Suggest installing rust-src when standard library source is unavailable

[sladyn98]

Why

A lot of diagnostics point at a span inside the standard library e.g. required by a bound in std::iter::Iterator::sum``, or JoinHandle::join takes ownership of the receiver. For toolchains installed via `rustup` std is shipped with its paths remapped to `/rustc//library/...` and the source itself is not included unless the `rust-src` component is installed.

So when an error references std, rustc tries to render that span, can't find the source on disk, and falls back to printing just the bare remapped path with no code:

  error[E0277]: a value of type String cannot be made by summing an iterator ...
   --> src/main.rs:2:20
    ...
  note: required by a bound in std::iter::Iterator::sum
   --> /rustc/<hash>/library/core/src/iter/traits/iterator.rs:3669:4

For the user this is a dead end: the path doesn't exist locally, there's no surrounding code. It is fixable: rustup component add rust-src makes the source available, after which the same diagnostic renders the real std snippet. But that capability is
invisible. This PR closes that discoverability gap by suggesting it at exactly the moment it would help.

Closes #156402

[rustbot]

rustc_errors::annotate_snippet_emitter_writer was changed

cc @Muscraft

[rustbot]

r? @tiif

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[sladyn98]

r? @estebank

[estebank]

This looks good! Would you have time to address the nitpicks? If not, we can likely land this as is.

View changes since this review

[sladyn98]

@estebank I think we are good to merge this ?

[sladyn98]

@estebank Any updates on this ?

[estebank]

@bors r+

[rust-bors]

📌 Commit 55b1daf has been approved by estebank

It is now in the queue for this repository.

🌲 The tree is currently closed for pull requests below priority 5. This pull request will be tested once the tree is reopened.

Reason for tree closure: error: could not lock gitconfig

[rust-bors]

💔 Test for 8a723e4 failed: CI. Failed job:

• auto - test-x86_64-gnu-llvm-22-2 (web logs, enhanced plaintext logs)

[rust-log-analyzer]

The job test-x86_64-gnu-llvm-22-2 failed! Check out the build log: (web) (plain enhanced) (plain)

Click to see the possible cause of the failure (guessed by this bot)

help: if you don't care about this missing field, you can explicitly ignore it
   |
67 -             theme,
68 -         } = self;
67 +             theme, rust_src_hint_shown: _ } = self;
   |
help: or always ignore missing fields here
   |
67 -             theme,
68 -         } = self;

[estebank]

@sladyn98 My apologies for the long delay. I was traveling for an extended period and I've only caught up with pings recently.

It seems like you might have rebase on top of a more recent main and apply a few small changes to get things working again:

error[E0027]: pattern does not mention field `rust_src_hint_shown`
  --> compiler/rustc_errors/src/annotate_snippet_emitter_writer.rs:57:13
   |
57 |           let AnnotateSnippetEmitter {
   |  _____________^
58 | |             dst,
59 | |             sm,
60 | |             short_message,
...  |
67 | |             theme,
68 | |         } = self;
   | |_________^ missing field `rust_src_hint_shown`
   |
help: include the missing field in the pattern
   |
67 -             theme,
68 -         } = self;
67 +             theme, rust_src_hint_shown } = self;
   |
help: if you don't care about this missing field, you can explicitly ignore it
   |
67 -             theme,
68 -         } = self;
67 +             theme, rust_src_hint_shown: _ } = self;
   |
help: or always ignore missing fields here
   |
67 -             theme,
68 -         } = self;
67 +             theme, .. } = self;
   |

r=me once that is addressed.

Thank you for working on this!