Warn on duplicate #suppress directives on the same node

[Copilot]

Duplicate #suppress directives for the same diagnostic on a single node were silently accepted. This change adds a compiler warning so redundant suppressions are surfaced whether or not they include a message.

• Compiler behavior

  • detect repeated #suppress "<code>" directives attached to the same node
  • emit a new duplicate-suppression warning on the later directive
  • preserve existing suppression behavior for the first matching directive

• Diagnostic definition

  • add a dedicated warning message for duplicate suppressions:
    • Diagnostic "<code>" is already suppressed on this node.

• Coverage

  • add focused tests for duplicate suppressions:
    • with a suppression message
    • without a suppression message

model Test {
  #suppress "deprecated" "Msg"
  #suppress "deprecated" "Msg"
  prop: Dep;
}

This now reports a duplicate-suppression warning on the second directive.

[pkg-pr-new]

Open in StackBlitz

npm i https://pkg.pr.new/@typespec/compiler@11113

commit: c34f798

[timotheeguerin]

@copilto add changelog

[timotheeguerin]

@copilot add changelog

[Copilot]

@copilot add changelog

Addressed in c34f798.

[azure-sdk-automation]

You can try these changes here

🛝 Playground	🌐 Website	🛝 VSCode Extension

[markcowl]

Looks good, need a PR to remove all the duplicate suppressions, which we should be able to do without this update