Add redundant else lint

camsteffen · camsteffen · commit f059febe85f4 · 2020-11-29T17:55:42.000-06:00
diff --git a/CHANGELOG.md b/CHANGELOG.md
@@ -2023,6 +2023,7 @@ Released 2018-09-13
 [`redundant_closure`]: https://rust-lang.github.io/rust-clippy/master/index.html#redundant_closure
 [`redundant_closure_call`]: https://rust-lang.github.io/rust-clippy/master/index.html#redundant_closure_call
 [`redundant_closure_for_method_calls`]: https://rust-lang.github.io/rust-clippy/master/index.html#redundant_closure_for_method_calls
+[`redundant_else`]: https://rust-lang.github.io/rust-clippy/master/index.html#redundant_else
 [`redundant_field_names`]: https://rust-lang.github.io/rust-clippy/master/index.html#redundant_field_names
 [`redundant_pattern`]: https://rust-lang.github.io/rust-clippy/master/index.html#redundant_pattern
 [`redundant_pattern_matching`]: https://rust-lang.github.io/rust-clippy/master/index.html#redundant_pattern_matching
diff --git a/clippy_lints/src/lib.rs b/clippy_lints/src/lib.rs
@@ -293,6 +293,7 @@ mod question_mark;
 mod ranges;
 mod redundant_clone;
 mod redundant_closure_call;
+mod redundant_else;
 mod redundant_field_names;
 mod redundant_pub_crate;
 mod redundant_static_lifetimes;
@@ -810,6 +811,7 @@ pub fn register_plugins(store: &mut rustc_lint::LintStore, sess: &Session, conf:
         &ranges::REVERSED_EMPTY_RANGES,
         &redundant_clone::REDUNDANT_CLONE,
         &redundant_closure_call::REDUNDANT_CLOSURE_CALL,
+        &redundant_else::REDUNDANT_ELSE,
         &redundant_field_names::REDUNDANT_FIELD_NAMES,
         &redundant_pub_crate::REDUNDANT_PUB_CRATE,
         &redundant_static_lifetimes::REDUNDANT_STATIC_LIFETIMES,
@@ -1113,6 +1115,7 @@ pub fn register_plugins(store: &mut rustc_lint::LintStore, sess: &Session, conf:
     store.register_early_pass(|| box items_after_statements::ItemsAfterStatements);
     store.register_early_pass(|| box precedence::Precedence);
     store.register_early_pass(|| box needless_continue::NeedlessContinue);
+    store.register_early_pass(|| box redundant_else::RedundantElse);
     store.register_late_pass(|| box create_dir::CreateDir);
     store.register_early_pass(|| box needless_arbitrary_self_type::NeedlessArbitrarySelfType);
     store.register_early_pass(|| box redundant_static_lifetimes::RedundantStaticLifetimes);
@@ -1294,6 +1297,7 @@ pub fn register_plugins(store: &mut rustc_lint::LintStore, sess: &Session, conf:
         LintId::of(&pass_by_ref_or_value::TRIVIALLY_COPY_PASS_BY_REF),
         LintId::of(&ranges::RANGE_MINUS_ONE),
         LintId::of(&ranges::RANGE_PLUS_ONE),
+        LintId::of(&redundant_else::REDUNDANT_ELSE),
         LintId::of(&ref_option_ref::REF_OPTION_REF),
         LintId::of(&shadow::SHADOW_UNRELATED),
         LintId::of(&strings::STRING_ADD_ASSIGN),
diff --git a/clippy_lints/src/redundant_else.rs b/clippy_lints/src/redundant_else.rs
@@ -0,0 +1,135 @@
+use crate::utils::span_lint_and_help;
+use rustc_ast::ast::{Block, Expr, ExprKind, Stmt, StmtKind};
+use rustc_ast::visit::{walk_expr, Visitor};
+use rustc_lint::{EarlyContext, EarlyLintPass};
+use rustc_middle::lint::in_external_macro;
+use rustc_session::{declare_lint_pass, declare_tool_lint};
+
+declare_clippy_lint! {
+    /// **What it does:** Checks for `else` blocks that can be removed without changing semantics.
+    ///
+    /// **Why is this bad?** The `else` block adds unnecessary indentation and verbosity.
+    ///
+    /// **Known problems:** Some may prefer to keep the `else` block for clarity.
