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

Author: llogiq

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,
             );
-        };
+        }
     }
 }