Improve diagnostics for buffer reuse with borrowed references

compiler/rustc_borrowck/src/diagnostics/conflict_errors.rs

@@ -3084,6 +3084,39 @@ impl<'infcx, 'tcx> MirBorrowckCtxt<'_, 'infcx, 'tcx> {
             });
 
             explanation.add_explanation_to_diagnostic(&self, &mut err, "", Some(borrow_span), None);
+
+            // Detect buffer reuse pattern
+            if let BorrowExplanation::UsedLater(_dropped_local, _, _, _) = explanation {
+                // Check all locals at the borrow location to find Vec<&T> types
+                for (local, local_decl) in self.body.local_decls.iter_enumerated() {
+                    if let ty::Adt(adt_def, args) = local_decl.ty.kind()
+                        && self.infcx.tcx.is_diagnostic_item(sym::Vec, adt_def.did())
+                        && args.len() > 0
+                    {
+                        let vec_inner_ty = args.type_at(0);
+                        // Check if Vec contains references
+                        if vec_inner_ty.is_ref() {
+                            let local_place = local.into();
+                            if let Some(local_name) = self.describe_place(local_place) {
+                                err.span_label(
+                                    local_decl.source_info.span,
+                                    format!("variable `{local_name}` declared here"),
+                                );
+                                err.note(
+                                    format!(
+                                        "`{local_name}` is a collection that stores borrowed references, \
+                                         but {name} does not live long enough to be stored in it"
+                                    )
+                                );
+                                err.help(
+                                    "buffer reuse with borrowed references requires unsafe code or restructuring"
+                                );
+                                break;
+                            }
+                        }
+                    }
+                }
+            }
         }
 
         err

tests/ui/borrowck/buffer-reuse-pattern-issue-147694.rs

@@ -0,0 +1,63 @@
+fn process_data(_: &[&[u8]]) {}
+
+fn test_buffer_cleared_after_use() {
+    let sources = vec![vec![1u8, 2, 3, 4, 5], vec![6, 7, 8, 9]];
+    let mut buffer: Vec<&[u8]> = vec![];
+    //~^ NOTE variable `buffer` declared here
+
+    for source in sources {
+        let data: Vec<u8> = source;
+        //~^ NOTE binding `data` declared here
+        buffer.extend(data.split(|x| *x == 3));
+        //~^ ERROR `data` does not live long enough
+        //~| NOTE borrowed value does not live long enough
+        //~| NOTE borrow later used here
+        //~| NOTE `buffer` is a collection that stores borrowed references, but `data` does not live long enough to be stored in it
+        //~| HELP buffer reuse with borrowed references requires unsafe code or restructuring
+        process_data(&buffer);
+        buffer.clear();
+    } //~ NOTE `data` dropped here while still borrowed
+}
+
+fn test_buffer_cleared_at_start() {
+    let sources = vec![vec![1u8, 2, 3, 4, 5], vec![6, 7, 8, 9]];
+    let mut buffer: Vec<&[u8]> = vec![];
+    //~^ NOTE variable `buffer` declared here
+
+    for source in sources {
+        buffer.clear();
+        //~^ NOTE borrow later used here
+        let data: Vec<u8> = source;
+        //~^ NOTE binding `data` declared here
+        buffer.extend(data.split(|x| *x == 3));
+        //~^ ERROR `data` does not live long enough
+        //~| NOTE borrowed value does not live long enough
+        //~| NOTE `buffer` is a collection that stores borrowed references, but `data` does not live long enough to be stored in it
+        //~| HELP buffer reuse with borrowed references requires unsafe code or restructuring
+        process_data(&buffer);
+    } //~ NOTE `data` dropped here while still borrowed
+}
+
+fn test_no_explicit_clear() {
+    let sources = vec![vec![1u8, 2, 3, 4, 5], vec![6, 7, 8, 9]];
+    let mut buffer: Vec<&[u8]> = vec![];
+    //~^ NOTE variable `buffer` declared here
+
+    for source in sources {
+        let data: Vec<u8> = source;
+        //~^ NOTE binding `data` declared here
+        buffer.extend(data.split(|x| *x == 3));
+        //~^ ERROR `data` does not live long enough
+        //~| NOTE borrowed value does not live long enough
+        //~| NOTE borrow later used here
+        //~| NOTE `buffer` is a collection that stores borrowed references, but `data` does not live long enough to be stored in it
+        //~| HELP buffer reuse with borrowed references requires unsafe code or restructuring
+        process_data(&buffer);
+    } //~ NOTE `data` dropped here while still borrowed
+}
+
+fn main() {
+    test_buffer_cleared_after_use();
+    test_buffer_cleared_at_start();
+    test_no_explicit_clear();
+}

