Label overwritten assignments for never read assignments

compiler/rustc_mir_transform/src/errors.rs

@@ -218,11 +218,29 @@ pub(crate) struct UnusedVarAssignedOnly {
 pub(crate) struct UnusedAssign {
     pub name: Symbol,
     #[subdiagnostic]
+    pub overwrite: Option<UnusedAssignOverwrite>,
+    #[subdiagnostic]
     pub suggestion: Option<UnusedAssignSuggestion>,
     #[help("maybe it is overwritten before being read?")]
     pub help: bool,
 }
 
+pub(crate) struct UnusedAssignOverwrite {
+    pub assigned_span: Span,
+    pub overwrite_span: Span,
+    pub name: Symbol,
+}
+
+impl Subdiagnostic for UnusedAssignOverwrite {
+    fn add_to_diag<G: EmissionGuarantee>(self, diag: &mut Diag<'_, G>) {
+        diag.span_label(self.assigned_span, "this value is reassigned later and never used");
+        diag.span_label(
+            self.overwrite_span,
+            format!("`{}` is overwritten here before the previous value is read", self.name),
+        );
+    }
+}
+
 #[derive(Subdiagnostic)]
 #[multipart_suggestion(
     "you might have meant to mutate the pointed at value being passed in, instead of changing the reference in the local binding",

compiler/rustc_mir_transform/src/liveness.rs

@@ -1094,29 +1094,49 @@ fn report_unused_assignments(self) {
                 self.body,
             );
 
-            // We probed MIR in reverse order for dataflow.
-            // We revert the vector to give a consistent order to the user.
-            for (source_info, Access { live, kind, is_direct }) in statements.into_iter().rev() {
+            // By convention, underscore-prefixed bindings are allowed to be unused explicitly.
+            if name.as_str().starts_with('_') {
+                continue;
+            }
+
+            let mut next_direct_assign = None;
+            let mut dead_statements = Vec::with_capacity(statements.len());
+
+            for (source_info, Access { live, kind, is_direct }) in statements.into_iter() {
+                let overwrite = match (kind, is_direct, next_direct_assign) {
+                    (AccessKind::Assign, true, Some(overwrite_span)) => {
+                        Some(errors::UnusedAssignOverwrite {
+                            assigned_span: source_info.span,
+                            overwrite_span,
+                            name,
+                        })
+                    }
+                    _ => None,
+                };
+
+                if kind == AccessKind::Assign && is_direct {
+                    next_direct_assign = Some(source_info.span);
+                }
+
                 if live {
                     continue;
                 }
-
                 // If this place was dropped and has non-trivial drop,
                 // skip reporting field assignments.
                 if !is_direct && is_maybe_drop_guard {
                     continue;
                 }
+                dead_statements.push((source_info, kind, is_direct, overwrite));
+            }
 
+            // We probed MIR in reverse order for dataflow.
+            // Emit diagnostics in source order instead.
+            for (source_info, kind, is_direct, overwrite) in dead_statements.into_iter().rev() {
                 // Report the dead assignment.
                 let Some(hir_id) = source_info.scope.lint_root(&self.body.source_scopes) else {
                     continue;
                 };
 
-                // By convention, underscore-prefixed bindings are allowed to be unused explicitly
-                if name.as_str().starts_with('_') {
-                    break;
-                }
-
                 match kind {
                     AccessKind::Assign => {
                         let suggestion = annotate_mut_binding_to_immutable_binding(
@@ -1126,11 +1146,14 @@ fn report_unused_assignments(self) {
                             source_info.span,
                             self.body,
                         );
+                        let overwrite =
+                            if suggestion.is_none() && is_direct { overwrite } else { None };
+                        let help = suggestion.is_none() && overwrite.is_none();
                         tcx.emit_node_span_lint(
                             lint::builtin::UNUSED_ASSIGNMENTS,
                             hir_id,
                             source_info.span,
-                            errors::UnusedAssign { name, help: suggestion.is_none(), suggestion },
+                            errors::UnusedAssign { name, overwrite, help, suggestion },
                         )
                     }
                     AccessKind::Param => tcx.emit_node_span_lint(

tests/ui/closures/2229_closure_analysis/diagnostics/liveness.stderr

@@ -80,9 +80,10 @@ warning: value assigned to `e.x` is never read
   --> $DIR/liveness.rs:74:13
    |
 LL |             e.x = Some("e1");
-   |             ^^^^^^^^^^^^^^^^
-   |
-   = help: maybe it is overwritten before being read?
+   |             ^^^^^^^^^^^^^^^^ this value is reassigned later and never used
+LL |
+LL |             e.x = Some("e2");
+   |             ---------------- `e.x` is overwritten here before the previous value is read
 
