Replace macro backtraces with labeled local uses

[sophiajt]

This PR (which builds on #35688) follows from the conversations on how best to handle the macro backtraces. The feeling there was that there were two different "groups" of users.

The first group, the macro users, rarely (and likely never) want to see the macro backtrace. This is often more confusing to users as it will be talking about code they didn't write.

The second group, the macro writers, are trying to debug a macro. They'll want to see something of the backtrace so that they can see where it's going wrong and what the steps were to get there.

For the first group, it seems clear that we don't want to show any macro backtrace. For the second group, we want to show enough to help the macro writer.

This PR uses a heuristic. It will only show any backtrace steps if they are in the same crate that is being compiled. This keeps errors in foreign crates from showing to users that didn't need them.

Additionally, in asking around I repeated heard that the middle steps of the backtrace are rarely, if ever, used in practice. This PR takes and applies this knowledge. Now, instead of a full backtrace, the user is given the error underline inside of a local macro as well as the use site as a secondary label. This effectively means seeing the root of the error and the top of the backtrace, eliding the middle steps.

Rather than being the "perfect solution", this PR opts to take an incremental step towards a better experience. Likely it would help to have additional macro debugging tools, as they could be much more verbose than we'd likely want to use in the error messages themselves.

Some examples follow.

Example 1

Before:

After:

Example 2

Before:

After:

[rust-highfive]

r? @pnkfelix

(rust_highfive has picked a reviewer for you, use r? to override)

[sophiajt]

r? @nikomatsakis

cc @nrc

[Mark-Simulacrum]

Why can't this be for child in &mut children?

[nikomatsakis]

This looks quite good; my only complaints are:

• I'd still like to have a comment explaining what the "Fixup macro span" thing is doing a bit
• we should have a cross-crate text, to show the behavior when macro is not local

To make a cross-crate text, you can create a test with an auxiliary directory, and add a // aux-build:foo.rs (to build auxiliary/foo.rs). So you'd have files like:

src/test/compile-fail/cross-crate-macro-backtrace/main.rs // <-- main file for the text
src/test/compile-fail/cross-crate-macro-backtrace/auxiliary/foo.rs // <-- crate

[Mark-Simulacrum]

Both of these should be for span in &mut .. compatible, I think.

[sophiajt]

@nikomatsakis - nits addressed, new ui test added, comment added.

[bors]

☔ The latest upstream changes (presumably #35747) made this pull request unmergeable. Please resolve the merge conflicts.

[nikomatsakis]

@jonathandturner wonderful, r=me, but needs rebase =)

[Mark-Simulacrum]

Why &mut children.iter_mut()? Shouldn't just &mut children be enough?

[sophiajt]

Probably a little overkill? &mut children doesn't compile:

error: the trait bound `std::vec::Vec<SubDiagnostic>: std::iter::Iterator` is not satisfied [--explain E0277]
   --> src/librustc_errors/emitter.rs:468:9
    |>
468 |>         for child in &mut children {
    |>         ^
note: `std::vec::Vec<SubDiagnostic>` is not an iterator; maybe try calling `.iter()` or a similar method
note: required because of the requirements on the impl of `std::iter::Iterator` for `&mut std::vec::Vec<SubDiagnostic>`

But I probably could drop the &mut in front.

[Mark-Simulacrum]

That's... odd, since it works in a simpler example: https://is.gd/Ep6Unm.

fn main() {
    let mut v = vec![1, 2];

    for el in &mut v {
        *el += 10;
    }

    println!("{:?}", v);
}

Edit: Actually, children is already &mut Vec<_>, so just for child in children should work, if I understand iterators correctly.

[sophiajt]

Also doesn't work...

error: use of moved value: `*children` [--explain E0382]
   --> src/librustc_errors/emitter.rs:472:13
    |>
468 |>         for child in children {
    |>                      -------- value moved here
...
472 |>             children.push(SubDiagnostic {
    |>             ^^^^^^^^ value used here after move
note: move occurs because `children` has type `&mut std::vec::Vec<SubDiagnostic>`, which does not implement the `Copy` trait

Sticking with the one that does :)

[sophiajt]

@bors r=nikomatsakis

[bors]

📌 Commit 54d42cc has been approved by nikomatsakis

[bors]

⌛ Testing commit 54d42cc with merge 169b612...

[bors]

☀️ Test successful - auto-linux-32-nopt-t, auto-linux-32-opt, auto-linux-64-cargotest, auto-linux-64-cross-freebsd, auto-linux-64-debug-opt, auto-linux-64-nopt-t, auto-linux-64-opt, auto-linux-64-opt-no-mir, auto-linux-64-opt-rustbuild, auto-linux-64-x-android-t, auto-linux-cross-opt, auto-linux-musl-64-opt, auto-mac-32-opt, auto-mac-64-opt, auto-mac-64-opt-rustbuild, auto-mac-cross-ios-opt, auto-win-gnu-32-opt, auto-win-gnu-32-opt-rustbuild, auto-win-gnu-64-opt, auto-win-msvc-32-opt, auto-win-msvc-64-cargotest, auto-win-msvc-64-opt, auto-win-msvc-64-opt-no-mir, auto-win-msvc-64-opt-rustbuild
Approved by: nikomatsakis
Pushing 169b612 to master...