show int2ptr warning once for each span (but don't duplicate the long help)

src/diagnostics.rs

@@ -69,7 +69,9 @@ pub enum NonHaltingDiagnostic {
     FreedAlloc(AllocId),
     RejectedIsolatedOp(String),
     ProgressReport,
-    Int2Ptr,
+    Int2Ptr {
+        details: bool,
+    },
 }
 
 /// Level of Miri specific diagnostics
@@ -451,13 +453,13 @@ fn process_diagnostics(&self, info: TopFrameInfo<'tcx>) {
                         format!("{op} was made to return an error due to isolation"),
                     ProgressReport =>
                         format!("progress report: current operation being executed is here"),
-                    Int2Ptr => format!("integer-to-pointer cast"),
+                    Int2Ptr { .. } => format!("integer-to-pointer cast"),
                 };
 
                 let (title, diag_level) = match e {
                     RejectedIsolatedOp(_) =>
                         ("operation rejected by isolation", DiagLevel::Warning),
-                    Int2Ptr => ("integer-to-pointer cast", DiagLevel::Warning),
+                    Int2Ptr { .. } => ("integer-to-pointer cast", DiagLevel::Warning),
                     CreatedPointerTag(..)
                     | PoppedPointerTag(..)
                     | CreatedCallId(..)
@@ -467,7 +469,7 @@ fn process_diagnostics(&self, info: TopFrameInfo<'tcx>) {
                 };
 
                 let helps = match e {
-                    Int2Ptr =>
+                    Int2Ptr { details: true } =>
                         vec![
                             (None, format!("this program is using integer-to-pointer casts or (equivalently) `from_exposed_addr`,")),
                             (None, format!("which means that Miri might miss pointer bugs in this program")),

src/intptrcast.rs

@@ -5,6 +5,7 @@
 use rand::Rng;
 
 use rustc_data_structures::fx::{FxHashMap, FxHashSet};
+use rustc_span::Span;
 use rustc_target::abi::{HasDataLayout, Size};
 
 use crate::*;
@@ -140,12 +141,18 @@ pub fn ptr_from_addr_cast(
 
         match global_state.provenance_mode {
             ProvenanceMode::Default => {
-                // The first time this happens, print a warning.
-                use std::sync::atomic::{AtomicBool, Ordering};
-                static FIRST_WARNING: AtomicBool = AtomicBool::new(true);
-                if FIRST_WARNING.swap(false, Ordering::Relaxed) {
-                    register_diagnostic(NonHaltingDiagnostic::Int2Ptr);
+                // The first time this happens at a particular location, print a warning.
+                thread_local! {
+                    // `Span` is non-`Send`, so we use a thread-local instead.
+                    static PAST_WARNINGS: RefCell<FxHashSet<Span>> = RefCell::default();
                 }
+                PAST_WARNINGS.with_borrow_mut(|past_warnings| {
+                    let first = past_warnings.is_empty();
+                    if past_warnings.insert(ecx.cur_span()) {
+                        // Newly inserted, so first time we see this span.
+                        register_diagnostic(NonHaltingDiagnostic::Int2Ptr { details: first });
+                    }
+                });
             }
             ProvenanceMode::Strict => {
                 throw_unsup_format!(

src/lib.rs

@@ -8,6 +8,7 @@
 #![feature(yeet_expr)]
 #![feature(is_some_with)]
 #![feature(nonzero_ops)]
+#![feature(local_key_cell_methods)]
 #![warn(rust_2018_idioms)]
 #![allow(
     clippy::collapsible_else_if,

tests/pass/box.stderr

@@ -18,3 +18,16 @@ note: inside `main` at $DIR/box.rs:LL:CC
 LL |     into_raw();
    |     ^^^^^^^^^^
 
+warning: integer-to-pointer cast
+  --> $DIR/box.rs:LL:CC
+   |
+LL |         let r = ((u.as_ptr() as usize) + 0) as *mut i32;
+   |                 ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^ integer-to-pointer cast
+   |
+   = note: inside `into_unique` at $DIR/box.rs:LL:CC
+note: inside `main` at $DIR/box.rs:LL:CC
+  --> $DIR/box.rs:LL:CC
+   |
+LL |     into_unique();
+   |     ^^^^^^^^^^^^^
+