Add a USELESS_LET_IF_SEQ lint

Author: mcarton

README.md

@@ -14,7 +14,7 @@ Table of contents:
 * [License](#license)
 
 ##Lints
-There are 139 lints included in this crate:
+There are 140 lints included in this crate:
 
 name                                                                                                                 | default | meaning
 ---------------------------------------------------------------------------------------------------------------------|---------|------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------
@@ -149,6 +149,7 @@ name
 [use_debug](https://github.com/Manishearth/rust-clippy/wiki#use_debug)                                               | allow   | use `Debug`-based formatting
 [used_underscore_binding](https://github.com/Manishearth/rust-clippy/wiki#used_underscore_binding)                   | warn    | using a binding which is prefixed with an underscore
 [useless_format](https://github.com/Manishearth/rust-clippy/wiki#useless_format)                                     | warn    | useless use of `format!`
+[useless_let_if_seq](https://github.com/Manishearth/rust-clippy/wiki#useless_let_if_seq)                             | warn    | TODO
 [useless_transmute](https://github.com/Manishearth/rust-clippy/wiki#useless_transmute)                               | warn    | transmutes that have the same to and from types
 [useless_vec](https://github.com/Manishearth/rust-clippy/wiki#useless_vec)                                           | warn    | useless `vec!`
 [while_let_loop](https://github.com/Manishearth/rust-clippy/wiki#while_let_loop)                                     | warn    | `loop { if let { ... } else break }` can be written as a `while let` loop

src/let_if_seq.rs

@@ -0,0 +1,141 @@
+use rustc::lint::*;
+use rustc_front::hir::*;
+use syntax::codemap;
+use utils::{snippet, span_lint_and_then};
+
+/// **What it does:** This lint checks for variable declarations immediatly followed by a
+/// conditionnal affectation.
+///
+/// **Why is this bad?** This is not idiomatic Rust.
+///
+/// **Known problems:** None.
+///
+/// **Example:** 
+/// ```rust,ignore
+/// let foo;
+///
+/// if bar() {
+///     foo = 42;
+/// } else {
+///     foo = 0;
+/// }
+///
+/// let mut baz = None;
+///
+/// if bar() {
+///     baz = Some(42);
+/// }
+/// ```
+///
+/// should be writen
+///
+/// ```rust,ignore
+/// let foo = if bar() {
+///     42;
+/// } else {
+///     0;
+/// };
+///
+/// let baz = if bar() {
+///     Some(42);
+/// } else {
+///     None
+/// };
+/// ```
+declare_lint! {
+    pub USELESS_LET_IF_SEQ,
+    Warn,
+    "TODO"
+}
+
+#[derive(Copy,Clone)]
+pub struct LetIfSeq;
+
+impl LintPass for LetIfSeq {
+    fn get_lints(&self) -> LintArray {
+        lint_array!(USELESS_LET_IF_SEQ)
+    }
+}
+
+impl LateLintPass for LetIfSeq {
+    fn check_block(&mut self, cx: &LateContext, block: &Block) {
+        let mut it = block.stmts.iter().peekable();
+        while let Some(ref stmt) = it.next() {
+            if_let_chain! {[
+                let StmtDecl(ref decl, _) = stmt.node,
+                let DeclLocal(ref decl) = decl.node,
+                let PatKind::Ident(mode, ref name, None) = decl.pat.node,
+                let Some(expr) = it.peek(),
+                let StmtExpr(ref if_, _) = expr.node,
+                let ExprIf(ref cond, ref then, ref else_) = if_.node,
+                let Some(value) = check_assign(name.node, then),
+            ], {
+                let span = codemap::mk_sp(stmt.span.lo, if_.span.hi);
+
+                let default = if let Some(ref else_) = *else_ {
+                    if let ExprBlock(ref else_) = else_.node {
+                        if let Some(default) = check_assign(name.node, else_) {
+                            default
+                        } else if let Some(ref default) = decl.init {
+                            &*default
+                        } else {
+                            return;
+                        }
+                    } else {
+                        return;
+                    }
+                } else if let Some(ref default) = decl.init {
+                    &*default
+                } else {
+                    return;
+                };
+
+                let mutability = match mode {
+                    BindByRef(MutMutable) | BindByValue(MutMutable) => "<mut> ",
+                    _ => "",
+                };
+
+                // FIXME: this should not suggest `mut` if we can detect that the variable is not
+                // use mutably after the `if`
+
+                let sug = format!(
+                    "let {mut}{name} = if {cond} {{{then} {value} }} else {{{else} {default} }};",
+                    mut=mutability,
+                    name=name.node.name,
+                    cond=snippet(cx, cond.span, "_"),
+                    then=if then.stmts.len() > 1 { " ..;" } else { "" },
+                    else=if else_.is_some() { " ..;" } else { "" },
+                    value=snippet(cx, value.span, "<value>"),
+                    default=snippet(cx, default.span, "<default>"),
+                );
+                span_lint_and_then(cx,
+                                   USELESS_LET_IF_SEQ,
+                                   span,
+                                   "`let foo;`/`if .. { foo =  ..; }` sequence detected",
+                                   |db| {
+                                       db.span_suggestion(span,
+                                                          "it is more idiomatic to write",
+                                                          sug);
+                                       if !mutability.is_empty() {
+                                           db.note("you might not need `mut` at all");
+                                       }
+                                   });
+            }}
+        }
+    }
+}
+
+fn check_assign(name: Ident, block: &Block) -> Option<&Expr> {
+    if_let_chain! {[
+        let Some(expr) = block.stmts.iter().last(),
+        let StmtSemi(ref expr, _) = expr.node,
+        let ExprAssign(ref var, ref value) = expr.node,
+        let ExprPath(None, ref path) = var.node,
+        path.segments.len() == 1,
+        name.name.as_str() == path.segments[0].identifier.name.as_str(),
+    ], {
+        return Some(value);
+    }}
+
+    None
+}

