Add lint for ifs that could be collapsed

"Collapsible" ifs are ones which contain only a then block, and the then
block consists of an if that only has a then block.

src/collapsible_if.rs

@@ -0,0 +1,80 @@
+//! Checks for if expressions that contain only an if expression.
+//!
+//! For example, the lint would catch:
+//!
+//! ```
+//! if x {
+//!     if y {
+//!         println!("Hello world");
+//!     }
+//! }
+//! ```
+//!
+//! This lint is **warn** by default
+
+use rustc::plugin::Registry;
+use rustc::lint::*;
+use rustc::middle::def::*;
+use syntax::ast::*;
+use syntax::ptr::P;
+use syntax::codemap::{Span, Spanned};
+use syntax::print::pprust::expr_to_string;
+
+declare_lint! {
+    pub COLLAPSIBLE_IF,
+    Warn,
+    "Warn on if expressions that can be collapsed"
+}
+
+#[derive(Copy,Clone)]
+pub struct CollapsibleIf;
+
+impl LintPass for CollapsibleIf {
+    fn get_lints(&self) -> LintArray {
+        lint_array!(COLLAPSIBLE_IF)
+    }
+
+    fn check_expr(&mut self, cx: &Context, e: &Expr) {
+        if let ExprIf(ref check, ref then_block, None) = e.node {
+            let expr = check_block(then_block);
+            let expr = match expr {
+                Some(e) => e,
+                None => return
+            };
+            if let ExprIf(ref check_inner, _, None) = expr.node {
+                let (check, check_inner) = (check_to_string(check), check_to_string(check_inner));
+                cx.span_lint(COLLAPSIBLE_IF, e.span,
+                             &format!("This if statement can be collapsed. Try: if {} && {}", check, check_inner));
+            }
+		    }
+    }
+}
+
+fn requires_brackets(e: &Expr) -> bool {
+    match e.node {
+        ExprBinary(Spanned {node: n, ..}, _, _) if n == BiEq => false,
+        _ => true
+    }
+}
+
+fn check_to_string(e: &Expr) -> String {
+    if requires_brackets(e) {
+        format!("({})", expr_to_string(e))
+    } else {
+        format!("{}", expr_to_string(e))
+    }
+}
+
+fn check_block(b: &Block) -> Option<&P<Expr>> {
+    if b.stmts.len() == 1 && b.expr.is_none() {
+        let stmt = &b.stmts[0];
+        return match stmt.node {
+            StmtExpr(ref e, _) => Some(e),
+            _ => None
+        };
+    }
+    if let Some(ref e) = b.expr {
+        return Some(e);
+    }
+    None
+}

src/lib.rs

@@ -25,6 +25,7 @@ pub mod eta_reduction;
 pub mod identity_op;
 pub mod mut_mut;
 pub mod len_zero;
+pub mod collapsible_if;
 
 #[plugin_registrar]
 pub fn plugin_registrar(reg: &mut Registry) {
@@ -45,6 +46,7 @@ pub fn plugin_registrar(reg: &mut Registry) {
     reg.register_lint_pass(box mut_mut::MutMut as LintPassObject);
     reg.register_lint_pass(box len_zero::LenZero as LintPassObject);
     reg.register_lint_pass(box misc::CmpOwned as LintPassObject);
+    reg.register_lint_pass(box collapsible_if::CollapsibleIf as LintPassObject);
 
     reg.register_lint_group("clippy", vec![types::BOX_VEC, types::LINKEDLIST,
                                            misc::SINGLE_MATCH, misc::STR_TO_STRING,
@@ -61,5 +63,6 @@ pub fn plugin_registrar(reg: &mut Registry) {
                                            mut_mut::MUT_MUT,
                                            len_zero::LEN_ZERO,
                                            len_zero::LEN_WITHOUT_IS_EMPTY,
+                                           collapsible_if::COLLAPSIBLE_IF,
                                            ]);
 }

tests/compile-fail/collapsible_if.rs

@@ -0,0 +1,37 @@
+#![feature(plugin)]
+#![plugin(clippy)]
+
+#[deny(collapsible_if)]
+fn main() {
+    let x = "hello";
+    let y = "world";
+    if x == "hello" { //~ERROR This if statement can be collapsed
+        if y == "world" {
+            println!("Hello world!");
+        }
+    }
+
+    if x == "hello" || x == "world" { //~ERROR This if statement can be collapsed
+        if y == "world" || y == "hello" {
+            println!("Hello world!");
+        }
+    }
+
+    // Works because any if with an else statement cannot be collapsed.
+    if x == "hello" {
+        if y == "world" {
+            println!("Hello world!");
+        }
+    } else {
+        println!("Not Hello world");
+    }
+
+    if x == "hello" {
+        if y == "world" {
+            println!("Hello world!");
+        } else {
+            println!("Hello something else");
+        }
+    }
+
+}