New lint for struct update that has no effect

src/lib.rs

@@ -51,6 +51,7 @@ pub mod mutex_atomic;
 pub mod zero_div_zero;
 pub mod open_options;
 pub mod needless_features;
+pub mod needless_update;
 
 mod reexport {
     pub use syntax::ast::{Name, Ident, NodeId};
@@ -96,6 +97,7 @@ pub fn plugin_registrar(reg: &mut Registry) {
     reg.register_late_lint_pass(box zero_div_zero::ZeroDivZeroPass);
     reg.register_late_lint_pass(box mutex_atomic::MutexAtomic);
     reg.register_late_lint_pass(box needless_features::NeedlessFeaturesPass);
+    reg.register_late_lint_pass(box needless_update::NeedlessUpdatePass);
 
     reg.register_lint_group("clippy_pedantic", vec![
         methods::OPTION_UNWRAP_USED,
@@ -155,6 +157,7 @@ pub fn plugin_registrar(reg: &mut Registry) {
         needless_bool::NEEDLESS_BOOL,
         needless_features::UNSTABLE_AS_MUT_SLICE,
         needless_features::UNSTABLE_AS_SLICE,
+        needless_update::NEEDLESS_UPDATE,
         open_options::NONSENSICAL_OPEN_OPTIONS,
         precedence::PRECEDENCE,
         ptr_arg::PTR_ARG,

src/needless_update.rs

@@ -0,0 +1,35 @@
+use rustc::lint::{LateContext, LateLintPass, LintArray, LintPass};
+use rustc::middle::ty::TyStruct;
+use rustc_front::hir::{Expr, ExprStruct};
+
+use utils::span_lint;
+
+declare_lint! {
+    pub NEEDLESS_UPDATE,
+    Warn,
+    "using `{ ..base }` when there are no missing fields"
+}
+
+#[derive(Copy, Clone)]
+pub struct NeedlessUpdatePass;
+
+impl LintPass for NeedlessUpdatePass {
+    fn get_lints(&self) -> LintArray {
+        lint_array!(NEEDLESS_UPDATE)
+    }
+}
+
+impl LateLintPass for NeedlessUpdatePass {
+    fn check_expr(&mut self, cx: &LateContext, expr: &Expr) {
+        if let ExprStruct(_, ref fields, Some(ref base)) = expr.node {
+            let ty = cx.tcx.expr_ty(expr);
+            if let TyStruct(def, _) = ty.sty {
+                if fields.len() == def.struct_variant().fields.len() {
+                    span_lint(cx, NEEDLESS_UPDATE, base.span,
+                              "struct update has no effect, all the fields \
+                              in the struct have already been specified");
+                }
+            }
+        }
+    }
+}