Rollup merge of rust-lang#132947 - lqd:trimmed-ice, r=compiler-errors

clarify `must_produce_diag` ICE for debugging

We have a sanity check to ensure the expensive `trimmed_def_paths` functions are called only when producing diagnostics, and not e.g. on the happy path. The panic often happens IME during development because of randomly printing stuff, causing an ICE if no diagnostics were also emitted.

I have this change locally but figured it could be useful to others, so this PR clarifies the message when this happens during development.

The output currently looks like this by default; it's a bit confusing with words missing:

```
thread 'rustc' panicked at compiler/rustc_errors/src/lib.rs:628:17:
must_produce_diag: `trimmed_def_paths` called but no diagnostics emitted; `with_no_trimmed_paths` for debugging. called at: disabled backtrace
stack backtrace:
   0:     0x7ffff79570f6 - std::backtrace_rs::backtrace::libunwind::trace::h33576c57327a3cea
                               at .../library/std/src/../../backtrace/src/backtrace/libunwind.rs:116:5
   1:     0x7ffff79570f6 - std::backtrace_rs::backtrace::trace_unsynchronized::h7972a09393b420db
                               at .../library/std/src/../../backtrace/src/backtrace/mod.rs:66:5
   2:     0x7ffff79570f6 - std::sys::backtrace::_print_fmt::hae8c5bbfbf7a8322
                               at .../library/std/src/sys/backtrace.rs:66:9
   3:     0x7ffff79570f6 - <std::sys::backtrace::BacktraceLock::print::DisplayBacktrace as core::fmt::Display>::fmt::h1fd6a7a210f5b535
...
```

The new output mentions how to get more information and locate where the `with_no_trimmed_paths` call needs to be added.

1. By default, backtraces are disabled:
```
thread 'rustc' panicked at compiler/rustc_errors/src/lib.rs:642:17:
`trimmed_def_paths` called, diagnostics were expected but none were emitted. Use `with_no_trimmed_paths` for debugging. Backtraces are currently disabled: set `RUST_BACKTRACE=1` and re-run to see where it happened.
stack backtrace:
   0:     0x7ffff79565f6 - std::backtrace_rs::backtrace::libunwind::trace::h33576c57327a3cea
...
```

2. With backtraces enabled:
```
thread 'rustc' panicked at compiler/rustc_errors/src/lib.rs:642:17:
`trimmed_def_paths` called, diagnostics were expected but none were emitted. Use `with_no_trimmed_paths` for debugging. This happened in the following `must_produce_diag` call's backtrace:
   0: <rustc_errors::DiagCtxtHandle>::set_must_produce_diag
             at .../compiler/rustc_errors/src/lib.rs:1133:58
   1: <rustc_session::session::Session>::record_trimmed_def_paths
             at .../compiler/rustc_session/src/session.rs:327:9
   2: rustc_middle::ty::print::pretty::trimmed_def_paths
             at .../compiler/rustc_middle/src/ty/print/pretty.rs:3351:5
...
```

A `\n` could be added here or there, but it didn't matter much whenever I hit this case with the new message.

compiler/rustc_errors/src/lib.rs

@@ -623,12 +623,25 @@ impl Drop for DiagCtxtInner {
             self.flush_delayed()
         }
 
+        // Sanity check: did we use some of the expensive `trimmed_def_paths` functions
+        // unexpectedly, that is, without producing diagnostics? If so, for debugging purposes, we
+        // suggest where this happened and how to avoid it.
         if !self.has_printed && !self.suppressed_expected_diag && !std::thread::panicking() {
             if let Some(backtrace) = &self.must_produce_diag {
+                let suggestion = match backtrace.status() {
+                    BacktraceStatus::Disabled => String::from(
+                        "Backtraces are currently disabled: set `RUST_BACKTRACE=1` and re-run \
+                        to see where it happened.",
+                    ),
+                    BacktraceStatus::Captured => format!(
+                        "This happened in the following `must_produce_diag` call's backtrace:\n\
+                        {backtrace}",
+                    ),
+                    _ => String::from("(impossible to capture backtrace where this happened)"),
+                };
                 panic!(
-                    "must_produce_diag: `trimmed_def_paths` called but no diagnostics emitted; \
-                     `with_no_trimmed_paths` for debugging. \
-                     called at: {backtrace}"
+                    "`trimmed_def_paths` called, diagnostics were expected but none were emitted. \
+                    Use `with_no_trimmed_paths` for debugging. {suggestion}"
                 );
             }
         }