 warning: value assigned to `e.x` is never read
   --> $DIR/liveness.rs:76:13
@@ -104,9 +105,10 @@ warning: value assigned to `b` is never read
   --> $DIR/liveness.rs:77:13
    |
 LL |             b = Some("e1");
-   |             ^^^^^^^^^^^^^^
-   |
-   = help: maybe it is overwritten before being read?
+   |             ^^^^^^^^^^^^^^ this value is reassigned later and never used
+LL |
+LL |             b = Some("e2");
+   |             -------------- `b` is overwritten here before the previous value is read
 
 warning: value assigned to `b` is never read
   --> $DIR/liveness.rs:79:13
@@ -120,17 +122,17 @@ warning: value assigned to `d.x` is never read
   --> $DIR/liveness.rs:68:13
    |
 LL |             d.x = Some("d1");
-   |             ^^^^^^^^^^^^^^^^
-   |
-   = help: maybe it is overwritten before being read?
+   |             ^^^^^^^^^^^^^^^^ this value is reassigned later and never used
+LL |             d.x = Some("d2");
+   |             ---------------- `d.x` is overwritten here before the previous value is read
 
 warning: value assigned to `a` is never read
   --> $DIR/liveness.rs:70:13
    |
 LL |             a = Some("d1");
-   |             ^^^^^^^^^^^^^^
-   |
-   = help: maybe it is overwritten before being read?
+   |             ^^^^^^^^^^^^^^ this value is reassigned later and never used
+LL |             a = Some("d2");
+   |             -------------- `a` is overwritten here before the previous value is read
 
 warning: 16 warnings emitted
 

tests/ui/destructuring-assignment/warn-unused-duplication.stderr

@@ -2,9 +2,10 @@ warning: value assigned to `a` is never read
   --> $DIR/warn-unused-duplication.rs:9:6
    |
 LL |     (a, a) = (0, 1);
-   |      ^
+   |      ^  - `a` is overwritten here before the previous value is read
+   |      |
+   |      this value is reassigned later and never used
    |
-   = help: maybe it is overwritten before being read?
 note: the lint level is defined here
   --> $DIR/warn-unused-duplication.rs:3:9
    |

tests/ui/lint/unused/overwritten-before-read-note-issue-154434.rs

@@ -0,0 +1,9 @@
+//@ check-pass
+//@ compile-flags: -W unused-assignments
+
+fn main() {
+    let mut d = String::from("hello"); //~ WARN value assigned to `d` is never read
+    d = String::from("ahoy");
+
+    println!("{d}, world!");
+}

tests/ui/lint/unused/overwritten-before-read-note-issue-154434.stderr

@@ -0,0 +1,12 @@
+warning: value assigned to `d` is never read
+  --> $DIR/overwritten-before-read-note-issue-154434.rs:5:17
+   |
+LL |     let mut d = String::from("hello");
+   |                 ^^^^^^^^^^^^^^^^^^^^^ this value is reassigned later and never used
+LL |     d = String::from("ahoy");
+   |     - `d` is overwritten here before the previous value is read
+   |
+   = note: requested on the command line with `-W unused-assignments`
+
+warning: 1 warning emitted
+

tests/ui/lint/unused/unused-assign-148960.stderr

@@ -2,9 +2,10 @@ error: value assigned to `value` is never read
   --> $DIR/unused-assign-148960.rs:6:21
    |
 LL |     let mut value = b"0".to_vec();
-   |                     ^^^^^^^^^^^^^
+   |                     ^^^^^^^^^^^^^ this value is reassigned later and never used
+LL |     value = b"1".to_vec();
+   |     ----- `value` is overwritten here before the previous value is read
    |
-   = help: maybe it is overwritten before being read?
 note: the lint level is defined here
   --> $DIR/unused-assign-148960.rs:2:9
    |
@@ -16,25 +17,25 @@ error: value assigned to `x` is never read
   --> $DIR/unused-assign-148960.rs:12:17
    |
 LL |     let mut x = 1;
-   |                 ^
-   |
-   = help: maybe it is overwritten before being read?
+   |                 ^ this value is reassigned later and never used
+LL |     x = 2;
+   |     ----- `x` is overwritten here before the previous value is read
 
 error: value assigned to `x` is never read
   --> $DIR/unused-assign-148960.rs:13:5
    |
 LL |     x = 2;
-   |     ^^^^^
-   |
-   = help: maybe it is overwritten before being read?
+   |     ^^^^^ this value is reassigned later and never used
+LL |     x = 3;
+   |     ----- `x` is overwritten here before the previous value is read
 
