diagnostics: do not repeat the entire message in the span label

Author: RalfJung

src/tools/miri/src/diagnostics.rs

@@ -426,11 +426,17 @@ pub fn report_error<'tcx>(
         _ => {}
     }
 
-    msg.insert(0, format_interp_error(ecx.tcx.dcx(), e));
+    let mut primary_msg = String::new();
+    if let Some(title) = title {
+        write!(primary_msg, "{title}: ").unwrap();
+    }
+    write!(primary_msg, "{}", format_interp_error(ecx.tcx.dcx(), e)).unwrap();
+
+    msg.insert(0, format!("{} occurred here", title.unwrap_or("error")));
 
     report_msg(
         DiagLevel::Error,
-        if let Some(title) = title { format!("{title}: {}", msg[0]) } else { msg[0].clone() },
+        primary_msg,
         msg,
         vec![],
         helps,

src/tools/miri/tests/fail-dep/concurrency/libc_pthread_cond_double_destroy.stderr

@@ -2,7 +2,7 @@ error: Undefined Behavior: using uninitialized data, but this operation requires
   --> tests/fail-dep/concurrency/libc_pthread_cond_double_destroy.rs:LL:CC
    |
 LL |         libc::pthread_cond_destroy(cond.as_mut_ptr());
-   |         ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^ using uninitialized data, but this operation requires initialized memory
+   |         ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^ Undefined Behavior occurred here
    |
    = help: this indicates a bug in the program: it performed an invalid operation, and caused Undefined Behavior
    = help: see https://doc.rust-lang.org/nightly/reference/behavior-considered-undefined.html for further information

src/tools/miri/tests/fail-dep/concurrency/libc_pthread_cond_move.init.stderr

@@ -2,7 +2,7 @@ error: Undefined Behavior: `pthread_cond_t` can't be moved after first use
   --> tests/fail-dep/concurrency/libc_pthread_cond_move.rs:LL:CC
    |
 LL |         libc::pthread_cond_destroy(cond2.as_mut_ptr());
-   |         ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^ `pthread_cond_t` can't be moved after first use
+   |         ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^ Undefined Behavior occurred here
    |
    = help: this indicates a bug in the program: it performed an invalid operation, and caused Undefined Behavior
    = help: see https://doc.rust-lang.org/nightly/reference/behavior-considered-undefined.html for further information

src/tools/miri/tests/fail-dep/concurrency/libc_pthread_cond_move.static_initializer.stderr

@@ -2,7 +2,7 @@ error: Undefined Behavior: `pthread_cond_t` can't be moved after first use
   --> tests/fail-dep/concurrency/libc_pthread_cond_move.rs:LL:CC
    |
 LL |         libc::pthread_cond_destroy(&mut cond2 as *mut _);
-   |         ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^ `pthread_cond_t` can't be moved after first use
+   |         ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^ Undefined Behavior occurred here
    |
    = help: this indicates a bug in the program: it performed an invalid operation, and caused Undefined Behavior
    = help: see https://doc.rust-lang.org/nightly/reference/behavior-considered-undefined.html for further information

src/tools/miri/tests/fail-dep/concurrency/libc_pthread_condattr_double_destroy.stderr

@@ -2,7 +2,7 @@ error: Undefined Behavior: using uninitialized data, but this operation requires
   --> tests/fail-dep/concurrency/libc_pthread_condattr_double_destroy.rs:LL:CC
    |
 LL |         libc::pthread_condattr_destroy(attr.as_mut_ptr());
-   |         ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^ using uninitialized data, but this operation requires initialized memory
+   |         ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^ Undefined Behavior occurred here
    |
    = help: this indicates a bug in the program: it performed an invalid operation, and caused Undefined Behavior
    = help: see https://doc.rust-lang.org/nightly/reference/behavior-considered-undefined.html for further information

src/tools/miri/tests/fail-dep/concurrency/libc_pthread_create_too_few_args.stderr

@@ -1,6 +1,6 @@
 error: Undefined Behavior: calling a function with more arguments than it expected
    |
-   = note: calling a function with more arguments than it expected
+   = note: Undefined Behavior occurred here
    = note: (no span available)
    = help: this indicates a bug in the program: it performed an invalid operation, and caused Undefined Behavior
    = help: see https://doc.rust-lang.org/nightly/reference/behavior-considered-undefined.html for further information

src/tools/miri/tests/fail-dep/concurrency/libc_pthread_create_too_many_args.stderr

@@ -1,6 +1,6 @@
 error: Undefined Behavior: calling a function with fewer arguments than it requires
    |
-   = note: calling a function with fewer arguments than it requires
+   = note: Undefined Behavior occurred here
    = note: (no span available)
    = help: this indicates a bug in the program: it performed an invalid operation, and caused Undefined Behavior
    = help: see https://doc.rust-lang.org/nightly/reference/behavior-considered-undefined.html for further information

src/tools/miri/tests/fail-dep/concurrency/libc_pthread_join_detached.stderr

@@ -2,7 +2,7 @@ error: Undefined Behavior: trying to join a detached thread
   --> tests/fail-dep/concurrency/libc_pthread_join_detached.rs:LL:CC
    |
 LL |         assert_eq!(libc::pthread_join(native, ptr::null_mut()), 0);
-   |                    ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^ trying to join a detached thread
+   |                    ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^ Undefined Behavior occurred here
    |
    = help: this indicates a bug in the program: it performed an invalid operation, and caused Undefined Behavior
    = help: see https://doc.rust-lang.org/nightly/reference/behavior-considered-undefined.html for further information

src/tools/miri/tests/fail-dep/concurrency/libc_pthread_join_joined.stderr

@@ -2,7 +2,7 @@ error: Undefined Behavior: trying to join an already joined thread
   --> tests/fail-dep/concurrency/libc_pthread_join_joined.rs:LL:CC
    |
 LL |         assert_eq!(libc::pthread_join(native, ptr::null_mut()), 0);
-   |                    ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^ trying to join an already joined thread
+   |                    ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^ Undefined Behavior occurred here
    |
    = help: this indicates a bug in the program: it performed an invalid operation, and caused Undefined Behavior
    = help: see https://doc.rust-lang.org/nightly/reference/behavior-considered-undefined.html for further information

src/tools/miri/tests/fail-dep/concurrency/libc_pthread_join_main.stderr

@@ -2,7 +2,7 @@ error: Undefined Behavior: trying to join a detached thread
   --> tests/fail-dep/concurrency/libc_pthread_join_main.rs:LL:CC
    |
 LL |             assert_eq!(libc::pthread_join(thread_id, ptr::null_mut()), 0);
-   |                        ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^ trying to join a detached thread
+   |                        ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^ Undefined Behavior occurred here
    |
    = help: this indicates a bug in the program: it performed an invalid operation, and caused Undefined Behavior
    = help: see https://doc.rust-lang.org/nightly/reference/behavior-considered-undefined.html for further information