new large_enum_variants lint

CHANGELOG.md

@@ -221,6 +221,7 @@ All notable changes to this project will be documented in this file.
 [`items_after_statements`]: https://github.com/Manishearth/rust-clippy/wiki#items_after_statements
 [`iter_next_loop`]: https://github.com/Manishearth/rust-clippy/wiki#iter_next_loop
 [`iter_nth`]: https://github.com/Manishearth/rust-clippy/wiki#iter_nth
+[`large_enum_variant`]: https://github.com/Manishearth/rust-clippy/wiki#large_enum_variant
 [`len_without_is_empty`]: https://github.com/Manishearth/rust-clippy/wiki#len_without_is_empty
 [`len_zero`]: https://github.com/Manishearth/rust-clippy/wiki#len_zero
 [`let_and_return`]: https://github.com/Manishearth/rust-clippy/wiki#let_and_return

README.md

@@ -17,7 +17,7 @@ Table of contents:
 
 ## Lints
 
-There are 167 lints included in this crate:
+There are 168 lints included in this crate:
 
 name                                                                                                                 | default | triggers on
 ---------------------------------------------------------------------------------------------------------------------|---------|----------------------------------------------------------------------------------------------------------------------------------
@@ -83,6 +83,7 @@ name
 [items_after_statements](https://github.com/Manishearth/rust-clippy/wiki#items_after_statements)                     | allow   | blocks where an item comes after a statement
 [iter_next_loop](https://github.com/Manishearth/rust-clippy/wiki#iter_next_loop)                                     | warn    | for-looping over `_.next()` which is probably not intended
 [iter_nth](https://github.com/Manishearth/rust-clippy/wiki#iter_nth)                                                 | warn    | using `.iter().nth()` on a standard library type with O(1) element access
+[large_enum_variant](https://github.com/Manishearth/rust-clippy/wiki#large_enum_variant)                             | warn    | large variants on an enum
 [len_without_is_empty](https://github.com/Manishearth/rust-clippy/wiki#len_without_is_empty)                         | warn    | traits and impls that have `.len()` but not `.is_empty()`
 [len_zero](https://github.com/Manishearth/rust-clippy/wiki#len_zero)                                                 | warn    | checking `.len() == 0` or `.len() > 0` (or similar) when `.is_empty()` could be used instead
 [let_and_return](https://github.com/Manishearth/rust-clippy/wiki#let_and_return)                                     | warn    | creating a let-binding and then immediately returning it like `let x = expr; x` at the end of a block

clippy_lints/src/large_enum_variant.rs

@@ -0,0 +1,71 @@
+//! lint when there are large variants on an enum
+
+use rustc::lint::*;
+use rustc::hir::*;
+use utils::span_help_and_lint;
+use rustc::ty::layout::TargetDataLayout;
+
+/// **What it does:** Checks for large variants on enums.
+///
+/// **Why is this bad?** Enum size is bounded by the largest variant. Having a large variant
+/// can penalize the memory layout of that enum.
+///
+/// **Known problems:** None.
+///
+/// **Example:**
+/// ```rust
+/// enum Test {
+///    A(i32),
+///    B([i32; 8000]),
+/// }
+/// ```
+declare_lint! {
+    pub LARGE_ENUM_VARIANT,
+    Warn,
+    "large variants on an enum"
+}
+
+#[derive(Copy,Clone)]
+pub struct LargeEnumVariant {
+    maximum_variant_size_allowed: u64,
+}
+
+impl LargeEnumVariant {
+    pub fn new(maximum_variant_size_allowed: u64) -> Self {
+        LargeEnumVariant {
+            maximum_variant_size_allowed: maximum_variant_size_allowed,
+        }
+    }
+}
+
+impl LintPass for LargeEnumVariant {
+    fn get_lints(&self) -> LintArray {
+        lint_array!(LARGE_ENUM_VARIANT)
+    }
+}
+
+impl LateLintPass for LargeEnumVariant {
+    fn check_variant(&mut self, cx: &LateContext, variant: &Variant, _ :&Generics) {
+        let data_layout = TargetDataLayout::parse(cx.sess());
+        let param_env = cx.tcx.empty_parameter_environment();
+        let infcx = cx.tcx.borrowck_fake_infer_ctxt(param_env);
+        let mut variant_size = 0;
+
+        for field in variant.node.data.fields() {
+            let ty = cx.tcx.node_id_to_type(field.id);
+            if let Ok(layout) = ty.layout(&infcx) {
+                variant_size += layout.size(&data_layout).bytes();
+            }
+        }
+
+        if variant_size > self.maximum_variant_size_allowed {
+            span_help_and_lint(
+                cx,
+                LARGE_ENUM_VARIANT,
+                variant.span,
+                &format!("large enum variant found on variant `{}`", variant.node.name),
+                "consider boxing the large branches to reduce the total size of the enum",
+            );
+        }
+    }
+}

