Rollup merge of #114819 - estebank:issue-78124, r=compiler-errors

Point at return type when it influences non-first `match` arm

When encountering code like

```rust
fn foo() -> i32 {
    match 0 {
        1 => return 0,
        2 => "",
        _ => 1,
    }
}
```

Point at the return type and not at the prior arm, as that arm has type `!` which isn't influencing the arm corresponding to arm `2`.

Fix #78124.

compiler/rustc_ast_lowering/src/expr.rs

@@ -1648,7 +1648,7 @@ impl<'hir> LoweringContext<'_, 'hir> {
         hir::ExprKind::Match(
             scrutinee,
             arena_vec![self; break_arm, continue_arm],
-            hir::MatchSource::TryDesugar,
+            hir::MatchSource::TryDesugar(scrutinee.hir_id),
         )
     }
 

compiler/rustc_hir/src/hir.rs

@@ -2148,7 +2148,7 @@ pub enum MatchSource {
     /// A desugared `for _ in _ { .. }` loop.
     ForLoopDesugar,
     /// A desugared `?` operator.
-    TryDesugar,
+    TryDesugar(HirId),
     /// A desugared `<expr>.await`.
     AwaitDesugar,
     /// A desugared `format_args!()`.
@@ -2162,7 +2162,7 @@ impl MatchSource {
         match self {
             Normal => "match",
             ForLoopDesugar => "for",
-            TryDesugar => "?",
+            TryDesugar(_) => "?",
             AwaitDesugar => ".await",
             FormatArgs => "format_args!()",
         }

compiler/rustc_hir_typeck/src/_match.rs

@@ -107,7 +107,9 @@ impl<'a, 'tcx> FnCtxt<'a, 'tcx> {
             let (span, code) = match prior_arm {
                 // The reason for the first arm to fail is not that the match arms diverge,
                 // but rather that there's a prior obligation that doesn't hold.
-                None => (arm_span, ObligationCauseCode::BlockTailExpression(arm.body.hir_id)),
+                None => {
+                    (arm_span, ObligationCauseCode::BlockTailExpression(arm.body.hir_id, match_src))
+                }
                 Some((prior_arm_block_id, prior_arm_ty, prior_arm_span)) => (
                     expr.span,
                     ObligationCauseCode::MatchExpressionArm(Box::new(MatchExpressionArmCause {
@@ -120,7 +122,6 @@ impl<'a, 'tcx> FnCtxt<'a, 'tcx> {
                         scrut_span: scrut.span,
                         source: match_src,
                         prior_arms: other_arms.clone(),
-                        scrut_hir_id: scrut.hir_id,
                         opt_suggest_box_span,
                     })),
                 ),
@@ -145,7 +146,12 @@ impl<'a, 'tcx> FnCtxt<'a, 'tcx> {
                 other_arms.remove(0);
             }
 
-            prior_arm = Some((arm_block_id, arm_ty, arm_span));
+            if !arm_ty.is_never() {
+                // When a match arm has type `!`, then it doesn't influence the expected type for
+                // the following arm. If all of the prior arms are `!`, then the influence comes
+                // from elsewhere and we shouldn't point to any previous arm.
+                prior_arm = Some((arm_block_id, arm_ty, arm_span));
+            }
         }
 
         // If all of the arms in the `match` diverge,

compiler/rustc_hir_typeck/src/coercion.rs

@@ -1603,7 +1603,7 @@ impl<'tcx, 'exprs, E: AsCoercionSite> CoerceMany<'tcx, 'exprs, E> {
                         );
                         err.span_label(cause.span, "return type is not `()`");
                     }
-                    ObligationCauseCode::BlockTailExpression(blk_id) => {
+                    ObligationCauseCode::BlockTailExpression(blk_id, ..) => {
                         let parent_id = fcx.tcx.hir().parent_id(blk_id);
                         err = self.report_return_mismatched_types(
                             cause,
@@ -1748,7 +1748,7 @@ impl<'tcx, 'exprs, E: AsCoercionSite> CoerceMany<'tcx, 'exprs, E> {
                 ) && !in_external_macro(fcx.tcx.sess, cond_expr.span)
                     && !matches!(
                         cond_expr.kind,
-                        hir::ExprKind::Match(.., hir::MatchSource::TryDesugar)
+                        hir::ExprKind::Match(.., hir::MatchSource::TryDesugar(_))
                     )
             {
                 err.span_label(cond_expr.span, "expected this to be `()`");

