New lint: unnecessary_semicolon (#14032)

This lint detects and removes the unnecessary semicolon after a `match`
or `if` statement returning `()`. It seems to be quite a common
"mistake", given the number of hits (88) we had in the Clippy sources
themselves.

The lint doesn't bother about loops, as `rustfmt` already removes the
extra semicolon. It doesn't handle blocks either, as an extra block
level, followed or not by a semicolon, is likely intentional.

I propose to put the lint in `pedantic`, as putting it in `style` seems
quite hazardous given the number of hits.

Note: there exists a `redundant-semicolon` lint in the compiler, but it
is an early lint and cannot check that the expression evaluates to `()`,
so it ignores the cases we're handling here.

----

changelog: [`unnecessary_semicolon`]: new lint

CHANGELOG.md

@@ -6174,6 +6174,7 @@ Released 2018-09-13
 [`unnecessary_safety_comment`]: https://rust-lang.github.io/rust-clippy/master/index.html#unnecessary_safety_comment
 [`unnecessary_safety_doc`]: https://rust-lang.github.io/rust-clippy/master/index.html#unnecessary_safety_doc
 [`unnecessary_self_imports`]: https://rust-lang.github.io/rust-clippy/master/index.html#unnecessary_self_imports
+[`unnecessary_semicolon`]: https://rust-lang.github.io/rust-clippy/master/index.html#unnecessary_semicolon
 [`unnecessary_sort_by`]: https://rust-lang.github.io/rust-clippy/master/index.html#unnecessary_sort_by
 [`unnecessary_struct_initialization`]: https://rust-lang.github.io/rust-clippy/master/index.html#unnecessary_struct_initialization
 [`unnecessary_to_owned`]: https://rust-lang.github.io/rust-clippy/master/index.html#unnecessary_to_owned

clippy_dev/src/setup/intellij.rs

@@ -62,7 +62,7 @@ fn check_and_get_rustc_dir(rustc_path: &str) -> Result<PathBuf, ()> {
                 eprintln!("error: unable to get the absolute path of rustc ({err})");
                 return Err(());
             },
-        };
+        }
     }
 
     let path = path.join("compiler");

clippy_dev/src/update_lints.rs

@@ -842,7 +842,7 @@ fn try_rename_file(old_name: &Path, new_name: &Path) -> bool {
         Ok(file) => drop(file),
         Err(e) if matches!(e.kind(), io::ErrorKind::AlreadyExists | io::ErrorKind::NotFound) => return false,
         Err(e) => panic_file(e, new_name, "create"),
-    };
+    }
     match fs::rename(old_name, new_name) {
         Ok(()) => true,
         Err(e) => {

clippy_lints/src/assigning_clones.rs

@@ -257,7 +257,7 @@ fn build_sugg<'tcx>(
                         // The receiver may have been a value type, so we need to add an `&` to
                         // be sure the argument to clone_from will be a reference.
                         arg_sugg = arg_sugg.addr();
-                    };
+                    }
 
                     format!("{receiver_sugg}.clone_from({arg_sugg})")
                 },

clippy_lints/src/attrs/mixed_attributes_style.rs

@@ -60,7 +60,7 @@ pub(super) fn check(cx: &EarlyContext<'_>, item_span: Span, attrs: &[Attribute])
                 }
                 outer_attr_kind.insert(kind);
             },
-        };
+        }
     }
 }
 

clippy_lints/src/casts/cast_possible_wrap.rs

@@ -84,6 +84,6 @@ pub(super) fn check(cx: &LateContext<'_>, expr: &Expr<'_>, cast_from: Ty<'_>, ca
             diag
                 .note("`usize` and `isize` may be as small as 16 bits on some platforms")
                 .note("for more information see https://doc.rust-lang.org/reference/types/numeric.html#machine-dependent-integer-types");
-        };
+        }
     });
 }

clippy_lints/src/casts/cast_sign_loss.rs

@@ -205,7 +205,7 @@ fn expr_muldiv_sign(cx: &LateContext<'_>, expr: &Expr<'_>) -> Sign {
             // - uncertain if there are any uncertain values (because they could be negative or positive),
             Sign::Uncertain => return Sign::Uncertain,
             Sign::ZeroOrPositive => (),
-        };
+        }
     }
 
     // A mul/div is:
@@ -236,7 +236,7 @@ fn expr_add_sign(cx: &LateContext<'_>, expr: &Expr<'_>) -> Sign {
             // - uncertain if there are any uncertain values (because they could be negative or positive),
             Sign::Uncertain => return Sign::Uncertain,
             Sign::ZeroOrPositive => positive_count += 1,
-        };
+        }
     }
 