src/lib.rs

@@ -76,6 +76,7 @@ pub mod identity_op;
 pub mod if_not_else;
 pub mod items_after_statements;
 pub mod len_zero;
+pub mod let_if_seq;
 pub mod lifetimes;
 pub mod loops;
 pub mod map_clone;
@@ -232,6 +233,7 @@ pub fn plugin_registrar(reg: &mut Registry) {
     reg.register_late_lint_pass(box blacklisted_name::BlackListedName::new(conf.blacklisted_names));
     reg.register_late_lint_pass(box functions::Functions::new(conf.too_many_arguments_threshold));
     reg.register_early_lint_pass(box doc::Doc);
+    reg.register_late_lint_pass(box let_if_seq::LetIfSeq);
 
     reg.register_lint_group("clippy_pedantic", vec![
         array_indexing::INDEXING_SLICING,
@@ -293,6 +295,7 @@ pub fn plugin_registrar(reg: &mut Registry) {
         items_after_statements::ITEMS_AFTER_STATEMENTS,
         len_zero::LEN_WITHOUT_IS_EMPTY,
         len_zero::LEN_ZERO,
+        let_if_seq::USELESS_LET_IF_SEQ,
         lifetimes::NEEDLESS_LIFETIMES,
         lifetimes::UNUSED_LIFETIMES,
         loops::EMPTY_LOOP,

tests/compile-fail/let_if_seq.rs

@@ -0,0 +1,51 @@
+#![feature(plugin)]
+#![plugin(clippy)]
+
+#![allow(unused_variables, unused_assignments, similar_names, blacklisted_name)]
+#![deny(useless_let_if_seq)]
+
+fn f() -> bool { true }
+
+fn main() {
+    let mut foo = 0;
+    //~^ ERROR `let foo;`/`if .. { foo =  ..; }` sequence detected
+    //~| HELP more idiomatic
+    //~| SUGGESTION let mut foo = if f() { 42 } else { 0 };
+    if f() {
+        foo = 42;
+    }
+
+    let mut bar = 0;
+    //~^ ERROR `let foo;`/`if .. { foo =  ..; }` sequence detected
+    //~| HELP more idiomatic
+    //~| SUGGESTION let mut bar = if f() { ..; 42 } else { ..; 0 };
+    if f() {
+        f();
+        bar = 42;
+    }
+    else {
+        f();
+    }
+
+    let quz;
+    //~^ ERROR `let foo;`/`if .. { foo =  ..; }` sequence detected
+    //~| HELP more idiomatic
+    //~| SUGGESTION let mut quz = if f() { ..; 42 } else { ..; 0 };
+
+    if f() {
+        quz = 42;
+    } else {
+        quz = 0;
+    }
+
+    // baz needs to be mut
+    let mut baz = 0;
+    //~^ ERROR `let foo;`/`if .. { foo =  ..; }` sequence detected
+    //~| HELP more idiomatic
+    //~| SUGGESTION let mut baz = if f() { ..; 42 } else { ..; 0 };
+    if f() {
+        baz = 42;
+    }
+
+    baz = 1337;
+}