swiftlang/swift

[SR-1553] [QOI] Warn on discarded binding of () result

Open

#44,162 opened on May 18, 2016

View on GitHub
 (13 comments) (0 reactions) (0 assignees)Swift (69,989 stars) (10,719 forks)batch import
compilerdiagnostics qualitygood first issueimprovement

Description
Previous ID SR-1553
Radar None
Original Reporter @ddunbar
Type Improvement
Votes 0
Component/s Compiler
Labels Improvement, DiagnosticsQoI
Assignee zsalloum (JIRA)
Priority Medium

md5: a541dcf4281d920a0880df5d3eabc2fa

Issue Description:

Swift should warn on the code below:

$ cat x.swift
func f0() -> () {}
let _ = f0()

$ swiftc -c x.swift
$

This is somewhat related to SE-0047, in that if you use let _ = ... to discard results, and then later refactor the code to not return a result, Swift won't help you find the other places in the code base to update.

Contributor guide

Tech stack
swift
Domain
developer experiencetooling
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.
1-2 days
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 understanding of Swift compilerfamiliarity with diagnostic emission
Newbie friendlinessA 1-100 score estimating how approachable this issue is for first-time contributors.
55
Research direction
Examine the Swift compiler's handling of `let =` bindings, particularly in the type checker's treatment of discardable results. Look at files like `lib/Sema/TypeCheckStmt.cpp` for where `let` patterns are processed and where warnings about unused results are emitted. Consider adding a new diagnostic when the type of the expression is `()`. The existing behavior for ` =` might already warn; check if `let =` is treated differently. Review the discussion in SE 0047 for context.