add new lint, stacked_ifs

This commit:
- Adds a new late pass lint
- Adds relevant UI tests

CHANGELOG.md

@@ -5704,6 +5704,7 @@ Released 2018-09-13
 [`skip_while_next`]: https://rust-lang.github.io/rust-clippy/master/index.html#skip_while_next
 [`slow_vector_initialization`]: https://rust-lang.github.io/rust-clippy/master/index.html#slow_vector_initialization
 [`stable_sort_primitive`]: https://rust-lang.github.io/rust-clippy/master/index.html#stable_sort_primitive
+[`stacked_ifs`]: https://rust-lang.github.io/rust-clippy/master/index.html#stacked_ifs
 [`std_instead_of_alloc`]: https://rust-lang.github.io/rust-clippy/master/index.html#std_instead_of_alloc
 [`std_instead_of_core`]: https://rust-lang.github.io/rust-clippy/master/index.html#std_instead_of_core
 [`str_split_at_newline`]: https://rust-lang.github.io/rust-clippy/master/index.html#str_split_at_newline

clippy_lints/src/declared_lints.rs

@@ -648,6 +648,7 @@ pub(crate) static LINTS: &[&crate::LintInfo] = &[
     crate::size_of_in_element_count::SIZE_OF_IN_ELEMENT_COUNT_INFO,
     crate::size_of_ref::SIZE_OF_REF_INFO,
     crate::slow_vector_initialization::SLOW_VECTOR_INITIALIZATION_INFO,
+    crate::stacked_ifs::STACKED_IFS_INFO,
     crate::std_instead_of_core::ALLOC_INSTEAD_OF_CORE_INFO,
     crate::std_instead_of_core::STD_INSTEAD_OF_ALLOC_INFO,
     crate::std_instead_of_core::STD_INSTEAD_OF_CORE_INFO,

clippy_lints/src/lib.rs

@@ -326,6 +326,7 @@ mod single_range_in_vec_init;
 mod size_of_in_element_count;
 mod size_of_ref;
 mod slow_vector_initialization;
+mod stacked_ifs;
 mod std_instead_of_core;
 mod strings;
 mod strlen_on_c_strings;
@@ -1129,6 +1130,7 @@ pub fn register_lints(store: &mut rustc_lint::LintStore, conf: &'static Conf) {
     store.register_late_pass(|_| Box::new(zero_repeat_side_effects::ZeroRepeatSideEffects));
     store.register_late_pass(|_| Box::new(manual_unwrap_or_default::ManualUnwrapOrDefault));
     store.register_late_pass(|_| Box::new(integer_division_remainder_used::IntegerDivisionRemainderUsed));
+    store.register_late_pass(|_| Box::new(stacked_ifs::StackedIfs));
     // add lints here, do not remove this comment, it's used in `new_lint`
 }
 

clippy_lints/src/stacked_ifs.rs

@@ -0,0 +1,94 @@
+use clippy_utils::diagnostics::span_lint_and_help;
+use rustc_hir::{Expr, ExprKind};
+use rustc_lint::{LateContext, LateLintPass};
+use rustc_session::declare_lint_pass;
+
+declare_clippy_lint! {
+    /// ### What it does
+    /// Detects `if` statements where the condition is another `if` statement.
+    /// ### Why is this bad?
+    /// This makes code hard to read.
+    /// ### Example
+    /// ```no_run
+    /// let a = 3;
+    /// let b = 4;
+    /// let c = 5;
+    /// if if a == b {
+    ///     4
+    /// } else {
+    ///     5
+    /// } == c {
+    ///     // Do something.
+    /// }
+    /// ```
+    /// Use instead:
+    /// ```no_run
+    /// let a = 3;
+    /// let b = 4;
+    /// let c = 5;
+    /// let value = if a == b {
+    ///     4
+    /// } else {
+    ///     5
+    /// };
+    /// if value == c {
+    ///     // Do something.
+    /// }
+    /// ```
+    #[clippy::version = "1.79.0"]
+    pub STACKED_IFS,
+    style,
+    "finds if statements with another if statement as the condition"
+}
+
+declare_lint_pass!(StackedIfs => [STACKED_IFS]);
+
+impl LateLintPass<'_> for StackedIfs {
+    fn check_expr(&mut self, cx: &LateContext<'_>, expr: &Expr<'_>) {
+        // Ensure that the expression isn't a part of a constant or macro expansion.
+        if !expr.span.from_expansion() {
+            stacked_ifs(cx, expr);
+        }
+    }
+}
+
+fn stacked_ifs(cx: &LateContext<'_>, expr: &Expr<'_>) {
+    // Check for if statements where the condition is another if statement.
+    let ExprKind::If(condition, _, _) = expr.kind else {
+        return;
+    };
+
+    let condition = condition.peel_drop_temps();
+
+    if condition.span.from_expansion() {
+        return;
+    }
+
+    if let ExprKind::If(..) = condition.kind {
+        emit_lint(cx, condition);
+    }
+
+    if let ExprKind::Binary(_, lhs, rhs) = condition.kind {
+        if let ExprKind::If(..) = lhs.kind
+            && !lhs.span.from_expansion()
+        {
+            emit_lint(cx, lhs);
+        }
+        if let ExprKind::If(..) = rhs.kind
+            && !rhs.span.from_expansion()
+        {
+            emit_lint(cx, rhs);
+        }
+    }
+}
+
+fn emit_lint(cx: &LateContext<'_>, expr: &Expr<'_>) {
+    span_lint_and_help(
+        cx,
+        STACKED_IFS,
+        expr.span,
+        "stacked `if` found",
+        None,
+        "avoid using an `if` statement as a condition for another `if` statement",
+    );
+}