tests/ui/borrowck/buffer-reuse-pattern-issue-147694.stderr

@@ -0,0 +1,64 @@
+error[E0597]: `data` does not live long enough
+  --> $DIR/buffer-reuse-pattern-issue-147694.rs:11:23
+   |
+LL |     let mut buffer: Vec<&[u8]> = vec![];
+   |         ---------- variable `buffer` declared here
+...
+LL |         let data: Vec<u8> = source;
+   |             ---- binding `data` declared here
+LL |
+LL |         buffer.extend(data.split(|x| *x == 3));
+   |         ------        ^^^^ borrowed value does not live long enough
+   |         |
+   |         borrow later used here
+...
+LL |     }
+   |     - `data` dropped here while still borrowed
+   |
+   = note: `buffer` is a collection that stores borrowed references, but `data` does not live long enough to be stored in it
+   = help: buffer reuse with borrowed references requires unsafe code or restructuring
+
+error[E0597]: `data` does not live long enough
+  --> $DIR/buffer-reuse-pattern-issue-147694.rs:32:23
+   |
+LL |     let mut buffer: Vec<&[u8]> = vec![];
+   |         ---------- variable `buffer` declared here
+...
+LL |         buffer.clear();
+   |         ------ borrow later used here
+LL |
+LL |         let data: Vec<u8> = source;
+   |             ---- binding `data` declared here
+LL |
+LL |         buffer.extend(data.split(|x| *x == 3));
+   |                       ^^^^ borrowed value does not live long enough
+...
+LL |     }
+   |     - `data` dropped here while still borrowed
+   |
+   = note: `buffer` is a collection that stores borrowed references, but `data` does not live long enough to be stored in it
+   = help: buffer reuse with borrowed references requires unsafe code or restructuring
+
+error[E0597]: `data` does not live long enough
+  --> $DIR/buffer-reuse-pattern-issue-147694.rs:49:23
+   |
+LL |     let mut buffer: Vec<&[u8]> = vec![];
+   |         ---------- variable `buffer` declared here
+...
+LL |         let data: Vec<u8> = source;
+   |             ---- binding `data` declared here
+LL |
+LL |         buffer.extend(data.split(|x| *x == 3));
+   |         ------        ^^^^ borrowed value does not live long enough
+   |         |
+   |         borrow later used here
+...
+LL |     }
+   |     - `data` dropped here while still borrowed
+   |
+   = note: `buffer` is a collection that stores borrowed references, but `data` does not live long enough to be stored in it
+   = help: buffer reuse with borrowed references requires unsafe code or restructuring
+
+error: aborting due to 3 previous errors
+
+For more information about this error, try `rustc --explain E0597`.

tests/ui/issues/issue-52126-assign-op-invariance.stderr

@@ -4,13 +4,18 @@ error[E0597]: `line` does not live long enough
 LL |     for line in vec!["123456789".to_string(), "12345678".to_string()] {
    |         ---- binding `line` declared here
 LL |         let v: Vec<&str> = line.split_whitespace().collect();
-   |                            ^^^^ borrowed value does not live long enough
+   |             -              ^^^^ borrowed value does not live long enough
+   |             |
+   |             variable `v` declared here
 ...
 LL |         acc += cnt2;
    |         --- borrow later used here
 ...
 LL |     }
    |     - `line` dropped here while still borrowed
+   |
+   = note: `v` is a collection that stores borrowed references, but `line` does not live long enough to be stored in it
+   = help: buffer reuse with borrowed references requires unsafe code or restructuring
 
