implement manual deduplication for isolation-error=warn-nobacktrace

Author: RalfJung

src/tools/miri/README.md

@@ -318,8 +318,8 @@ environment variable. We first document the most relevant and most commonly used
   and `warn-nobacktrace` are the supported actions. The default is to `abort`,
   which halts the machine. Some (but not all) operations also support continuing
   execution with a "permission denied" error being returned to the program.
-  `warn` prints a full backtrace when that happens; `warn-nobacktrace` is less
-  verbose. `hide` hides the warning entirely.
+  `warn` prints a full backtrace each time that happens; `warn-nobacktrace` is less
+  verbose and shown at most once per operation. `hide` hides the warning entirely.
 * `-Zmiri-num-cpus` states the number of available CPUs to be reported by miri. By default, the
   number of available CPUs is `1`. Note that this flag does not affect how miri handles threads in
   any way.

src/tools/miri/src/helpers.rs

@@ -1,6 +1,8 @@
 use std::cmp;
+use std::collections::BTreeSet;
 use std::iter;
 use std::num::NonZero;
+use std::sync::Mutex;
 use std::time::Duration;
 
 use rustc_apfloat::ieee::{Double, Single};
@@ -603,9 +605,18 @@ pub trait EvalContextExt<'mir, 'tcx: 'mir>: crate::MiriInterpCxExt<'mir, 'tcx> {
         match reject_with {
             RejectOpWith::Abort => isolation_abort_error(op_name),
             RejectOpWith::WarningWithoutBacktrace => {
-                this.tcx
-                    .dcx()
-                    .warn(format!("{op_name} was made to return an error due to isolation"));
+                // This exists to reduce verbosity; make sure we emit the warning at most once per
+                // operation.
+                static EMITTED_WARNINGS: Mutex<BTreeSet<String>> = Mutex::new(BTreeSet::new());
+
+                let mut emitted_warnings = EMITTED_WARNINGS.lock().unwrap();
+                if !emitted_warnings.contains(op_name) {
+                    // First time we are seeing this.
+                    emitted_warnings.insert(op_name.to_owned());
+                    this.tcx
+                        .dcx()
+                        .warn(format!("{op_name} was made to return an error due to isolation"));
+                }
                 Ok(())
             }
             RejectOpWith::Warning => {