     // A sum is:

clippy_lints/src/checked_conversions.rs

@@ -273,7 +273,7 @@ fn get_types_from_cast<'a>(
             },
             _ => {},
         }
-    };
+    }
     None
 }
 

clippy_lints/src/declared_lints.rs

@@ -757,6 +757,7 @@ pub static LINTS: &[&crate::LintInfo] = &[
     crate::unnecessary_map_on_constructor::UNNECESSARY_MAP_ON_CONSTRUCTOR_INFO,
     crate::unnecessary_owned_empty_strings::UNNECESSARY_OWNED_EMPTY_STRINGS_INFO,
     crate::unnecessary_self_imports::UNNECESSARY_SELF_IMPORTS_INFO,
+    crate::unnecessary_semicolon::UNNECESSARY_SEMICOLON_INFO,
     crate::unnecessary_struct_initialization::UNNECESSARY_STRUCT_INITIALIZATION_INFO,
     crate::unnecessary_wraps::UNNECESSARY_WRAPS_INFO,
     crate::unneeded_struct_pattern::UNNEEDED_STRUCT_PATTERN_INFO,

clippy_lints/src/default_constructed_unit_structs.rs

@@ -80,6 +80,6 @@ impl LateLintPass<'_> for DefaultConstructedUnitStructs {
                 String::new(),
                 Applicability::MachineApplicable,
             );
-        };
+        }
     }
 }

clippy_lints/src/dereference.rs

@@ -331,7 +331,7 @@ impl<'tcx> LateLintPass<'tcx> for Dereferencing<'tcx> {
                                     deref_count += 1;
                                 },
                                 None => break None,
-                            };
+                            }
                         };
 
                         let use_node = use_cx.use_node(cx);

clippy_lints/src/enum_clike.rs

@@ -70,7 +70,7 @@ impl<'tcx> LateLintPass<'tcx> for UnportableVariant {
                             var.span,
                             "C-like enum variant discriminant is not portable to 32-bit targets",
                         );
-                    };
+                    }
                 }
             }
         }

clippy_lints/src/extra_unused_type_parameters.rs

@@ -183,7 +183,7 @@ impl<'cx, 'tcx> TypeWalker<'cx, 'tcx> {
                     .collect()
             };
             self.emit_sugg(spans, msg, help);
-        };
+        }
     }
 }
 

clippy_lints/src/functions/impl_trait_in_params.rs

@@ -45,7 +45,7 @@ pub(super) fn check_fn<'tcx>(cx: &LateContext<'_>, kind: &'tcx FnKind<'_>, body:
         for param in generics.params {
             if param.is_impl_trait() {
                 report(cx, param, generics);
-            };
+            }
         }
     }
 }

clippy_lints/src/functions/must_use.rs

@@ -160,7 +160,7 @@ fn check_needless_must_use(
                 && !is_must_use_ty(cx, future_ty)
             {
                 return;
-            };
+            }
         }
 
         span_lint_and_help(

clippy_lints/src/implicit_saturating_add.rs

@@ -120,7 +120,7 @@ fn get_const<'tcx>(cx: &LateContext<'tcx>, expr: &Expr<'tcx>) -> Option<(u128, B
         let ecx = ConstEvalCtxt::new(cx);
         if let Some(Constant::Int(c)) = ecx.eval(r) {
             return Some((c, op.node, l));
-        };
+        }
         if let Some(Constant::Int(c)) = ecx.eval(l) {
             return Some((c, invert_op(op.node)?, r));
         }

clippy_lints/src/implicit_saturating_sub.rs

@@ -350,7 +350,7 @@ fn check_with_condition<'tcx>(
                     if cx.typeck_results().expr_ty(cond_left).is_signed() {
                     } else {
                         print_lint_and_sugg(cx, var_name, expr);
-                    };
+                    }
                 }
             },
             ExprKind::Path(QPath::TypeRelative(_, name)) => {

clippy_lints/src/iter_over_hash_type.rs

@@ -65,6 +65,6 @@ impl LateLintPass<'_> for IterOverHashType {
                 expr.span,
                 "iteration over unordered hash-based type",
             );
-        };
+        }
     }
 }

clippy_lints/src/let_with_type_underscore.rs

@@ -41,6 +41,6 @@ impl<'tcx> LateLintPass<'tcx> for UnderscoreTyped {
                 Some(ty.span.with_lo(local.pat.span.hi())),
                 "remove the explicit type `_` declaration",
             );
-        };
+        }
     }
 }