 error: aborting due to 1 previous error
 

tests/ui/span/borrowck-let-suggestion-suffixes.rs

@@ -3,6 +3,7 @@ fn id<T>(x: T) -> T { x }
 fn f() {
     let old = ['o'];         // statement 0
     let mut v1 = Vec::new(); // statement 1
+    //~^ NOTE variable `v1` declared here
 
     let mut v2 = Vec::new(); // statement 2
 
@@ -13,6 +14,8 @@ fn f() {
         v2.push(&young[0]);      // statement 4
         //~^ ERROR `young[_]` does not live long enough
         //~| NOTE borrowed value does not live long enough
+        //~| NOTE `v1` is a collection that stores borrowed references, but `young[_]` does not live long enough to be stored in it
+        //~| HELP buffer reuse with borrowed references requires unsafe code or restructuring
     } //~ NOTE `young[_]` dropped here while still borrowed
 
     let mut v3 = Vec::new(); // statement 5

tests/ui/span/borrowck-let-suggestion-suffixes.stderr

@@ -1,6 +1,9 @@
 error[E0597]: `young[_]` does not live long enough
-  --> $DIR/borrowck-let-suggestion-suffixes.rs:13:17
+  --> $DIR/borrowck-let-suggestion-suffixes.rs:14:17
    |
+LL |     let mut v1 = Vec::new(); // statement 1
+   |         ------ variable `v1` declared here
+...
 LL |         let young = ['y'];       // statement 3
    |             ----- binding `young` declared here
 ...
@@ -12,9 +15,12 @@ LL |     }
 ...
 LL |     (v1, v2, v3, /* v4 is above. */ v5).use_ref();
    |          -- borrow later used here
+   |
+   = note: `v1` is a collection that stores borrowed references, but `young[_]` does not live long enough to be stored in it
+   = help: buffer reuse with borrowed references requires unsafe code or restructuring
 
 error[E0716]: temporary value dropped while borrowed
-  --> $DIR/borrowck-let-suggestion-suffixes.rs:20:14
+  --> $DIR/borrowck-let-suggestion-suffixes.rs:23:14
    |
 LL |     v3.push(&id('x'));           // statement 6
    |              ^^^^^^^ - temporary value is freed at the end of this statement
@@ -31,7 +37,7 @@ LL ~     v3.push(&binding);           // statement 6
    |
 
 error[E0716]: temporary value dropped while borrowed
-  --> $DIR/borrowck-let-suggestion-suffixes.rs:30:18
+  --> $DIR/borrowck-let-suggestion-suffixes.rs:33:18
    |
 LL |         v4.push(&id('y'));
    |                  ^^^^^^^ - temporary value is freed at the end of this statement
@@ -44,7 +50,7 @@ LL |         v4.use_ref();
    = note: consider using a `let` binding to create a longer lived value
 
 error[E0716]: temporary value dropped while borrowed
-  --> $DIR/borrowck-let-suggestion-suffixes.rs:41:14
+  --> $DIR/borrowck-let-suggestion-suffixes.rs:44:14
    |
 LL |     v5.push(&id('z'));
    |              ^^^^^^^ - temporary value is freed at the end of this statement

tests/ui/span/regions-escape-loop-via-vec.stderr

@@ -23,6 +23,9 @@ LL |         _y.push(&mut z);
 error[E0597]: `z` does not live long enough
   --> $DIR/regions-escape-loop-via-vec.rs:7:17
    |
+LL |     let mut _y = vec![&mut x];
+   |         ------ variable `_y` declared here
+LL |     while x < 10 {
 LL |         let mut z = x;
    |             ----- binding `z` declared here
 LL |         _y.push(&mut z);
@@ -32,6 +35,9 @@ LL |         _y.push(&mut z);
 ...
 LL |     }
    |     - `z` dropped here while still borrowed
+   |
+   = note: `_y` is a collection that stores borrowed references, but `z` does not live long enough to be stored in it
+   = help: buffer reuse with borrowed references requires unsafe code or restructuring
 
 error[E0503]: cannot use `x` because it was mutably borrowed
   --> $DIR/regions-escape-loop-via-vec.rs:9:9