Suggest removing superfluous semicolon when statements used as expression

compiler/rustc_hir_typeck/src/fn_ctxt/checks.rs

@@ -1744,7 +1744,11 @@ impl<'a, 'tcx> FnCtxt<'a, 'tcx> {
                                         Ty::new_unit(self.tcx),
                                     );
                                 }
-                                if !self.consider_removing_semicolon(blk, expected_ty, err) {
+                                if !self.err_ctxt().consider_removing_semicolon(
+                                    blk,
+                                    expected_ty,
+                                    err,
+                                ) {
                                     self.err_ctxt().consider_returning_binding(
                                         blk,
                                         expected_ty,

compiler/rustc_hir_typeck/src/fn_ctxt/suggestions.rs

@@ -22,7 +22,7 @@ use rustc_hir::{
     Path, QPath, Stmt, StmtKind, TyKind, WherePredicate,
 };
 use rustc_hir_analysis::astconv::AstConv;
-use rustc_infer::traits::{self, StatementAsExpression};
+use rustc_infer::traits::{self};
 use rustc_middle::lint::in_external_macro;
 use rustc_middle::middle::stability::EvalResult;
 use rustc_middle::ty::print::with_no_trimmed_paths;
@@ -1791,45 +1791,6 @@ impl<'a, 'tcx> FnCtxt<'a, 'tcx> {
         }
     }
 
-    /// A common error is to add an extra semicolon:
-    ///
-    /// ```compile_fail,E0308
-    /// fn foo() -> usize {
-    ///     22;
-    /// }
-    /// ```
-    ///
-    /// This routine checks if the final statement in a block is an
-    /// expression with an explicit semicolon whose type is compatible
-    /// with `expected_ty`. If so, it suggests removing the semicolon.
-    pub(crate) fn consider_removing_semicolon(
-        &self,
-        blk: &'tcx hir::Block<'tcx>,
-        expected_ty: Ty<'tcx>,
-        err: &mut Diag<'_>,
-    ) -> bool {
-        if let Some((span_semi, boxed)) = self.err_ctxt().could_remove_semicolon(blk, expected_ty) {
-            if let StatementAsExpression::NeedsBoxing = boxed {
-                err.span_suggestion_verbose(
-                    span_semi,
-                    "consider removing this semicolon and boxing the expression",
-                    "",
-                    Applicability::HasPlaceholders,
-                );
-            } else {
-                err.span_suggestion_short(
-                    span_semi,
-                    "remove this semicolon to return this value",
-                    "",
-                    Applicability::MachineApplicable,
-                );
-            }
-            true
-        } else {
-            false
-        }
-    }
-
     pub(crate) fn is_field_suggestable(
         &self,
         field: &ty::FieldDef,

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

@@ -1976,6 +1976,7 @@ impl<'tcx> TypeErrCtxt<'_, 'tcx> {
                 self.suggest_accessing_field_where_appropriate(cause, &exp_found, diag);
                 self.suggest_await_on_expect_found(cause, span, &exp_found, diag);
                 self.suggest_function_pointers(cause, span, &exp_found, diag);
+                self.suggest_turning_stmt_into_expr(cause, &exp_found, diag);
             }
         }
 

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

@@ -1,8 +1,13 @@
+use crate::infer::error_reporting::hir::Path;
 use hir::def::CtorKind;
 use hir::intravisit::{walk_expr, walk_stmt, Visitor};
+use hir::{Local, QPath};
 use rustc_data_structures::fx::FxIndexSet;
 use rustc_errors::{Applicability, Diag};
 use rustc_hir as hir;
+use rustc_hir::def::Res;
+use rustc_hir::MatchSource;
+use rustc_hir::Node;
 use rustc_middle::traits::{
     IfExpressionCause, MatchExpressionArmCause, ObligationCause, ObligationCauseCode,
     StatementAsExpression,
@@ -293,6 +298,97 @@ impl<'tcx> TypeErrCtxt<'_, 'tcx> {
         }
     }
 
