Add a flag to disable leak backtraces

src/tools/miri/README.md

@@ -301,6 +301,15 @@ environment variable. We first document the most relevant and most commonly used
 * `-Zmiri-disable-isolation` disables host isolation.  As a consequence,
   the program has access to host resources such as environment variables, file
   systems, and randomness.
+* `-Zmiri-disable-leak-backtraces` disables backtraces reports for memory leaks. By default, a
+  backtrace is captured for every allocation when it is created, just in case it leaks. This incurs
+  some memory overhead to store data that is almost never used. This flag is implied by
+  `-Zmiri-ignore-leaks`.
+* `-Zmiri-env-forward=<var>` forwards the `var` environment variable to the interpreted program. Can
+  be used multiple times to forward several variables. Execution will still be deterministic if the
+  value of forwarded variables stays the same. Has no effect if `-Zmiri-disable-isolation` is set.
+* `-Zmiri-ignore-leaks` disables the memory leak checker, and also allows some
+  remaining threads to exist when the main thread exits.
 * `-Zmiri-isolation-error=<action>` configures Miri's response to operations
   requiring host access while isolation is enabled. `abort`, `hide`, `warn`,
   and `warn-nobacktrace` are the supported actions. The default is to `abort`,
@@ -308,11 +317,6 @@ environment variable. We first document the most relevant and most commonly used
   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.
-* `-Zmiri-env-forward=<var>` forwards the `var` environment variable to the interpreted program. Can
-  be used multiple times to forward several variables. Execution will still be deterministic if the
-  value of forwarded variables stays the same. Has no effect if `-Zmiri-disable-isolation` is set.
-* `-Zmiri-ignore-leaks` disables the memory leak checker, and also allows some
-  remaining threads to exist when the main thread exits.
 * `-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/bin/miri.rs

@@ -359,6 +359,8 @@ fn main() {
                 isolation_enabled = Some(false);
             }
             miri_config.isolated_op = miri::IsolatedOp::Allow;
+        } else if arg == "-Zmiri-disable-leak-backtraces" {
+            miri_config.collect_leak_backtraces = false;
         } else if arg == "-Zmiri-disable-weak-memory-emulation" {
             miri_config.weak_memory_emulation = false;
         } else if arg == "-Zmiri-track-weak-memory-loads" {
@@ -385,6 +387,7 @@ fn main() {
             };
         } else if arg == "-Zmiri-ignore-leaks" {
             miri_config.ignore_leaks = true;
+            miri_config.collect_leak_backtraces = false;
         } else if arg == "-Zmiri-panic-on-unsupported" {
             miri_config.panic_on_unsupported = true;
         } else if arg == "-Zmiri-tag-raw-pointers" {

src/tools/miri/src/eval.rs

@@ -146,6 +146,8 @@ pub struct MiriConfig {
     pub num_cpus: u32,
     /// Requires Miri to emulate pages of a certain size
     pub page_size: Option<u64>,
+    /// Whether to collect a backtrace when each allocation is created, just in case it leaks.
+    pub collect_leak_backtraces: bool,
 }
 
 impl Default for MiriConfig {
@@ -180,6 +182,7 @@ fn default() -> MiriConfig {
             gc_interval: 10_000,
             num_cpus: 1,
             page_size: None,
+            collect_leak_backtraces: true,
         }
     }
 }
@@ -461,9 +464,14 @@ pub fn eval_entry<'tcx>(
         let leaks = ecx.find_leaked_allocations(&ecx.machine.static_roots);
         if !leaks.is_empty() {
             report_leaks(&ecx, leaks);
-            tcx.sess.note_without_error(
-                "the evaluated program leaked memory, pass `-Zmiri-ignore-leaks` to disable this check",
-            );
+            let leak_message = "the evaluated program leaked memory, pass `-Zmiri-ignore-leaks` to disable this check";
+            if ecx.machine.collect_leak_backtraces {
+                // If we are collecting leak backtraces, each leak is a distinct error diagnostic.
+                tcx.sess.note_without_error(leak_message);
+            } else {
+                // If we do not have backtraces, we just report an error without any span.
+                tcx.sess.err(leak_message);
+            };
             // Ignore the provided return code - let the reported error
             // determine the return code.
             return None;

