Adds a new lint that checks if there is a semicolon on the last block statement if it returns nothing

CHANGELOG.md

@@ -2185,6 +2185,7 @@ Released 2018-09-13
 [`same_item_push`]: https://rust-lang.github.io/rust-clippy/master/index.html#same_item_push
 [`search_is_some`]: https://rust-lang.github.io/rust-clippy/master/index.html#search_is_some
 [`self_assignment`]: https://rust-lang.github.io/rust-clippy/master/index.html#self_assignment
+[`semicolon_if_nothing_returned`]: https://rust-lang.github.io/rust-clippy/master/index.html#semicolon_if_nothing_returned
 [`serde_api_misuse`]: https://rust-lang.github.io/rust-clippy/master/index.html#serde_api_misuse
 [`shadow_reuse`]: https://rust-lang.github.io/rust-clippy/master/index.html#shadow_reuse
 [`shadow_same`]: https://rust-lang.github.io/rust-clippy/master/index.html#shadow_same

clippy_lints/src/lib.rs

@@ -310,6 +310,7 @@ mod regex;
 mod repeat_once;
 mod returns;
 mod self_assignment;
+mod semicolon_if_nothing_returned;
 mod serde_api;
 mod shadow;
 mod single_component_path_imports;
@@ -876,6 +877,7 @@ pub fn register_plugins(store: &mut rustc_lint::LintStore, sess: &Session, conf:
         &returns::LET_AND_RETURN,
         &returns::NEEDLESS_RETURN,
         &self_assignment::SELF_ASSIGNMENT,
+        &semicolon_if_nothing_returned::SEMICOLON_IF_NOTHING_RETURNED,
         &serde_api::SERDE_API_MISUSE,
         &shadow::SHADOW_REUSE,
         &shadow::SHADOW_SAME,
@@ -1237,6 +1239,7 @@ pub fn register_plugins(store: &mut rustc_lint::LintStore, sess: &Session, conf:
     store.register_late_pass(|| box manual_unwrap_or::ManualUnwrapOr);
     store.register_late_pass(|| box manual_ok_or::ManualOkOr);
     store.register_late_pass(|| box float_equality_without_abs::FloatEqualityWithoutAbs);
+    store.register_late_pass(|| box semicolon_if_nothing_returned::SemicolonIfNothingReturned);
     store.register_late_pass(|| box async_yields_async::AsyncYieldsAsync);
     let disallowed_methods = conf.disallowed_methods.iter().cloned().collect::<FxHashSet<_>>();
     store.register_late_pass(move || box disallowed_method::DisallowedMethod::new(&disallowed_methods));
@@ -1291,6 +1294,7 @@ pub fn register_plugins(store: &mut rustc_lint::LintStore, sess: &Session, conf:
         LintId::of(&panic_unimplemented::UNIMPLEMENTED),
         LintId::of(&panic_unimplemented::UNREACHABLE),
         LintId::of(&pattern_type_mismatch::PATTERN_TYPE_MISMATCH),
+        LintId::of(&semicolon_if_nothing_returned::SEMICOLON_IF_NOTHING_RETURNED),
         LintId::of(&shadow::SHADOW_REUSE),
         LintId::of(&shadow::SHADOW_SAME),
         LintId::of(&strings::STRING_ADD),