+    pub(super) fn suggest_turning_stmt_into_expr(
+        &self,
+        cause: &ObligationCause<'tcx>,
+        exp_found: &ty::error::ExpectedFound<Ty<'tcx>>,
+        diag: &mut Diag<'_>,
+    ) {
+        let ty::error::ExpectedFound { expected, found } = exp_found;
+        if !found.peel_refs().is_unit() {
+            return;
+        }
+
+        let ObligationCauseCode::BlockTailExpression(hir_id, MatchSource::Normal) = cause.code()
+        else {
+            return;
+        };
+
+        let node = self.tcx.hir_node(*hir_id);
+        let mut blocks = vec![];
+        if let hir::Node::Block(block) = node
+            && let Some(expr) = block.expr
+            && let hir::ExprKind::Path(QPath::Resolved(_, Path { res, .. })) = expr.kind
+            && let Res::Local(local) = res
+            && let Node::Local(Local { init: Some(init), .. }) = self.tcx.parent_hir_node(*local)
+        {
+            fn collect_blocks<'hir>(expr: &hir::Expr<'hir>, blocks: &mut Vec<&hir::Block<'hir>>) {
+                match expr.kind {
+                    // `blk1` and `blk2` must be have the same types, it will be reported before reaching here
+                    hir::ExprKind::If(_, blk1, Some(blk2)) => {
+                        collect_blocks(blk1, blocks);
+                        collect_blocks(blk2, blocks);
+                    }
+                    hir::ExprKind::Match(_, arms, _) => {
+                        // all arms must have same types
+                        for arm in arms.iter() {
+                            collect_blocks(arm.body, blocks);
+                        }
+                    }
+                    hir::ExprKind::Block(blk, _) => {
+                        blocks.push(blk);
+                    }
+                    _ => {}
+                }
+            }
+            collect_blocks(init, &mut blocks);
+        }
+
+        let expected_inner: Ty<'_> = expected.peel_refs();
+        for block in blocks.iter() {
+            self.consider_removing_semicolon(block, expected_inner, diag);
+        }
+    }
+
+    /// A common error is to add an extra semicolon:
+    ///
+    /// ```compile_fail,E0308
+    /// fn foo() -> usize {
+    ///     22;
+    /// }
+    /// ```
+    ///
+    /// This routine checks if the final statement in a block is an
+    /// expression with an explicit semicolon whose type is compatible
+    /// with `expected_ty`. If so, it suggests removing the semicolon.
+    pub fn consider_removing_semicolon(
+        &self,
+        blk: &'tcx hir::Block<'tcx>,
+        expected_ty: Ty<'tcx>,
+        diag: &mut Diag<'_>,
+    ) -> bool {
+        if let Some((span_semi, boxed)) = self.could_remove_semicolon(blk, expected_ty) {
+            if let StatementAsExpression::NeedsBoxing = boxed {
+                diag.span_suggestion_verbose(
+                    span_semi,
+                    "consider removing this semicolon and boxing the expression",
+                    "",
+                    Applicability::HasPlaceholders,
+                );
+            } else {
+                diag.span_suggestion_short(
+                    span_semi,
+                    "remove this semicolon to return this value",
+                    "",
+                    Applicability::MachineApplicable,
+                );
+            }
+            true
+        } else {
+            false
+        }
+    }
+
     pub(super) fn suggest_function_pointers(
         &self,
         cause: &ObligationCause<'tcx>,

tests/ui/inference/stmts-as-exp-105431.rs

@@ -0,0 +1,76 @@
+#![allow(unused)]
+
+fn test_if() -> i32 {
+    let x = if true {
+        eprintln!("hello");
+        3;
+    }
+    else {
+        4;
+    };
+    x //~ ERROR mismatched types
+}
+
+fn test_if_without_binding() -> i32 {
+    if true { //~ ERROR mismatched types
+        eprintln!("hello");
+        3;
+    }
+    else { //~ ERROR mismatched types
+        4;
+    }
+}
+
+fn test_match() -> i32 {
+    let v = 1;
+    let res = match v {
+        1 => { 1; }
+        _ => { 2; }
+    };
+    res //~ ERROR mismatched types
+}
+
+fn test_match_match_without_binding() -> i32 {
+    let v = 1;
+    match v {
+        1 => { 1; } //~ ERROR mismatched types
+        _ => { 2; } //~ ERROR mismatched types
+    }
+}
+
+fn test_match_arm_different_types() -> i32 {
+    let v = 1;
+    let res = match v {
+        1 => { if 1 < 2 { 1 } else { 2 } }
+        _ => { 2; } //~ ERROR `match` arms have incompatible types
+    };
+    res
+}
+
+fn test_if_match_mixed() -> i32 {
+    let x = if true {
+        3;
+    } else {
+        match 1 {
+            1 => { 1 }
+            _ => { 2 }
+        };
+    };
+    x //~ ERROR mismatched types
+}
+
+fn test_if_match_mixed_failed() -> i32 {
+    let x = if true {
+        3;
+    } else {
+        // because this is a tailed expr, so we won't check deeper
+        match 1 {
+            1 => { 33; }
+            _ => { 44; }
+        }
+    };
+    x //~ ERROR mismatched types
+}
+
+
+fn main() {}