compiler/rustc_hir_typeck/src/fn_ctxt/checks.rs

@@ -1580,7 +1580,10 @@ impl<'a, 'tcx> FnCtxt<'a, 'tcx> {
             let coerce = ctxt.coerce.as_mut().unwrap();
             if let Some((tail_expr, tail_expr_ty)) = tail_expr_ty {
                 let span = self.get_expr_coercion_span(tail_expr);
-                let cause = self.cause(span, ObligationCauseCode::BlockTailExpression(blk.hir_id));
+                let cause = self.cause(
+                    span,
+                    ObligationCauseCode::BlockTailExpression(blk.hir_id, hir::MatchSource::Normal),
+                );
                 let ty_for_diagnostic = coerce.merged_ty();
                 // We use coerce_inner here because we want to augment the error
                 // suggesting to wrap the block in square brackets if it might've

compiler/rustc_infer/src/infer/error_reporting/mod.rs

@@ -743,6 +743,35 @@ impl<'tcx> TypeErrCtxt<'_, 'tcx> {
             ObligationCauseCode::Pattern { origin_expr: false, span: Some(span), .. } => {
                 err.span_label(span, "expected due to this");
             }
+            ObligationCauseCode::BlockTailExpression(
+                _,
+                hir::MatchSource::TryDesugar(scrut_hir_id),
+            ) => {
+                if let Some(ty::error::ExpectedFound { expected, .. }) = exp_found {
+                    let scrut_expr = self.tcx.hir().expect_expr(scrut_hir_id);
+                    let scrut_ty = if let hir::ExprKind::Call(_, args) = &scrut_expr.kind {
+                        let arg_expr = args.first().expect("try desugaring call w/out arg");
+                        self.typeck_results.as_ref().and_then(|typeck_results| {
+                            typeck_results.expr_ty_opt(arg_expr)
+                        })
+                    } else {
+                        bug!("try desugaring w/out call expr as scrutinee");
+                    };
+
+                    match scrut_ty {
+                        Some(ty) if expected == ty => {
+                            let source_map = self.tcx.sess.source_map();
+                            err.span_suggestion(
+                                source_map.end_point(cause.span()),
+                                "try removing this `?`",
+                                "",
+                                Applicability::MachineApplicable,
+                            );
+                        }
+                        _ => {}
+                    }
+                }
+            },
             ObligationCauseCode::MatchExpressionArm(box MatchExpressionArmCause {
                 arm_block_id,
                 arm_span,
@@ -752,12 +781,11 @@ impl<'tcx> TypeErrCtxt<'_, 'tcx> {
                 prior_arm_ty,
                 source,
                 ref prior_arms,
-                scrut_hir_id,
                 opt_suggest_box_span,
                 scrut_span,
                 ..
             }) => match source {
-                hir::MatchSource::TryDesugar => {
+                hir::MatchSource::TryDesugar(scrut_hir_id) => {
                     if let Some(ty::error::ExpectedFound { expected, .. }) = exp_found {
                         let scrut_expr = self.tcx.hir().expect_expr(scrut_hir_id);
                         let scrut_ty = if let hir::ExprKind::Call(_, args) = &scrut_expr.kind {
@@ -1978,7 +2006,7 @@ impl<'tcx> TypeErrCtxt<'_, 'tcx> {
         trace: &TypeTrace<'tcx>,
         terr: TypeError<'tcx>,
     ) -> Vec<TypeErrorAdditionalDiags> {
-        use crate::traits::ObligationCauseCode::MatchExpressionArm;
+        use crate::traits::ObligationCauseCode::{BlockTailExpression, MatchExpressionArm};
         let mut suggestions = Vec::new();
         let span = trace.cause.span();
         let values = self.resolve_vars_if_possible(trace.values);
@@ -1996,11 +2024,17 @@ impl<'tcx> TypeErrCtxt<'_, 'tcx> {
                 // specify a byte literal
                 (ty::Uint(ty::UintTy::U8), ty::Char) => {
                     if let Ok(code) = self.tcx.sess().source_map().span_to_snippet(span)