clippy_lints/src/lib.rs

@@ -372,6 +372,7 @@ mod unnecessary_literal_bound;
 mod unnecessary_map_on_constructor;
 mod unnecessary_owned_empty_strings;
 mod unnecessary_self_imports;
+mod unnecessary_semicolon;
 mod unnecessary_struct_initialization;
 mod unnecessary_wraps;
 mod unneeded_struct_pattern;
@@ -972,5 +973,6 @@ pub fn register_lints(store: &mut rustc_lint::LintStore, conf: &'static Conf) {
     store.register_late_pass(|_| Box::new(unnecessary_literal_bound::UnnecessaryLiteralBound));
     store.register_late_pass(move |_| Box::new(arbitrary_source_item_ordering::ArbitrarySourceItemOrdering::new(conf)));
     store.register_late_pass(|_| Box::new(unneeded_struct_pattern::UnneededStructPattern));
+    store.register_late_pass(|_| Box::new(unnecessary_semicolon::UnnecessarySemicolon));
     // add lints here, do not remove this comment, it's used in `new_lint`
 }

clippy_lints/src/lifetimes.rs

@@ -784,7 +784,7 @@ fn report_elidable_lifetimes(
         |diag| {
             if !include_suggestions {
                 return;
-            };
+            }
 
             if let Some(suggestions) = elision_suggestions(cx, generics, elidable_lts, usages) {
                 diag.multipart_suggestion("elide the lifetimes", suggestions, Applicability::MachineApplicable);

clippy_lints/src/literal_representation.rs

@@ -251,7 +251,7 @@ impl LiteralDigitGrouping {
                     );
                     if !consistent {
                         return Err(WarningType::InconsistentDigitGrouping);
-                    };
+                    }
                 }
 
                 Ok(())

clippy_lints/src/manual_let_else.rs

@@ -106,7 +106,7 @@ impl<'tcx> QuestionMark {
                     emit_manual_let_else(cx, stmt.span, match_expr, &ident_map, pat_arm.pat, diverging_arm.body);
                 },
             }
-        };
+        }
     }
 }
 
@@ -295,7 +295,7 @@ fn pat_allowed_for_else(cx: &LateContext<'_>, pat: &'_ Pat<'_>, check_types: boo
             PatKind::Struct(..) | PatKind::TupleStruct(..) | PatKind::Path(..)
         ) {
             return;
-        };
+        }
         let ty = typeck_results.pat_ty(pat);
         // Option and Result are allowed, everything else isn't.
         if !(is_type_diagnostic_item(cx, ty, sym::Option) || is_type_diagnostic_item(cx, ty, sym::Result)) {

clippy_lints/src/manual_rem_euclid.rs

@@ -85,7 +85,7 @@ impl<'tcx> LateLintPass<'tcx> for ManualRemEuclid {
                     }
                 },
                 _ => return,
-            };
+            }
 
             let mut app = Applicability::MachineApplicable;
             let rem_of = snippet_with_context(cx, rem2_lhs.span, ctxt, "_", &mut app).0;

clippy_lints/src/manual_strip.rs

@@ -86,7 +86,7 @@ impl<'tcx> LateLintPass<'tcx> for ManualStrip {
             let target_res = cx.qpath_res(target_path, target_arg.hir_id);
             if target_res == Res::Err {
                 return;
-            };
+            }
 
             if let Res::Local(hir_id) = target_res
                 && let Some(used_mutably) = mutated_variables(then, cx)

clippy_lints/src/matches/manual_filter.rs

@@ -34,7 +34,7 @@ fn get_cond_expr<'tcx>(
             needs_negated: is_none_expr(cx, then_expr), /* if the `then_expr` resolves to `None`, need to negate the
                                                          * cond */
         });
-    };
+    }
     None
 }
 
@@ -45,7 +45,7 @@ fn peels_blocks_incl_unsafe_opt<'a>(expr: &'a Expr<'a>) -> Option<&'a Expr<'a>>
         if block.stmts.is_empty() {
             return block.expr;
         }
-    };
+    }
     None
 }
 