 error: value assigned to `p` is never read
   --> $DIR/unused-assign-148960.rs:24:17
    |
 LL |     let mut p = Point { x: 1, y: 1 };
-   |                 ^^^^^^^^^^^^^^^^^^^^
-   |
-   = help: maybe it is overwritten before being read?
+   |                 ^^^^^^^^^^^^^^^^^^^^ this value is reassigned later and never used
+LL |     p = Point { x: 2, y: 2 };
+   |     ------------------------ `p` is overwritten here before the previous value is read
 
 error: variable `foo` is assigned to, but never used
   --> $DIR/unused-assign-148960.rs:38:9

tests/ui/liveness/liveness-consts.stderr

@@ -44,17 +44,17 @@ warning: value assigned to `b` is never read
   --> $DIR/liveness-consts.rs:18:17
    |
 LL |     let mut b = 1;
-   |                 ^
-   |
-   = help: maybe it is overwritten before being read?
+   |                 ^ this value is reassigned later and never used
+LL |     b += 1;
+   |     ------ `b` is overwritten here before the previous value is read
 
 warning: value assigned to `b` is never read
   --> $DIR/liveness-consts.rs:19:5
    |
 LL |     b += 1;
-   |     ^^^^^^
-   |
-   = help: maybe it is overwritten before being read?
+   |     ^^^^^^ this value is reassigned later and never used
+LL |     b = 42;
+   |     ------ `b` is overwritten here before the previous value is read
 
 warning: unused variable: `z`
   --> $DIR/liveness-consts.rs:62:13

tests/ui/liveness/liveness-dead.stderr

@@ -2,9 +2,10 @@ error: value assigned to `x` is never read
   --> $DIR/liveness-dead.rs:9:24
    |
 LL |     let mut x: isize = 3;
-   |                        ^
+   |                        ^ this value is reassigned later and never used
+LL |     x = 4;
+   |     ----- `x` is overwritten here before the previous value is read
    |
-   = help: maybe it is overwritten before being read?
 note: the lint level is defined here
   --> $DIR/liveness-dead.rs:2:9
    |

tests/ui/liveness/liveness-unused.stderr

@@ -167,49 +167,55 @@ error: value assigned to `a` is never read
   --> $DIR/liveness-unused.rs:183:5
    |
 LL |     a += b;
-   |     ^^^^^^
-   |
-   = help: maybe it is overwritten before being read?
+   |     ^^^^^^ this value is reassigned later and never used
+LL |
+LL |     a -= c;
+   |     ------ `a` is overwritten here before the previous value is read
 
 error: value assigned to `a` is never read
   --> $DIR/liveness-unused.rs:185:5
    |
 LL |     a -= c;
-   |     ^^^^^^
-   |
-   = help: maybe it is overwritten before being read?
+   |     ^^^^^^ this value is reassigned later and never used
+LL |
+LL |     a *= d;
+   |     ------ `a` is overwritten here before the previous value is read
 
 error: value assigned to `a` is never read
   --> $DIR/liveness-unused.rs:187:5
    |
 LL |     a *= d;
-   |     ^^^^^^
-   |
-   = help: maybe it is overwritten before being read?
+   |     ^^^^^^ this value is reassigned later and never used
+LL |
+LL |     a /= e;
+   |     ------ `a` is overwritten here before the previous value is read
 
 error: value assigned to `a` is never read
   --> $DIR/liveness-unused.rs:189:5
    |
 LL |     a /= e;
-   |     ^^^^^^
-   |
-   = help: maybe it is overwritten before being read?
+   |     ^^^^^^ this value is reassigned later and never used
+LL |
+LL |     a |= f;
+   |     ------ `a` is overwritten here before the previous value is read
 
 error: value assigned to `a` is never read
   --> $DIR/liveness-unused.rs:191:5
    |
 LL |     a |= f;
-   |     ^^^^^^
-   |
-   = help: maybe it is overwritten before being read?
+   |     ^^^^^^ this value is reassigned later and never used
+LL |
+LL |     a &= g;
+   |     ------ `a` is overwritten here before the previous value is read
 
 error: value assigned to `a` is never read
   --> $DIR/liveness-unused.rs:193:5
    |
 LL |     a &= g;
-   |     ^^^^^^
-   |
-   = help: maybe it is overwritten before being read?
+   |     ^^^^^^ this value is reassigned later and never used
+LL |
+LL |     a %= h;
+   |     ------ `a` is overwritten here before the previous value is read
 
 error: value assigned to `a` is never read
   --> $DIR/liveness-unused.rs:195:5
@@ -231,33 +237,37 @@ error: value assigned to `a` is never read
   --> $DIR/liveness-unused.rs:229:5
    |
 LL |     a += b;
-   |     ^^^^^^
-   |
-   = help: maybe it is overwritten before being read?
+   |     ^^^^^^ this value is reassigned later and never used
+LL |
+LL |     a -= c;
+   |     ------ `a` is overwritten here before the previous value is read
 