-                        && let Some(code) = code.strip_prefix('\'').and_then(|s| s.strip_suffix('\''))
-                        && !code.starts_with("\\u") // forbid all Unicode escapes
-                        && code.chars().next().is_some_and(|c| c.is_ascii()) // forbids literal Unicode characters beyond ASCII
+                        && let Some(code) =
+                            code.strip_prefix('\'').and_then(|s| s.strip_suffix('\''))
+                        // forbid all Unicode escapes
+                        && !code.starts_with("\\u")
+                        // forbids literal Unicode characters beyond ASCII
+                        && code.chars().next().is_some_and(|c| c.is_ascii())
                     {
-                        suggestions.push(TypeErrorAdditionalDiags::MeantByteLiteral { span, code: escape_literal(code) })
+                        suggestions.push(TypeErrorAdditionalDiags::MeantByteLiteral {
+                            span,
+                            code: escape_literal(code),
+                        })
                     }
                 }
                 // If a character was expected and the found expression is a string literal
@@ -2011,7 +2045,10 @@ impl<'tcx> TypeErrCtxt<'_, 'tcx> {
                         && let Some(code) = code.strip_prefix('"').and_then(|s| s.strip_suffix('"'))
                         && code.chars().count() == 1
                     {
-                        suggestions.push(TypeErrorAdditionalDiags::MeantCharLiteral { span, code: escape_literal(code) })
+                        suggestions.push(TypeErrorAdditionalDiags::MeantCharLiteral {
+                            span,
+                            code: escape_literal(code),
+                        })
                     }
                 }
                 // If a string was expected and the found expression is a character literal,
@@ -2021,7 +2058,10 @@ impl<'tcx> TypeErrCtxt<'_, 'tcx> {
                         if let Some(code) =
                             code.strip_prefix('\'').and_then(|s| s.strip_suffix('\''))
                         {
-                            suggestions.push(TypeErrorAdditionalDiags::MeantStrLiteral { span, code: escape_literal(code) })
+                            suggestions.push(TypeErrorAdditionalDiags::MeantStrLiteral {
+                                span,
+                                code: escape_literal(code),
+                            })
                         }
                     }
                 }
@@ -2030,17 +2070,24 @@ impl<'tcx> TypeErrCtxt<'_, 'tcx> {
                 (ty::Bool, ty::Tuple(list)) => if list.len() == 0 {
                     suggestions.extend(self.suggest_let_for_letchains(&trace.cause, span));
                 }
-                (ty::Array(_, _), ty::Array(_, _)) => suggestions.extend(self.suggest_specify_actual_length(terr, trace, span)),
+                (ty::Array(_, _), ty::Array(_, _)) => {
+                    suggestions.extend(self.suggest_specify_actual_length(terr, trace, span))
+                }
                 _ => {}
             }
         }
         let code = trace.cause.code();
-        if let &MatchExpressionArm(box MatchExpressionArmCause { source, .. }) = code
-                    && let hir::MatchSource::TryDesugar = source
-                    && let Some((expected_ty, found_ty, _, _)) = self.values_str(trace.values)
-                {
-                    suggestions.push(TypeErrorAdditionalDiags::TryCannotConvert { found: found_ty.content(), expected: expected_ty.content() });
-                }
+        if let &(MatchExpressionArm(box MatchExpressionArmCause { source, .. })
+            | BlockTailExpression(.., source)
+        ) = code
+            && let hir::MatchSource::TryDesugar(_) = source
+            && let Some((expected_ty, found_ty, _, _)) = self.values_str(trace.values)
+        {
+            suggestions.push(TypeErrorAdditionalDiags::TryCannotConvert {
+                found: found_ty.content(),
+                expected: expected_ty.content(),
+            });
+        }
         suggestions
     }
 
