Update PureAttribute documentation to mention CA1806 analyzer enforcement

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Summary

This PR updates the documentation for System.Diagnostics.Contracts.PureAttribute to clarify that the CA1806 code analyzer enforces this attribute.

Changes

The previous documentation stated:

This attribute is not enforced by the current analysis tools; you should use this attribute only if you are sure that the methods are pure.

This statement is outdated. The CA1806 analyzer does check for the [Pure] attribute and emits diagnostics when methods marked as pure have their return values ignored.

The updated documentation now states:

The CA1806 code analyzer checks for this attribute and emits a diagnostic when the return value of a pure method isn't used.

Context

When a method is marked with [Pure], CA1806 emits the following diagnostic if the return value isn't used:

{0} calls {1} but does not use the value the method returns. Because {1} is marked as a Pure method, it cannot have side effects. Use the result in a conditional statement, assign the result to a variable, or pass it as an argument to another method.

This change helps developers understand that applying the [Pure] attribute has practical effects in modern code analysis tooling.

Fixes #(issue number from dotnet/docs repository)

Original prompt

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This section details on the original issue you should resolve

<issue_title>Mention this analyzer also checks for the [Pure] attribute</issue_title>
<issue_description>### Type of issue

Typo

Description

CA1806 emits the following diagnostic for [Pure] methods:

{0} calls {1} but does not use the value the method returns. Because {1} is marked as a Pure method, it cannot have side effects. Use the result in a conditional statement, assign the result to a variable, or pass it as an argument to another method.

It would also be useful to mention this in the PureAttribute docs, since it currently says:

This attribute is not enforced by the current analysis tools; you should use this attribute only if you are sure that the methods are pure.

https://learn.microsoft.com/en-us/dotnet/api/system.diagnostics.contracts.pureattribute

Page URL

https://learn.microsoft.com/en-us/dotnet/fundamentals/code-analysis/quality-rules/ca1806

Content source URL

https://github.com/dotnet/docs/blob/main/docs/fundamentals/code-analysis/quality-rules/ca1806.md

Document Version Independent Id

59d0b85d-87f3-c4e4-6891-3002e2f60158

Platform Id

a91b8153-7420-d312-601a-b29f874cf8ae

Article author

@gewarren

Metadata

• ID: 0f51f0a0-29e7-be15-ffdc-ee210cf7ef1c
• PlatformId: a91b8153-7420-d312-601a-b29f874cf8ae
• Service: dotnet-fundamentals

Related Issues</issue_description>

Comments on the Issue (you are @copilot in this section)

Fixes dotnet/docs#48767

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Pull Request Overview

This PR updates the documentation for System.Diagnostics.Contracts.PureAttribute to reflect that the CA1806 code analyzer actively enforces this attribute by emitting diagnostics when return values of pure methods are ignored.

Key Changes:

• Removed outdated statement claiming the attribute is not enforced by analysis tools
• Added reference to CA1806 analyzer and its diagnostic behavior for [Pure] methods

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Tagging subscribers to this area: @tommcdon