Rollup merge of rust-lang#99539 - compiler-errors:bidirectional-block-suggestions, r=fee1-dead

Improve suggestions for returning binding

Fixes rust-lang#99525

Also reworks the cause codes for match and if a bit, I think cleaning them up in a positive way.
We no longer need to call `could_remove_semicolon` in successful code, which might save a few cycles?

Author: Dylan-DPC

compiler/rustc_hir/src/hir.rs

@@ -954,6 +954,16 @@ pub struct Block<'hir> {
     pub targeted_by_break: bool,
 }
 
+impl<'hir> Block<'hir> {
+    pub fn innermost_block(&self) -> &Block<'hir> {
+        let mut block = self;
+        while let Some(Expr { kind: ExprKind::Block(inner_block, _), .. }) = block.expr {
+            block = inner_block;
+        }
+        block
+    }
+}
+
 #[derive(Debug, HashStable_Generic)]
 pub struct Pat<'hir> {
     #[stable_hasher(ignore)]

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

@@ -351,7 +351,7 @@ pub enum ObligationCauseCode<'tcx> {
     ConstPatternStructural,
 
     /// Computing common supertype in an if expression
-    IfExpression(Box<IfExpressionCause>),
+    IfExpression(Box<IfExpressionCause<'tcx>>),
 
     /// Computing common supertype of an if expression with no else counter-part
     IfExpressionWithNoElse,
@@ -488,22 +488,27 @@ impl<'tcx> ty::Lift<'tcx> for StatementAsExpression {
 
 #[derive(Clone, Debug, PartialEq, Eq, Hash, Lift)]
 pub struct MatchExpressionArmCause<'tcx> {
+    pub arm_block_id: Option<hir::HirId>,
+    pub arm_ty: Ty<'tcx>,
     pub arm_span: Span,
+    pub prior_arm_block_id: Option<hir::HirId>,
+    pub prior_arm_ty: Ty<'tcx>,
+    pub prior_arm_span: Span,
     pub scrut_span: Span,
-    pub semi_span: Option<(Span, StatementAsExpression)>,
     pub source: hir::MatchSource,
     pub prior_arms: Vec<Span>,
-    pub last_ty: Ty<'tcx>,
     pub scrut_hir_id: hir::HirId,
     pub opt_suggest_box_span: Option<Span>,
 }
 
-#[derive(Clone, Debug, PartialEq, Eq, Hash)]
-pub struct IfExpressionCause {
-    pub then: Span,
-    pub else_sp: Span,
-    pub outer: Option<Span>,
-    pub semicolon: Option<(Span, StatementAsExpression)>,
+#[derive(Copy, Clone, Debug, PartialEq, Eq, Hash)]
+#[derive(Lift, TypeFoldable, TypeVisitable)]
+pub struct IfExpressionCause<'tcx> {
+    pub then_id: hir::HirId,
+    pub else_id: hir::HirId,
+    pub then_ty: Ty<'tcx>,
+    pub else_ty: Ty<'tcx>,
+    pub outer_span: Option<Span>,
     pub opt_suggest_box_span: Option<Span>,
 }
 

compiler/rustc_middle/src/traits/mod.rs

@@ -130,7 +130,6 @@ impl<N: fmt::Debug> fmt::Debug for traits::ImplSourceConstDestructData<N> {
 // Lift implementations
 
 TrivialTypeTraversalAndLiftImpls! {
-    super::IfExpressionCause,
     super::ImplSourceDiscriminantKindData,
     super::ImplSourcePointeeData,
 }

compiler/rustc_middle/src/traits/structural_impls.rs

@@ -22,7 +22,6 @@ use rustc_hir::intravisit::Visitor;
 use rustc_hir::GenericParam;
 use rustc_hir::Item;
 use rustc_hir::Node;
-use rustc_infer::infer::error_reporting::same_type_modulo_infer;
 use rustc_infer::traits::TraitEngine;
 use rustc_middle::traits::select::OverflowError;
 use rustc_middle::ty::abstract_const::NotConstEvaluatable;
@@ -640,7 +639,7 @@ impl<'a, 'tcx> InferCtxtExt<'tcx> for InferCtxt<'a, 'tcx> {
                                             if expected.len() == 1 { "" } else { "s" },
                                         )
                                     );
-                                } else if !same_type_modulo_infer(given_ty, expected_ty) {
+                                } else if !self.same_type_modulo_infer(given_ty, expected_ty) {
                                     // Print type mismatch
                                     let (expected_args, given_args) =
                                         self.cmp(given_ty, expected_ty);

