add new lint: stacked_if_match

Author: promptkp

CHANGELOG.md

@@ -5872,6 +5872,7 @@ 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_if_match`]: https://rust-lang.github.io/rust-clippy/master/index.html#stacked_if_match
 [`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

@@ -656,6 +656,7 @@ pub 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_if_match::STACKED_IF_MATCH_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

@@ -331,6 +331,7 @@ mod single_range_in_vec_init;
 mod size_of_in_element_count;
 mod size_of_ref;
 mod slow_vector_initialization;
+mod stacked_if_match;
 mod std_instead_of_core;
 mod string_patterns;
 mod strings;
@@ -933,5 +934,6 @@ pub fn register_lints(store: &mut rustc_lint::LintStore, conf: &'static Conf) {
     store.register_late_pass(|_| Box::new(set_contains_or_insert::SetContainsOrInsert));
     store.register_early_pass(|| Box::new(byte_char_slices::ByteCharSlice));
     store.register_early_pass(|| Box::new(cfg_not_test::CfgNotTest));
+    store.register_late_pass(|_| Box::new(stacked_if_match::StackedIfMatch));
     // add lints here, do not remove this comment, it's used in `new_lint`
 }

clippy_lints/src/stacked_if_match.rs

@@ -0,0 +1,51 @@
+use rustc_hir::*;
+use rustc_lint::{LateContext, LateLintPass};
+use rustc_session::declare_lint_pass;
+
+declare_clippy_lint! {
+    /// ### What it does
+    /// Checks for stacked `if` and `match`, e.g., `if if`.
+    ///
+    /// ### Why is this bad?
+    /// Stacked `if`'s and `match`'s are hard to read.
+    ///
+    /// ### Example
+    /// ```no_run
+    /// if if a == b {
+    ///     c == d
+    /// } else {
+    ///     e == f
+    /// } {
+    ///     println!("true");
+    /// } else {
+    ///     println!("false");
+    /// }
+    /// ```
+    ///
+    /// Use instead:
+    /// ```no_run
+    /// let cond = if a == b {
+    ///     c == d
+    /// } else {
+    ///     e == f
+    /// };
+    ///
+    /// if cond {
+    ///     println!("true");
+    /// } else {
+    ///     println!("false");
+    /// }
+    /// ```
+    #[clippy::version = "1.82.0"]
+    pub STACKED_IF_MATCH,
+    style,
+    "`if if` and `match match` that can be eliminated"
+}
+
+declare_lint_pass!(StackedIfMatch => [STACKED_IF_MATCH]);
+
+impl<'tcx> LateLintPass<'tcx> for StackedIfMatch {
+   fn check_expr(&mut self, cx: &LateContext<'tcx>, expr: &'tcx Expr<'_>) {
+
+    }
+}

tests/ui/stacked_if_match.rs

@@ -0,0 +1,5 @@
+#![warn(clippy::stacked_if_match)]
+
+fn main() {
+    // test code goes here
+}