@@ -68,14 +68,14 @@ fn is_some_expr(cx: &LateContext<'_>, target: HirId, ctxt: SyntaxContext, expr:
                 && is_res_lang_ctor(cx, path_res(cx, callee), OptionSome)
                 && path_to_local_id(arg, target);
         }
-    };
+    }
     false
 }
 
 fn is_none_expr(cx: &LateContext<'_>, expr: &Expr<'_>) -> bool {
     if let Some(inner_expr) = peels_blocks_incl_unsafe_opt(expr) {
         return is_res_lang_ctor(cx, path_res(cx, inner_expr), OptionNone);
-    };
+    }
     false
 }
 

clippy_lints/src/matches/manual_utils.rs

@@ -109,7 +109,7 @@ where
             }
         },
         None => return None,
-    };
+    }
 
     let mut app = Applicability::MachineApplicable;
 

clippy_lints/src/matches/match_like_matches.rs

@@ -117,7 +117,7 @@ where
             if let ty::Ref(..) = cx.typeck_results().expr_ty(ex_inner).kind() {
                 ex_new = ex_inner;
             }
-        };
+        }
         span_lint_and_sugg(
             cx,
             MATCH_LIKE_MATCHES_MACRO,

clippy_lints/src/matches/match_wild_enum.rs

@@ -170,7 +170,7 @@ pub(crate) fn check(cx: &LateContext<'_>, ex: &Expr<'_>, arms: &[Arm<'_>]) {
                 );
             });
         },
-    };
+    }
 }
 
 enum CommonPrefixSearcher<'a> {

clippy_lints/src/matches/try_err.rs

@@ -46,7 +46,7 @@ pub(super) fn check<'tcx>(cx: &LateContext<'tcx>, expr: &'tcx Expr<'_>, scrutine
             err_ty = ty;
         } else {
             return;
-        };
+        }
 
         span_lint_and_then(
             cx,

clippy_lints/src/matches/wild_in_or_pats.rs

@@ -13,7 +13,7 @@ pub(crate) fn check(cx: &LateContext<'_>, expr: &Expr<'_>, arms: &[Arm<'_>]) {
         && has_non_exhaustive_attr(cx.tcx, *adt_def)
     {
         return;
-    };
+    }
     for arm in arms {
         if let PatKind::Or(fields) = arm.pat.kind {
             // look for multiple fields in this arm that contains at least one Wild pattern

clippy_lints/src/methods/bytecount.rs

@@ -62,5 +62,5 @@ pub(super) fn check<'tcx>(
             ),
             applicability,
         );
-    };
+    }
 }

clippy_lints/src/methods/bytes_count_to_len.rs

@@ -32,5 +32,5 @@ pub(super) fn check<'tcx>(
             ),
             applicability,
         );
-    };
+    }
 }

clippy_lints/src/methods/bytes_nth.rs

@@ -46,5 +46,5 @@ pub(super) fn check<'tcx>(cx: &LateContext<'tcx>, expr: &Expr<'_>, recv: &'tcx E
             format!("{receiver}.as_bytes().get({n}).copied()"),
             applicability,
         );
-    };
+    }
 }

clippy_lints/src/methods/cloned_instead_of_copied.rs

@@ -32,7 +32,7 @@ pub fn check(cx: &LateContext<'_>, expr: &Expr<'_>, recv: &Expr<'_>, span: Span,
         // &T where T: Copy
         ty::Ref(_, ty, _) if is_copy(cx, *ty) => {},
         _ => return,
-    };
+    }
     span_lint_and_sugg(
         cx,
         CLONED_INSTEAD_OF_COPIED,

clippy_lints/src/methods/err_expect.rs

@@ -37,7 +37,7 @@ pub(super) fn check(
             "expect_err".to_string(),
             Applicability::MachineApplicable,
         );
-    };
+    }
 }
 
 /// Given a `Result<T, E>` type, return its data (`T`).

clippy_lints/src/methods/expect_fun_call.rs

@@ -58,7 +58,7 @@ pub(super) fn check<'tcx>(
             if ty.is_str() && can_be_static_str(cx, arg) {
                 return false;
             }
-        };
+        }
         true
     }
 

clippy_lints/src/methods/iter_with_drain.rs

@@ -25,5 +25,5 @@ pub(super) fn check(cx: &LateContext<'_>, expr: &Expr<'_>, recv: &Expr<'_>, span
             "into_iter()".to_string(),
             Applicability::MaybeIncorrect,
         );
-    };
+    }
 }

clippy_lints/src/methods/path_ends_with_ext.rs

@@ -37,7 +37,7 @@ pub(super) fn check(
             let _ = write!(sugg, r#".extension().is_some_and(|ext| ext == "{path}")"#);
         } else {
             let _ = write!(sugg, r#".extension().map_or(false, |ext| ext == "{path}")"#);
-        };
+        }
 
         span_lint_and_sugg(
             cx,

clippy_lints/src/methods/unnecessary_iter_cloned.rs

@@ -87,7 +87,7 @@ pub fn check_for_loop_iter(
                 // skip lint
                 return true;
             }
-        };
+        }
 
         // the lint should not be executed if no violation happens
         let snippet = if let ExprKind::MethodCall(maybe_iter_method_name, collection, [], _) = receiver.kind