@@ -2910,8 +2957,11 @@ impl<'tcx> ObligationCauseExt<'tcx> for ObligationCause<'tcx> {
             CompareImplItemObligation { kind: ty::AssocKind::Const, .. } => {
                 ObligationCauseFailureCode::ConstCompat { span, subdiags }
             }
+            BlockTailExpression(.., hir::MatchSource::TryDesugar(_)) => {
+                ObligationCauseFailureCode::TryCompat { span, subdiags }
+            }
             MatchExpressionArm(box MatchExpressionArmCause { source, .. }) => match source {
-                hir::MatchSource::TryDesugar => {
+                hir::MatchSource::TryDesugar(_) => {
                     ObligationCauseFailureCode::TryCompat { span, subdiags }
                 }
                 _ => ObligationCauseFailureCode::MatchCompat { span, subdiags },

compiler/rustc_infer/src/infer/error_reporting/nice_region_error/static_impl_trait.rs

@@ -146,7 +146,7 @@ impl<'a, 'tcx> NiceRegionError<'a, 'tcx> {
 
         if let SubregionOrigin::Subtype(box TypeTrace { cause, .. }) = sub_origin {
             if let ObligationCauseCode::ReturnValue(hir_id)
-            | ObligationCauseCode::BlockTailExpression(hir_id) = cause.code()
+            | ObligationCauseCode::BlockTailExpression(hir_id, ..) = cause.code()
             {
                 let parent_id = tcx.hir().get_parent_item(*hir_id);
                 if let Some(fn_decl) = tcx.hir().fn_decl_by_hir_id(parent_id.into()) {

compiler/rustc_middle/src/thir.rs

@@ -346,6 +346,7 @@ pub enum ExprKind<'tcx> {
     /// A `match` expression.
     Match {
         scrutinee: ExprId,
+        scrutinee_hir_id: hir::HirId,
         arms: Box<[ArmId]>,
     },
     /// A block.

compiler/rustc_middle/src/thir/visit.rs

@@ -70,7 +70,7 @@ pub fn walk_expr<'a, 'tcx: 'a, V: Visitor<'a, 'tcx>>(visitor: &mut V, expr: &Exp
             visitor.visit_expr(&visitor.thir()[expr]);
         }
         Loop { body } => visitor.visit_expr(&visitor.thir()[body]),
-        Match { scrutinee, ref arms } => {
+        Match { scrutinee, ref arms, .. } => {
             visitor.visit_expr(&visitor.thir()[scrutinee]);
             for &arm in &**arms {
                 visitor.visit_arm(&visitor.thir()[arm]);

compiler/rustc_middle/src/traits/mod.rs

@@ -402,7 +402,7 @@ pub enum ObligationCauseCode<'tcx> {
     OpaqueReturnType(Option<(Ty<'tcx>, Span)>),
 
     /// Block implicit return
-    BlockTailExpression(hir::HirId),
+    BlockTailExpression(hir::HirId, hir::MatchSource),
 
     /// #[feature(trivial_bounds)] is not enabled
     TrivialBound,
@@ -543,7 +543,6 @@ pub struct MatchExpressionArmCause<'tcx> {
     pub scrut_span: Span,
     pub source: hir::MatchSource,
     pub prior_arms: Vec<Span>,
-    pub scrut_hir_id: hir::HirId,
     pub opt_suggest_box_span: Option<Span>,
 }
 

compiler/rustc_mir_build/src/build/custom/parse/instruction.rs

@@ -65,7 +65,7 @@ impl<'tcx, 'body> ParseCtxt<'tcx, 'body> {
                 let target = self.parse_block(args[1])?;
                 self.parse_call(args[2], destination, target)
             },
-            ExprKind::Match { scrutinee, arms } => {
+            ExprKind::Match { scrutinee, arms, .. } => {
                 let discr = self.parse_operand(*scrutinee)?;
                 self.parse_match(arms, expr.span).map(|t| TerminatorKind::SwitchInt { discr, targets: t })
             },

compiler/rustc_mir_build/src/build/expr/into.rs

@@ -47,7 +47,7 @@ impl<'a, 'tcx> Builder<'a, 'tcx> {
             ExprKind::Block { block: ast_block } => {
                 this.ast_block(destination, block, ast_block, source_info)
             }
-            ExprKind::Match { scrutinee, ref arms } => {
+            ExprKind::Match { scrutinee, ref arms, .. } => {
                 this.match_expr(destination, expr_span, block, &this.thir[scrutinee], arms)
             }
             ExprKind::If { cond, then, else_opt, if_then_scope } => {

