Add a USELESS_LET_IF_SEQ lint

tests/compile-fail/let_if_seq.rs

@@ -0,0 +1,79 @@
+#![feature(plugin)]
+#![plugin(clippy)]
+
+#![allow(unused_variables, unused_assignments, similar_names, blacklisted_name)]
+#![deny(useless_let_if_seq)]
+
+fn f() -> bool { true }
+
+fn early_return() -> u8 {
+    // FIXME: we could extend the lint to include such cases:
+    let foo;
+
+    if f() {
+        return 42;
+    } else {
+        foo = 0;
+    }
+
+    foo
+}
+
+fn main() {
+    early_return();
+
+    let mut foo = 0;
+    //~^ ERROR `if _ { .. } else { .. }` is an expression
+    //~| HELP more idiomatic
+    //~| SUGGESTION let <mut> foo = if f() { 42 } else { 0 };
+    if f() {
+        foo = 42;
+    }
+
+    let mut bar = 0;
+    //~^ ERROR `if _ { .. } else { .. }` is an expression
+    //~| HELP more idiomatic
+    //~| SUGGESTION let <mut> bar = if f() { ..; 42 } else { ..; 0 };
+    if f() {
+        f();
+        bar = 42;
+    }
+    else {
+        f();
+    }
+
+    let quz;
+    //~^ ERROR `if _ { .. } else { .. }` is an expression
+    //~| HELP more idiomatic
+    //~| SUGGESTION let quz = if f() { 42 } else { 0 };
+
+    if f() {
+        quz = 42;
+    } else {
+        quz = 0;
+    }
+
+    // `toto` is used several times
+    let mut toto;
+
+    if f() {
+        toto = 42;
+    } else {
+        for i in &[1, 2] {
+            toto = *i;
+        }
+
+        toto = 2;
+    }
+
+    // baz needs to be mut
+    let mut baz = 0;
+    //~^ ERROR `if _ { .. } else { .. }` is an expression
+    //~| HELP more idiomatic
+    //~| SUGGESTION let <mut> baz = if f() { 42 } else { 0 };
+    if f() {
+        baz = 42;
+    }
+
+    baz = 1337;
+}