+    ///
+    /// **Example:**
+    ///
+    /// ```rust
+    /// fn my_func(count: u32) {
+    ///     if count == 0 {
+    ///         print!("Nothing to do");
+    ///         return;
+    ///     } else {
+    ///         print!("Moving on...");
+    ///     }
+    /// }
+    /// ```
+    /// Use instead:
+    /// ```rust
+    /// fn my_func(count: u32) {
+    ///     if count == 0 {
+    ///         print!("Nothing to do");
+    ///         return;
+    ///     }
+    ///     print!("Moving on...");
+    /// }
+    /// ```
+    pub REDUNDANT_ELSE,
+    pedantic,
+    "`else` branch that can be removed without changing semantics"
+}
+
+declare_lint_pass!(RedundantElse => [REDUNDANT_ELSE]);
+
+impl EarlyLintPass for RedundantElse {
+    fn check_stmt(&mut self, cx: &EarlyContext<'_>, stmt: &Stmt) {
+        if in_external_macro(cx.sess, stmt.span) {
+            return;
+        }
+        // Only look at expressions that are a whole statement
+        let expr: &Expr = match &stmt.kind {
+            StmtKind::Expr(expr) | StmtKind::Semi(expr) => expr,
+            _ => return,
+        };
+        // if else
+        let (mut then, mut els): (&Block, &Expr) = match &expr.kind {
+            ExprKind::If(_, then, Some(els)) => (then, els),
+            _ => return,
+        };
+        loop {
+            if !BreakVisitor::default().check_block(then) {
+                // then block does not always break
+                return;
+            }
+            match &els.kind {
+                // else if else
+                ExprKind::If(_, next_then, Some(next_els)) => {
+                    then = next_then;
+                    els = next_els;
+                    continue;
+                },
+                // else if without else
+                ExprKind::If(..) => return,
+                // done
+                _ => break,
+            }
+        }
+        span_lint_and_help(
+            cx,
+            REDUNDANT_ELSE,
+            els.span,
+            "redundant else block",
+            None,
+            "remove the `else` block and move the contents out",
+        );
+    }
+}
+
+/// Call `check` functions to check if an expression always breaks control flow
+#[derive(Default)]
+struct BreakVisitor {
+    is_break: bool,
+}
+
+impl<'ast> Visitor<'ast> for BreakVisitor {
+    fn visit_block(&mut self, block: &'ast Block) {
+        self.is_break = match block.stmts.as_slice() {
+            [.., last] => self.check_stmt(last),
+            _ => false,
+        };
+    }
+
+    fn visit_expr(&mut self, expr: &'ast Expr) {
+        self.is_break = match expr.kind {
+            ExprKind::Break(..) | ExprKind::Continue(..) | ExprKind::Ret(..) => true,
+            ExprKind::Match(_, ref arms) => arms.iter().all(|arm| self.check_expr(&arm.body)),
+            ExprKind::If(_, ref then, Some(ref els)) => self.check_block(then) && self.check_expr(els),
+            ExprKind::If(_, _, None)
+            // ignore loops for simplicity
+            | ExprKind::While(..) | ExprKind::ForLoop(..) | ExprKind::Loop(..) => false,
+            _ => {
+                walk_expr(self, expr);
+                return;
+            },
+        };
+    }
+}
+
+impl BreakVisitor {
+    fn check<T>(&mut self, item: T, visit: fn(&mut Self, T)) -> bool {
+        visit(self, item);
+        std::mem::replace(&mut self.is_break, false)
+    }
+
+    fn check_block(&mut self, block: &Block) -> bool {
+        self.check(block, Self::visit_block)
+    }
+
+    fn check_expr(&mut self, expr: &Expr) -> bool {
+        self.check(expr, Self::visit_expr)
+    }
+
+    fn check_stmt(&mut self, stmt: &Stmt) -> bool {
+        self.check(stmt, Self::visit_stmt)
+    }
+}
diff --git a/tests/ui/redundant_else.rs b/tests/ui/redundant_else.rs
@@ -0,0 +1,154 @@
+#![warn(clippy::redundant_else)]
+#![allow(clippy::needless_return)]
+
+fn main() {
+    loop {
+        // break
+        if foo() {
+            println!("Love your neighbor;");
+            break;
+        } else {
+            println!("yet don't pull down your hedge.");
+        }
+        // continue
+        if foo() {
+            println!("He that lies down with Dogs,");
+            continue;
+        } else {
+            println!("shall rise up with fleas.");
+        }
+        // match block
+        if foo() {
+            match foo() {
+                1 => break,
+                _ => return,
+            }
+        } else {
+            println!("You may delay, but time will not.");
+        }
+    }
+    // else if
+    if foo() {
+        return;
+    } else if foo() {
+        return;
+    } else {
+        println!("A fat kitchen makes a lean will.");
+    }
+    // let binding outside of block
+    let _ = {
+        if foo() {
+            return;
+        } else {
+            1
+        }
+    };
+    // else if with let binding outside of block
+    let _ = {
+        if foo() {
+            return;
+        } else if foo() {
+            return;
+        } else {
+            2
+        }
+    };
+    // inside if let
+    let _ = if let Some(1) = foo() {
+        let _ = 1;
+        if foo() {
+            return;
+        } else {
+            1
+        }
+    } else {
+        1
+    };
+
+    //
+    // non-lint cases
+    //
+
+    // sanity check
+    if foo() {
+        let _ = 1;
+    } else {
+        println!("Who is wise? He that learns from every one.");
+    }
+    // else if without else
+    if foo() {
+        return;
+    } else if foo() {
+        foo()
+    };
+    // nested if return
+    if foo() {
+        if foo() {
+            return;
+        }
+    } else {
+        foo()
+    };
+    // match with non-breaking branch
+    if foo() {
+        match foo() {
+            1 => foo(),
+            _ => return,
+        }
+    } else {
+        println!("Three may keep a secret, if two of them are dead.");
+    }
+    // let binding
+    let _ = if foo() {
+        return;
+    } else {
+        1
+    };
+    // assign
+    let a;
+    a = if foo() {
+        return;
+    } else {
+        1
+    };
+    // assign-op
+    a += if foo() {
+        return;
+    } else {
+        1
+    };
+    // if return else if else
+    if foo() {
+        return;
+    } else if foo() {
+        1
+    } else {
+        2
+    };
+    // if else if return else
+    if foo() {
+        1
+    } else if foo() {
+        return;
+    } else {
+        2
+    };
+    // else if with let binding
+    let _ = if foo() {
+        return;
+    } else if foo() {
+        return;
+    } else {
+        2
+    };
+    // inside function call
+    Box::new(if foo() {
+        return;
+    } else {
+        1
+    });
+}
+
+fn foo<T>() -> T {
+    unimplemented!("I'm not Santa Claus")
+}
diff --git a/tests/ui/redundant_else.stderr b/tests/ui/redundant_else.stderr