clippy_lints/src/lib.rs

@@ -85,6 +85,7 @@ pub mod functions;
 pub mod identity_op;
 pub mod if_not_else;
 pub mod items_after_statements;
+pub mod large_enum_variant;
 pub mod len_zero;
 pub mod let_if_seq;
 pub mod lifetimes;
@@ -260,6 +261,7 @@ pub fn register_plugins(reg: &mut rustc_plugin::Registry) {
     reg.register_late_lint_pass(box assign_ops::AssignOps);
     reg.register_late_lint_pass(box let_if_seq::LetIfSeq);
     reg.register_late_lint_pass(box eval_order_dependence::EvalOrderDependence);
+    reg.register_late_lint_pass(box large_enum_variant::LargeEnumVariant::new(conf.enum_variant_size_threshold));
 
     reg.register_lint_group("clippy_restrictions", vec![
         arithmetic::FLOAT_ARITHMETIC,
@@ -337,6 +339,7 @@ pub fn register_plugins(reg: &mut rustc_plugin::Registry) {
         functions::NOT_UNSAFE_PTR_ARG_DEREF,
         functions::TOO_MANY_ARGUMENTS,
         identity_op::IDENTITY_OP,
+        large_enum_variant::LARGE_ENUM_VARIANT,
         len_zero::LEN_WITHOUT_IS_EMPTY,
         len_zero::LEN_ZERO,
         let_if_seq::USELESS_LET_IF_SEQ,

clippy_lints/src/utils/conf.rs

@@ -162,6 +162,8 @@ define_Conf! {
     ("too-large-for-stack", too_large_for_stack, 200 => u64),
     /// Lint: ENUM_VARIANT_NAMES. The minimum number of enum variants for the lints about variant names to trigger
     ("enum-variant-name-threshold", enum_variant_name_threshold, 3 => u64),
+    /// Lint: LARGE_ENUM_VARIANT. The maximum size of a emum's variant to avoid box suggestion
+    ("enum-variant-size-threshold", enum_variant_size_threshold, 200 => u64),
 }
 
 /// Read the `toml` configuration file. The function will ignore “File not found” errors iif

tests/compile-fail/large_enum_variant.rs

@@ -0,0 +1,24 @@
+#![feature(plugin)]
+#![plugin(clippy)]
+
+#![allow(dead_code)]
+#![allow(unused_variables)]
+#![deny(large_enum_variant)]
+
+enum LargeEnum {
+    A(i32),
+    B([i32; 8000]), //~ ERROR large enum variant found on variant `B`
+}
+
+enum AnotherLargeEnum {
+    VariantOk(i32, u32),
+    ContainingLargeEnum(LargeEnum), //~ ERROR large enum variant found on variant `ContainingLargeEnum`
+    ContainingMoreThanOneField(i32, [i32; 8000], [i32; 9500]), //~ ERROR large enum variant found on variant `ContainingMoreThanOneField`
+    VoidVariant,
+    StructLikeLittle { x: i32, y: i32 },
+    StructLikeLarge { x: [i32; 8000], y: i32 }, //~ ERROR large enum variant found on variant `StructLikeLarge`
+}
+
+fn main() {
+
+}