rust-lang/rust-clippy

large_enum_variant triggered in error-chain macro has unhelpful help message

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#2121 opened on Oct 9, 2017

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C-enhancementE-mediumT-middlegood first issue

Description

You know how I like to suggest E-hard new lints? Sorry, this time I only have an E-hard L-enhancement to report.

warning: large size difference between variants
  --> src/main.rs:66:1
   |
66 | / error_chain! {
67 | |     foreign_links {
68 | |         Io(::std::io::Error);
69 | |         Handlebars(::handlebars::TemplateRenderError);
70 | |         Yaml(::yaml::Error);
71 | |     }
72 | | }
   | |_^
   |
   = note: #[warn(large_enum_variant)] on by default
   = help: for further information visit https://rust-lang-nursery.github.io/rust-clippy/v0.0.165/index.html#large_enum_variant
help: consider boxing the large fields to reduce the total size of the enum
   |
113| err : Box<$ foreign_link_error_path> ) {
   |       ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
   = note: this error originates in a macro outside of the current crate
   = issue author's remark: this help message is not helpful

Is there a way to (easily) print the enum variants with their sizes? This might be helpful even in rather obvious cases.

Contributor guide

Tech stack
rust
Domain
toolingdeveloper experience
Issue type
feature
DifficultyEstimated implementation difficulty for a new contributor, from 1 for very small changes to 5 for expert-level work.
3
Estimated timeA rough time range for an experienced contributor to investigate, implement, test, and prepare a pull request.
1-3 hours
Activity statusHow available the issue appears right now: fresh, active, stale, blocked, or waiting on maintainer input.
stale
ClarityHow clearly the issue explains the expected change, acceptance criteria, and next step.
clear
Prerequisites
Basic RustFamiliarity with clippy internals
Newbie friendlinessA 1-100 score estimating how approachable this issue is for first-time contributors.
40
Research direction
First, locate the implementation of the `large enum variant` lint in the clippy codebase. The help message currently suggests boxing large fields but does not display the sizes of variants. The issue is to modify the lint to print the size of each variant in the diagnostic, perhaps using `std::mem::size of` or similar. Look at the lint's source file (likely `clippy lints/src/large enum variant.rs`) and the diagnostic emission code. Consider how to gather and display the variant sizes without breaking existing behavior.