rust-lang/rust-clippy

transmute_ptr_to_ref suggestion doesn't with some outer expression types

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#1754 opened on May 12, 2017

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C-bugT-middlegood first issue

Description

clippy suggests the following replacement

-                mem::transmute::<*mut c_void, &mut Box<FnMut(&Button)>>(data)(&button)
+                &mut *(data as *mut Box<FnMut(&Button)>)(&button)

but we should suggest parens around the &mut *...

bonus points if the parens only appear if necessary

Contributor guide

Tech stack
rust
Domain
tooling
Issue type
bug
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.
mostly clear
Prerequisites
Basic Rust knowledgeUnderstanding of clippy lint structure
Newbie friendlinessA 1-100 score estimating how approachable this issue is for first-time contributors.
40
Research direction
The issue describes a clippy lint `transmute ptr to ref` that incorrectly suggests replacing `mem::transmute::<*mut c void, &mut Box<FnMut(&Button)>>(data)(&button)` with `&mut *(data as *mut Box<FnMut(&Button)>)(&button)` without parentheses. The fix involves modifying the lint's suggestion code in the clippy repository (likely in `clippy lints/src/transmute.rs`) to wrap the `&mut *...` in parentheses when it appears in a context like a function call. The observation is that the suggestion is syntactically ambiguous without parens. Look for the `transmute ptr to ref` lint implementation, examine how the suggestion is generated (likely using `Applicability` and `span lint and sugg`), and add logic to detect if the expression is used as a callee (e.g., inside a function call) and then add parentheses around the suggested code. Test the change with the provided example and similar cases.