compiler/rustc_mir_build/src/thir/cx/expr.rs

@@ -732,6 +732,7 @@ impl<'tcx> Cx<'tcx> {
             },
             hir::ExprKind::Match(ref discr, ref arms, _) => ExprKind::Match {
                 scrutinee: self.mirror_expr(discr),
+                scrutinee_hir_id: discr.hir_id,
                 arms: arms.iter().map(|a| self.convert_arm(a)).collect(),
             },
             hir::ExprKind::Loop(ref body, ..) => {

compiler/rustc_mir_build/src/thir/pattern/check_match.rs

@@ -135,10 +135,12 @@ impl<'a, 'tcx> Visitor<'a, 'tcx> for MatchVisitor<'a, '_, 'tcx> {
                 });
                 return;
             }
-            ExprKind::Match { scrutinee, box ref arms } => {
+            ExprKind::Match { scrutinee, scrutinee_hir_id, box ref arms } => {
                 let source = match ex.span.desugaring_kind() {
                     Some(DesugaringKind::ForLoop) => hir::MatchSource::ForLoopDesugar,
-                    Some(DesugaringKind::QuestionMark) => hir::MatchSource::TryDesugar,
+                    Some(DesugaringKind::QuestionMark) => {
+                        hir::MatchSource::TryDesugar(scrutinee_hir_id)
+                    }
                     Some(DesugaringKind::Await) => hir::MatchSource::AwaitDesugar,
                     _ => hir::MatchSource::Normal,
                 };
@@ -277,7 +279,7 @@ impl<'p, 'tcx> MatchVisitor<'_, 'p, 'tcx> {
             | hir::MatchSource::FormatArgs => report_arm_reachability(&cx, &report),
             // Unreachable patterns in try and await expressions occur when one of
             // the arms are an uninhabited type. Which is OK.
-            hir::MatchSource::AwaitDesugar | hir::MatchSource::TryDesugar => {}
+            hir::MatchSource::AwaitDesugar | hir::MatchSource::TryDesugar(_) => {}
         }
 
         // Check if the match is exhaustive.

compiler/rustc_mir_build/src/thir/print.rs

@@ -321,7 +321,7 @@ impl<'a, 'tcx> ThirPrinter<'a, 'tcx> {
                 print_indented!(self, format!("pat: {:?}", pat), depth_lvl + 1);
                 print_indented!(self, "}", depth_lvl);
             }
-            Match { scrutinee, arms } => {
+            Match { scrutinee, arms, .. } => {
                 print_indented!(self, "Match {", depth_lvl);
                 print_indented!(self, "scrutinee:", depth_lvl + 1);
                 self.print_expr(*scrutinee, depth_lvl + 2);

compiler/rustc_passes/src/check_const.rs

@@ -45,7 +45,7 @@ impl NonConstExpr {
 
             Self::Loop(ForLoop) | Self::Match(ForLoopDesugar) => &[sym::const_for],
 
-            Self::Match(TryDesugar) => &[sym::const_try],
+            Self::Match(TryDesugar(_)) => &[sym::const_try],
 
             // All other expressions are allowed.
             Self::Loop(Loop | While) | Self::Match(Normal | FormatArgs) => &[],

compiler/rustc_trait_selection/src/traits/error_reporting/suggestions.rs

@@ -2700,7 +2700,7 @@ impl<'tcx> TypeErrCtxtExt<'tcx> for TypeErrCtxt<'_, 'tcx> {
             | ObligationCauseCode::MatchImpl(..)
             | ObligationCauseCode::ReturnType
             | ObligationCauseCode::ReturnValue(_)
-            | ObligationCauseCode::BlockTailExpression(_)
+            | ObligationCauseCode::BlockTailExpression(..)
             | ObligationCauseCode::AwaitableExpr(_)
             | ObligationCauseCode::ForLoopIterator
             | ObligationCauseCode::QuestionMark