tests/ui/inference/stmts-as-exp-105431.stderr

@@ -0,0 +1,129 @@
+error[E0308]: mismatched types
+  --> $DIR/stmts-as-exp-105431.rs:11:5
+   |
+LL | fn test_if() -> i32 {
+   |                 --- expected `i32` because of return type
+...
+LL |     x
+   |     ^ expected `i32`, found `()`
+   |
+help: remove this semicolon to return this value
+   |
+LL -         3;
+LL +         3
+   |
+help: remove this semicolon to return this value
+   |
+LL -         4;
+LL +         4
+   |
+
+error[E0308]: mismatched types
+  --> $DIR/stmts-as-exp-105431.rs:15:13
+   |
+LL |       if true {
+   |  _____________^
+LL | |         eprintln!("hello");
+LL | |         3;
+   | |          - help: remove this semicolon to return this value
+LL | |     }
+   | |_____^ expected `i32`, found `()`
+
+error[E0308]: mismatched types
+  --> $DIR/stmts-as-exp-105431.rs:19:10
+   |
+LL |       else {
+   |  __________^
+LL | |         4;
+   | |          - help: remove this semicolon to return this value
+LL | |     }
+   | |_____^ expected `i32`, found `()`
+
+error[E0308]: mismatched types
+  --> $DIR/stmts-as-exp-105431.rs:30:5
+   |
+LL | fn test_match() -> i32 {
+   |                    --- expected `i32` because of return type
+...
+LL |     res
+   |     ^^^ expected `i32`, found `()`
+   |
+help: remove this semicolon to return this value
+   |
+LL -         1 => { 1; }
+LL +         1 => { 1 }
+   |
+help: remove this semicolon to return this value
+   |
+LL -         _ => { 2; }
+LL +         _ => { 2 }
+   |
+
+error[E0308]: mismatched types
+  --> $DIR/stmts-as-exp-105431.rs:36:14
+   |
+LL |         1 => { 1; }
+   |              ^^^-^^
+   |              |  |
+   |              |  help: remove this semicolon to return this value
+   |              expected `i32`, found `()`
+
+error[E0308]: mismatched types
+  --> $DIR/stmts-as-exp-105431.rs:37:14
+   |
+LL |         _ => { 2; }
+   |              ^^^-^^
+   |              |  |
+   |              |  help: remove this semicolon to return this value
+   |              expected `i32`, found `()`
+
+error[E0308]: `match` arms have incompatible types
+  --> $DIR/stmts-as-exp-105431.rs:45:16
+   |
+LL |       let res = match v {
+   |  _______________-
+LL | |         1 => { if 1 < 2 { 1 } else { 2 } }
+   | |                ------------------------- this is found to be of type `{integer}`
+LL | |         _ => { 2; }
+   | |                ^-
+   | |                ||
+   | |                |help: consider removing this semicolon
+   | |                expected integer, found `()`
+LL | |     };
+   | |_____- `match` arms have incompatible types
+
+error[E0308]: mismatched types
+  --> $DIR/stmts-as-exp-105431.rs:59:5
+   |
+LL | fn test_if_match_mixed() -> i32 {
+   |                             --- expected `i32` because of return type
+...
+LL |     x
+   |     ^ expected `i32`, found `()`
+   |
+help: remove this semicolon to return this value
+   |
+LL -         3;
+LL +         3
+   |
+help: remove this semicolon to return this value
+   |
+LL -         };
+LL +         }
+   |
+
+error[E0308]: mismatched types
+  --> $DIR/stmts-as-exp-105431.rs:72:5
+   |
+LL | fn test_if_match_mixed_failed() -> i32 {
+   |                                    --- expected `i32` because of return type
+LL |     let x = if true {
+LL |         3;
+   |          - help: remove this semicolon to return this value
+...
+LL |     x
+   |     ^ expected `i32`, found `()`
+
+error: aborting due to 9 previous errors
+
+For more information about this error, try `rustc --explain E0308`.