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

@@ -9,7 +9,6 @@ use rustc_span::Span;
 use rustc_trait_selection::traits::query::evaluate_obligation::InferCtxtExt;
 use rustc_trait_selection::traits::{
     IfExpressionCause, MatchExpressionArmCause, ObligationCause, ObligationCauseCode,
-    StatementAsExpression,
 };
 
 impl<'a, 'tcx> FnCtxt<'a, 'tcx> {
@@ -75,8 +74,8 @@ impl<'a, 'tcx> FnCtxt<'a, 'tcx> {
         };
 
         let mut other_arms = vec![]; // Used only for diagnostics.
-        let mut prior_arm_ty = None;
-        for (i, arm) in arms.iter().enumerate() {
+        let mut prior_arm = None;
+        for arm in arms {
             if let Some(g) = &arm.guard {
                 self.diverges.set(Diverges::Maybe);
                 match g {
@@ -96,21 +95,28 @@ impl<'a, 'tcx> FnCtxt<'a, 'tcx> {
 
             let opt_suggest_box_span = self.opt_suggest_box_span(arm_ty, orig_expected);
 
-            let (arm_span, semi_span) =
-                self.get_appropriate_arm_semicolon_removal_span(&arms, i, prior_arm_ty, arm_ty);
-            let (span, code) = match i {
+            let (arm_block_id, arm_span) = if let hir::ExprKind::Block(blk, _) = arm.body.kind {
+                (Some(blk.hir_id), self.find_block_span(blk))
+            } else {
+                (None, arm.body.span)
+            };
+
+            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.
-                0 => (arm_span, ObligationCauseCode::BlockTailExpression(arm.body.hir_id)),
-                _ => (
+                None => (arm_span, ObligationCauseCode::BlockTailExpression(arm.body.hir_id)),
+                Some((prior_arm_block_id, prior_arm_ty, prior_arm_span)) => (
                     expr.span,
                     ObligationCauseCode::MatchExpressionArm(Box::new(MatchExpressionArmCause {
+                        arm_block_id,
                         arm_span,
+                        arm_ty,
+                        prior_arm_block_id,
+                        prior_arm_ty,
+                        prior_arm_span,
                         scrut_span: scrut.span,
-                        semi_span,
                         source: match_src,
                         prior_arms: other_arms.clone(),
-                        last_ty: prior_arm_ty.unwrap(),
                         scrut_hir_id: scrut.hir_id,
                         opt_suggest_box_span,
                     })),
@@ -139,7 +145,7 @@ impl<'a, 'tcx> FnCtxt<'a, 'tcx> {
                             let ret_ty = ret_coercion.borrow().expected_ty();
                             let ret_ty = self.inh.infcx.shallow_resolve(ret_ty);
                             self.can_coerce(arm_ty, ret_ty)
-                                && prior_arm_ty.map_or(true, |t| self.can_coerce(t, ret_ty))
+                                && prior_arm.map_or(true, |(_, t, _)| self.can_coerce(t, ret_ty))
                                 // The match arms need to unify for the case of `impl Trait`.
                                 && !matches!(ret_ty.kind(), ty::Opaque(..))
                         }
@@ -181,7 +187,8 @@ impl<'a, 'tcx> FnCtxt<'a, 'tcx> {
             if other_arms.len() > 5 {
                 other_arms.remove(0);
             }
-            prior_arm_ty = Some(arm_ty);
+
+            prior_arm = Some((arm_block_id, arm_ty, arm_span));
         }
 
         // If all of the arms in the `match` diverge,
@@ -207,28 +214,6 @@ impl<'a, 'tcx> FnCtxt<'a, 'tcx> {
         match_ty
     }
 
-    fn get_appropriate_arm_semicolon_removal_span(
-        &self,
-        arms: &'tcx [hir::Arm<'tcx>],
-        i: usize,
-        prior_arm_ty: Option<Ty<'tcx>>,
-        arm_ty: Ty<'tcx>,
-    ) -> (Span, Option<(Span, StatementAsExpression)>) {
-        let arm = &arms[i];
-        let (arm_span, mut semi_span) = if let hir::ExprKind::Block(blk, _) = &arm.body.kind {
-            self.find_block_span(blk, prior_arm_ty)
-        } else {
-            (arm.body.span, None)
-        };
-        if semi_span.is_none() && i > 0 {
-            if let hir::ExprKind::Block(blk, _) = &arms[i - 1].body.kind {
-                let (_, semi_span_prev) = self.find_block_span(blk, Some(arm_ty));
-                semi_span = semi_span_prev;
-            }
-        }
-        (arm_span, semi_span)
-    }
-
     /// When the previously checked expression (the scrutinee) diverges,
     /// warn the user about the match arms being unreachable.
     fn warn_arms_when_scrutinee_diverges(&self, arms: &'tcx [hir::Arm<'tcx>]) {
@@ -313,7 +298,7 @@ impl<'a, 'tcx> FnCtxt<'a, 'tcx> {
         else_ty: Ty<'tcx>,
         opt_suggest_box_span: Option<Span>,
     ) -> ObligationCause<'tcx> {
-        let mut outer_sp = if self.tcx.sess.source_map().is_multiline(span) {
+        let mut outer_span = if self.tcx.sess.source_map().is_multiline(span) {
             // The `if`/`else` isn't in one line in the output, include some context to make it
             // clear it is an if/else expression:
             // ```
@@ -339,69 +324,67 @@ impl<'a, 'tcx> FnCtxt<'a, 'tcx> {
             None
         };
 