clippy_lints/src/semicolon_if_nothing_returned.rs

@@ -0,0 +1,66 @@
+use crate::utils::{in_macro, snippet_with_macro_callsite, span_lint_and_sugg, sugg};
+use if_chain::if_chain;
+use rustc_errors::Applicability;
+use rustc_hir::{Block, ExprKind};
+use rustc_lint::{LateContext, LateLintPass};
+use rustc_session::{declare_lint_pass, declare_tool_lint};
+
+declare_clippy_lint! {
+    /// **What it does:** Looks for blocks of expressions and fires if the last expression returns `()`
+    /// but is not followed by a semicolon.
+    ///
+    /// **Why is this bad?** The semicolon might be optional but when
+    /// extending the block with new code, it doesn't require a change in previous last line.
+    ///
+    /// **Known problems:** None.
+    ///
+    /// **Example:**
+    ///
+    /// ```rust
+    /// fn main() {
+    ///     println!("Hello world")
+    /// }
+    /// ```
+    /// Use instead:
+    /// ```rust
+    /// fn main() {
+    ///     println!("Hello world");
+    /// }
+    /// ```
+    pub SEMICOLON_IF_NOTHING_RETURNED,
+    restriction,
+    "add a semicolon if nothing is returned"
+}
+
+declare_lint_pass!(SemicolonIfNothingReturned => [SEMICOLON_IF_NOTHING_RETURNED]);
+
+impl LateLintPass<'_> for SemicolonIfNothingReturned {
+    fn check_block(&mut self, cx: &LateContext<'tcx>, block: &'tcx Block<'tcx>) {
+        if_chain! {
+            if !in_macro(block.span);
+            if let Some(expr) = block.expr;
+            let t_expr = cx.typeck_results().expr_ty(expr);
+            if t_expr.is_unit();
+            if let snippet = snippet_with_macro_callsite(cx, expr.span, "}");
+            if !snippet.ends_with('}');
+            then {
+                // filter out the desugared `for` loop
+                if let ExprKind::DropTemps(..) = &expr.kind {
+                    return;
+                }
+
+                let sugg = sugg::Sugg::hir_with_macro_callsite(cx, &expr, "..");
+                let suggestion = format!("{0};", sugg);
+                span_lint_and_sugg(
+                    cx,
+                    SEMICOLON_IF_NOTHING_RETURNED,
+                    expr.span.source_callsite(),
+                    "consider adding a `;` to the last statement for consistent formatting",
+                    "add a `;` here",
+                    suggestion,
+                    Applicability::MaybeIncorrect,
+                );
+            }
+        }
+    }
+}

tests/ui/semicolon_if_nothing_returned.rs

@@ -0,0 +1,55 @@
+#![warn(clippy::semicolon_if_nothing_returned)]
+#![feature(label_break_value)]
+
+fn get_unit() {}
+
+// the functions below trigger the lint
+fn main() {
+    println!("Hello")
+}
+
+fn hello() {
+    get_unit()
+}
+
+fn basic101(x: i32) {
+    let y: i32;
+    y = x + 1
+}
+
+// this is fine
+fn print_sum(a: i32, b: i32) {
+    println!("{}", a + b);
+    assert_eq!(true, false);
+}
+
+fn foo(x: i32) {
+    let y: i32;
+    if x < 1 {
+        y = 4;
+    } else {
+        y = 5;
+    }
+}
+
+fn bar(x: i32) {
+    let y: i32;
+    match x {
+        1 => y = 4,
+        _ => y = 32,
+    }
+}
+
+fn foobar(x: i32) {
+    let y: i32;
+    'label: {
+        y = x + 1;
+    }
+}
+
+fn loop_test(x: i32) {
+    let y: i32;
+    for &ext in &["stdout", "stderr", "fixed"] {
+        println!("{}", ext);
+    }
+}

tests/ui/semicolon_if_nothing_returned.stderr

@@ -0,0 +1,22 @@
+error: consider adding a `;` to the last statement for consistent formatting
+  --> $DIR/semicolon_if_nothing_returned.rs:8:5
+   |
+LL |     println!("Hello")
+   |     ^^^^^^^^^^^^^^^^^ help: add a `;` here: `println!("Hello");`
+   |
+   = note: `-D clippy::semicolon-if-nothing-returned` implied by `-D warnings`
+
+error: consider adding a `;` to the last statement for consistent formatting
+  --> $DIR/semicolon_if_nothing_returned.rs:12:5
+   |
+LL |     get_unit()
+   |     ^^^^^^^^^^ help: add a `;` here: `get_unit();`
+
+error: consider adding a `;` to the last statement for consistent formatting
+  --> $DIR/semicolon_if_nothing_returned.rs:17:5
+   |
+LL |     y = x + 1
+   |     ^^^^^^^^^ help: add a `;` here: `y = x + 1;`
+
+error: aborting due to 3 previous errors
+