tests/ui/stacked_ifs.rs

@@ -0,0 +1,92 @@
+#![warn(clippy::stacked_ifs)]
+#![allow(clippy::manual_unwrap_or_default)]
+
+macro_rules! m {
+    () => {
+        let a = true;
+        let b = false;
+        if if a { b } else { true } {
+            println!("True");
+        }
+    };
+}
+
+fn main() {
+    let a = 3;
+    let b = 4;
+    if a == b { // No error
+        // Do something.
+    } else {
+        // Do something else.
+    }
+
+    if if a % 2 == 0 {
+        //~^ ERROR: stacked `if` found
+        a - 1
+    } else {
+        a + 1
+    } == b
+    {
+        // Do something.
+    }
+
+    if if a % 2 == 0 {
+        //~^ ERROR: stacked `if` found
+        a - 1
+    } else {
+        a + 1
+    } == b
+    {
+        // Do something.
+    } else {
+        // Do something else.
+    }
+
+    if b == if a % 2 == 0 {
+        //~^ ERROR: stacked `if` found
+        a - 1
+    } else {
+        a + 1
+    } {
+        // Do something.
+    } else {
+        // Do something else.
+    }
+
+    let c = 5;
+    let d = 6;
+
+    // More complex statements (example from issue 12483):
+    if if if a == b {
+        //~^ ERROR: stacked `if` found
+        //~^^ ERROR: stacked `if` found
+        b == c
+    } else {
+        a == c
+    } {
+        a == d
+    } else {
+        c == d
+    } {
+        println!("True!");
+    } else {
+        println!("False!");
+    }
+
+    let a = true;
+    let b = false;
+
+    if if a { b } else { true } {
+        //~^ ERROR: stacked `if` found
+        // Do something.
+    }
+
+    let x = Some(10);
+
+    if if let Some(num) = x { num } else { 0 } == 0 {
+        //~^ ERROR: stacked `if` found
+        // Do something.
+    }
+
+    m!(); // No error
+}

tests/ui/stacked_ifs.stderr

@@ -0,0 +1,92 @@
+error: stacked `if` found
+  --> tests/ui/stacked_ifs.rs:23:8
+   |
+LL |       if if a % 2 == 0 {
+   |  ________^
+LL | |
+LL | |         a - 1
+LL | |     } else {
+LL | |         a + 1
+LL | |     } == b
+   | |_____^
+   |
+   = help: avoid using an `if` statement as a condition for another `if` statement
+   = note: `-D clippy::stacked-ifs` implied by `-D warnings`
+   = help: to override `-D warnings` add `#[allow(clippy::stacked_ifs)]`
+
+error: stacked `if` found
+  --> tests/ui/stacked_ifs.rs:33:8
+   |
+LL |       if if a % 2 == 0 {
+   |  ________^
+LL | |
+LL | |         a - 1
+LL | |     } else {
+LL | |         a + 1
+LL | |     } == b
+   | |_____^
+   |
+   = help: avoid using an `if` statement as a condition for another `if` statement
+
+error: stacked `if` found
+  --> tests/ui/stacked_ifs.rs:45:13
+   |
+LL |       if b == if a % 2 == 0 {
+   |  _____________^
+LL | |
+LL | |         a - 1
+LL | |     } else {
+LL | |         a + 1
+LL | |     } {
+   | |_____^
+   |
+   = help: avoid using an `if` statement as a condition for another `if` statement
+
+error: stacked `if` found
+  --> tests/ui/stacked_ifs.rs:60:8
+   |
+LL |       if if if a == b {
+   |  ________^
+LL | |
+LL | |
+LL | |         b == c
+...  |
+LL | |         c == d
+LL | |     } {
+   | |_____^
+   |
+   = help: avoid using an `if` statement as a condition for another `if` statement
+
+error: stacked `if` found
+  --> tests/ui/stacked_ifs.rs:60:11
+   |
+LL |       if if if a == b {
+   |  ___________^
+LL | |
+LL | |
+LL | |         b == c
+LL | |     } else {
+LL | |         a == c
+LL | |     } {
+   | |_____^
+   |
+   = help: avoid using an `if` statement as a condition for another `if` statement
+
+error: stacked `if` found
+  --> tests/ui/stacked_ifs.rs:79:8
+   |
+LL |     if if a { b } else { true } {
+   |        ^^^^^^^^^^^^^^^^^^^^^^^^
+   |
+   = help: avoid using an `if` statement as a condition for another `if` statement
+
+error: stacked `if` found
+  --> tests/ui/stacked_ifs.rs:86:8
+   |
+LL |     if if let Some(num) = x { num } else { 0 } == 0 {
+   |        ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
+   |
+   = help: avoid using an `if` statement as a condition for another `if` statement
+
+error: aborting due to 7 previous errors
+