-        let mut remove_semicolon = None;
-        let error_sp = if let ExprKind::Block(block, _) = &else_expr.kind {
-            let (error_sp, semi_sp) = self.find_block_span(block, Some(then_ty));
-            remove_semicolon = semi_sp;
-            if block.expr.is_none() && block.stmts.is_empty() {
-                // Avoid overlapping spans that aren't as readable:
-                // ```
-                // 2 |        let x = if true {
-                //   |   _____________-
-                // 3 |  |         3
-                //   |  |         - expected because of this
-                // 4 |  |     } else {
-                //   |  |____________^
-                // 5 | ||
-                // 6 | ||     };
-                //   | ||     ^
-                //   | ||_____|
-                //   | |______if and else have incompatible types
-                //   |        expected integer, found `()`
-                // ```
-                // by not pointing at the entire expression:
-                // ```
-                // 2 |       let x = if true {
-                //   |               ------- `if` and `else` have incompatible types
-                // 3 |           3
-                //   |           - expected because of this
-                // 4 |       } else {
-                //   |  ____________^
-                // 5 | |
-                // 6 | |     };
-                //   | |_____^ expected integer, found `()`
-                // ```
-                if outer_sp.is_some() {
-                    outer_sp = Some(self.tcx.sess.source_map().guess_head_span(span));
-                }
+        let (error_sp, else_id) = if let ExprKind::Block(block, _) = &else_expr.kind {
+            let block = block.innermost_block();
+
+            // Avoid overlapping spans that aren't as readable:
+            // ```
+            // 2 |        let x = if true {
+            //   |   _____________-
+            // 3 |  |         3
+            //   |  |         - expected because of this
+            // 4 |  |     } else {
+            //   |  |____________^
+            // 5 | ||
+            // 6 | ||     };
+            //   | ||     ^
+            //   | ||_____|
+            //   | |______if and else have incompatible types
+            //   |        expected integer, found `()`
+            // ```
+            // by not pointing at the entire expression:
+            // ```
+            // 2 |       let x = if true {
+            //   |               ------- `if` and `else` have incompatible types
+            // 3 |           3
+            //   |           - expected because of this
+            // 4 |       } else {
+            //   |  ____________^
+            // 5 | |
+            // 6 | |     };
+            //   | |_____^ expected integer, found `()`
+            // ```
+            if block.expr.is_none() && block.stmts.is_empty()
+                && let Some(outer_span) = &mut outer_span
+            {
+                *outer_span = self.tcx.sess.source_map().guess_head_span(*outer_span);
             }
-            error_sp
+
+            (self.find_block_span(block), block.hir_id)
         } else {
-            // shouldn't happen unless the parser has done something weird
-            else_expr.span
+            (else_expr.span, else_expr.hir_id)
         };
 
-        // Compute `Span` of `then` part of `if`-expression.
-        let then_sp = if let ExprKind::Block(block, _) = &then_expr.kind {
-            let (then_sp, semi_sp) = self.find_block_span(block, Some(else_ty));
-            remove_semicolon = remove_semicolon.or(semi_sp);
+        let then_id = if let ExprKind::Block(block, _) = &then_expr.kind {
+            let block = block.innermost_block();
+            // Exclude overlapping spans
             if block.expr.is_none() && block.stmts.is_empty() {
-                outer_sp = None; // same as in `error_sp`; cleanup output
+                outer_span = None;
             }
-            then_sp
+            block.hir_id
         } else {
-            // shouldn't happen unless the parser has done something weird
-            then_expr.span
+            then_expr.hir_id
         };
 
         // Finally construct the cause:
         self.cause(
             error_sp,
             ObligationCauseCode::IfExpression(Box::new(IfExpressionCause {
-                then: then_sp,
-                else_sp: error_sp,
-                outer: outer_sp,
-                semicolon: remove_semicolon,
+                else_id,
+                then_id,
+                then_ty,
+                else_ty,
+                outer_span,
                 opt_suggest_box_span,
             })),
         )
@@ -482,22 +465,6 @@ impl<'a, 'tcx> FnCtxt<'a, 'tcx> {
         }
     }
 
-    fn find_block_span(
-        &self,
-        block: &'tcx hir::Block<'tcx>,
-        expected_ty: Option<Ty<'tcx>>,
-    ) -> (Span, Option<(Span, StatementAsExpression)>) {
-        if let Some(expr) = &block.expr {
-            (expr.span, None)
-        } else if let Some(stmt) = block.stmts.last() {
-            // possibly incorrect trailing `;` in the else arm
-            (stmt.span, expected_ty.and_then(|ty| self.could_remove_semicolon(block, ty)))
-        } else {
-            // empty block; point at its entirety
-            (block.span, None)
-        }
-    }
-
     // When we have a `match` as a tail expression in a `fn` with a returned `impl Trait`
     // we check if the different arms would work with boxed trait objects instead and
     // provide a structured suggestion in that case.