ExprKind::Match(_, [Arm { body, .. }], _), .. } => body,
+                    _ => return false, // Not really a for loop
+                }
+            }
+            _ => return false, // Not a while or a for loop
+        };
+
+        // If the expr is the loop expr in the tail
+        // then make the suggestion
+        if expr.hir_id == loop_expr_in_tail.hir_id {
+            let span = expr.span;
+
+            let (msg, suggestion) = if expected.is_never() {
+                (
+                    "consider adding a diverging expression here",
+                    "`loop {}` or `panic!(\"...\")`".to_string(),
+                )
+            } else {
+                ("consider returning a value here", format!("`{expected}` value"))
+            };
+
+            let src_map = self.tcx.sess.source_map();
+            let suggestion = if src_map.is_multiline(expr.span) {
+                let indentation = src_map.indentation_before(span).unwrap_or_else(String::new);
+                format!("\n{indentation}/* {suggestion} */")
+            } else {
+                // If the entire expr is on a single line
+                // put the suggestion also on the same line
+                format!(" /* {suggestion} */")
+            };
+
+            err.span_suggestion_verbose(
+                span.shrink_to_hi(),
+                msg,
+                suggestion,
+                Applicability::MaybeIncorrect,
+            );
+
+            true
+        } else {
+            false
+        }
+    }
+
     /// If the expected type is an enum (Issue #55250) with any variants whose
     /// sole field is of the found type, suggest such variants. (Issue #42764)
     pub(crate) fn suggest_compatible_variants(