Add label to lint for lifetimes used once

[estebank]

error: lifetime parameter `'a` only used once
  --> $DIR/fn-types.rs:19:10
   |
LL |   a: for<'a> fn(&'a u32), //~ ERROR `'a` only used once
   |          ^^      -- ...is used only here
   |          |
   |          this lifetime...

[rust-highfive]

r? @eddyb

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[eddyb]

r? @nikomatsakis

[oli-obk]

r? @oli-obk

Do you think it's worth making the removal a suggestion? It can get a little messy if the generics end up being empty afterwards, But since this is already an error, moving to another error seems fine, as long as the other error also has a suggestion

[estebank]

@oli-obk I was thinking of doing that as a follow up, as getting the right span for the suggestion requires dealing with leading and trailing commas, as well as the empty <> case. We also have to account for the total lifetime count introduced by the scope signature to suggest '_ only when appropriate.

[oli-obk]

@bors r+

Sgtm

[bors]

📌 Commit 973baaa has been approved by oli-obk

[bors]

⌛ Testing commit 973baaa with merge f968633...

[bors]

☀️ Test successful - status-appveyor, status-travis
Approved by: oli-obk
Pushing f968633 to master...