src/tools/miri/src/machine.rs

@@ -471,12 +471,17 @@ pub struct MiriMachine<'mir, 'tcx> {
     pub(crate) gc_interval: u32,
     /// The number of blocks that passed since the last BorTag GC pass.
     pub(crate) since_gc: u32,
+
     /// The number of CPUs to be reported by miri.
     pub(crate) num_cpus: u32,
+
     /// Determines Miri's page size and associated values
     pub(crate) page_size: u64,
     pub(crate) stack_addr: u64,
     pub(crate) stack_size: u64,
+
+    /// Whether to collect a backtrace when each allocation is created, just in case it leaks.
+    pub(crate) collect_leak_backtraces: bool,
 }
 
 impl<'mir, 'tcx> MiriMachine<'mir, 'tcx> {
@@ -585,6 +590,7 @@ pub(crate) fn new(config: &MiriConfig, layout_cx: LayoutCx<'tcx, TyCtxt<'tcx>>)
             page_size,
             stack_addr,
             stack_size,
+            collect_leak_backtraces: config.collect_leak_backtraces,
         }
     }
 
@@ -732,6 +738,7 @@ fn visit_tags(&self, visit: &mut dyn FnMut(BorTag)) {
             page_size: _,
             stack_addr: _,
             stack_size: _,
+            collect_leak_backtraces: _,
         } = self;
 
         threads.visit_tags(visit);
@@ -975,7 +982,11 @@ fn adjust_allocation<'b>(
         // If an allocation is leaked, we want to report a backtrace to indicate where it was
         // allocated. We don't need to record a backtrace for allocations which are allowed to
         // leak.
-        let backtrace = if kind.may_leak() { None } else { Some(ecx.generate_stacktrace()) };
+        let backtrace = if kind.may_leak() || !ecx.machine.collect_leak_backtraces {
+            None
+        } else {
+            Some(ecx.generate_stacktrace())
+        };
 
         let alloc: Allocation<Provenance, Self::AllocExtra> = alloc.adjust_from_tcx(
             &ecx.tcx,

src/tools/miri/tests/fail/memleak.stderr

@@ -15,6 +15,8 @@ note: inside `main`
 LL |     std::mem::forget(Box::new(42));
    |                      ^^^^^^^^^^^^
 
+note: some details are omitted, run with `MIRIFLAGS=-Zmiri-backtrace=full` for a verbose backtrace
+
 note: the evaluated program leaked memory, pass `-Zmiri-ignore-leaks` to disable this check
 
 error: aborting due to previous error

src/tools/miri/tests/fail/memleak_no_backtrace.rs

@@ -0,0 +1,7 @@
+//@compile-flags: -Zmiri-disable-leak-backtraces
+//@error-pattern: the evaluated program leaked memory
+//@normalize-stderr-test: ".*│.*" -> "$$stripped$$"
+
+fn main() {
+    std::mem::forget(Box::new(42));
+}

src/tools/miri/tests/fail/memleak_no_backtrace.stderr

@@ -0,0 +1,4 @@
+error: the evaluated program leaked memory, pass `-Zmiri-ignore-leaks` to disable this check
+
+error: aborting due to previous error
+

src/tools/miri/tests/fail/memleak_rc.32bit.stderr

@@ -16,6 +16,8 @@ note: inside `main`
 LL |     let x = Dummy(Rc::new(RefCell::new(None)));
    |                   ^^^^^^^^^^^^^^^^^^^^^^^^^^^
 
+note: some details are omitted, run with `MIRIFLAGS=-Zmiri-backtrace=full` for a verbose backtrace
+
 note: the evaluated program leaked memory, pass `-Zmiri-ignore-leaks` to disable this check
 
 error: aborting due to previous error

src/tools/miri/tests/fail/memleak_rc.64bit.stderr

@@ -16,6 +16,8 @@ note: inside `main`
 LL |     let x = Dummy(Rc::new(RefCell::new(None)));
    |                   ^^^^^^^^^^^^^^^^^^^^^^^^^^^
 
+note: some details are omitted, run with `MIRIFLAGS=-Zmiri-backtrace=full` for a verbose backtrace
+
 note: the evaluated program leaked memory, pass `-Zmiri-ignore-leaks` to disable this check
 
 error: aborting due to previous error