 error: value assigned to `a` is never read
   --> $DIR/liveness-unused.rs:231:5
    |
 LL |     a -= c;
-   |     ^^^^^^
-   |
-   = help: maybe it is overwritten before being read?
+   |     ^^^^^^ this value is reassigned later and never used
+LL |
+LL |     a *= d;
+   |     ------ `a` is overwritten here before the previous value is read
 
 error: value assigned to `a` is never read
   --> $DIR/liveness-unused.rs:233:5
    |
 LL |     a *= d;
-   |     ^^^^^^
-   |
-   = help: maybe it is overwritten before being read?
+   |     ^^^^^^ this value is reassigned later and never used
+LL |
+LL |     a /= e;
+   |     ------ `a` is overwritten here before the previous value is read
 
 error: value assigned to `a` is never read
   --> $DIR/liveness-unused.rs:235:5
    |
 LL |     a /= e;
-   |     ^^^^^^
-   |
-   = help: maybe it is overwritten before being read?
+   |     ^^^^^^ this value is reassigned later and never used
+LL |
+LL |     a %= f;
+   |     ------ `a` is overwritten here before the previous value is read
 
 error: value assigned to `a` is never read
   --> $DIR/liveness-unused.rs:237:5

tests/ui/liveness/liveness-upvars.stderr

@@ -112,9 +112,10 @@ warning: value assigned to `e` is never read
   --> $DIR/liveness-upvars.rs:82:13
    |
 LL |             e = Some("e1");
-   |             ^^^^^^^^^^^^^^
-   |
-   = help: maybe it is overwritten before being read?
+   |             ^^^^^^^^^^^^^^ this value is reassigned later and never used
+LL |
+LL |             e = Some("e2");
+   |             -------------- `e` is overwritten here before the previous value is read
 
 warning: value assigned to `e` is never read
   --> $DIR/liveness-upvars.rs:84:13
@@ -128,9 +129,9 @@ warning: value assigned to `d` is never read
   --> $DIR/liveness-upvars.rs:78:13
    |
 LL |             d = Some("d1");
-   |             ^^^^^^^^^^^^^^
-   |
-   = help: maybe it is overwritten before being read?
+   |             ^^^^^^^^^^^^^^ this value is reassigned later and never used
+LL |             d = Some("d2");
+   |             -------------- `d` is overwritten here before the previous value is read
 
 warning: value assigned to `v` is never read
   --> $DIR/liveness-upvars.rs:92:13
@@ -176,9 +177,10 @@ warning: value assigned to `state` is never read
   --> $DIR/liveness-upvars.rs:131:9
    |
 LL |         state = 4;
-   |         ^^^^^^^^^
-   |
-   = help: maybe it is overwritten before being read?
+   |         ^^^^^^^^^ this value is reassigned later and never used
+...
+LL |         state = 5;
+   |         --------- `state` is overwritten here before the previous value is read
 
 warning: value assigned to `state` is never read
   --> $DIR/liveness-upvars.rs:134:9
@@ -192,17 +194,17 @@ warning: value assigned to `s` is never read
   --> $DIR/liveness-upvars.rs:143:9
    |
 LL |         s = 1;
-   |         ^^^^^
-   |
-   = help: maybe it is overwritten before being read?
+   |         ^^^^^ this value is reassigned later and never used
+LL |         yield (s = 2);
+   |               ------- `s` is overwritten here before the previous value is read
 
 warning: value assigned to `s` is never read
   --> $DIR/liveness-upvars.rs:145:9
    |
 LL |         s = yield ();
-   |         ^^^^^^^^^^^^
-   |
-   = help: maybe it is overwritten before being read?
+   |         ^^^^^^^^^^^^ this value is reassigned later and never used
+LL |         s = 3;
+   |         ----- `s` is overwritten here before the previous value is read
 
 warning: 25 warnings emitted
 

tests/ui/rfcs/rfc-2005-default-binding-mode/general.stderr

@@ -2,9 +2,11 @@ warning: value assigned to `b` is never read
   --> $DIR/general.rs:60:25
    |
 LL |         (Some(a), &Some(mut b)) => {
-   |                         ^^^^^
+   |                         ^^^^^ this value is reassigned later and never used
+...
+LL |             b = 7;
+   |             ----- `b` is overwritten here before the previous value is read
    |
-   = help: maybe it is overwritten before being read?
    = note: `#[warn(unused_assignments)]` (part of `#[warn(unused)]`) on by default
 
 warning: 1 warning emitted