Point at the string inside literal and mention if we need string interpolation

modified:   compiler/rustc_passes/src/liveness.rs

	new file:   src/test/ui/type/issue-100584.rs
	new file:   src/test/ui/type/issue-100584.stderr

compiler/rustc_passes/src/liveness.rs

@@ -188,6 +188,19 @@ enum VarKind {
     Upvar(HirId, Symbol),
 }
 
+struct CollectLitsVisitor<'tcx> {
+    lit_exprs: Vec<&'tcx hir::Expr<'tcx>>,
+}
+
+impl<'tcx> Visitor<'tcx> for CollectLitsVisitor<'tcx> {
+    fn visit_expr(&mut self, expr: &'tcx Expr<'tcx>) {
+        if let hir::ExprKind::Lit(_) = expr.kind {
+            self.lit_exprs.push(expr);
+        }
+        intravisit::walk_expr(self, expr);
+    }
+}
+
 struct IrMaps<'tcx> {
     tcx: TyCtxt<'tcx>,
     live_node_map: HirIdMap<LiveNode>,
@@ -1342,7 +1355,7 @@ impl<'a, 'tcx> Liveness<'a, 'tcx> {
 
 impl<'a, 'tcx> Visitor<'tcx> for Liveness<'a, 'tcx> {
     fn visit_local(&mut self, local: &'tcx hir::Local<'tcx>) {
-        self.check_unused_vars_in_pat(&local.pat, None, |spans, hir_id, ln, var| {
+        self.check_unused_vars_in_pat(&local.pat, None, None, |spans, hir_id, ln, var| {
             if local.init.is_some() {
                 self.warn_about_dead_assign(spans, hir_id, ln, var);
             }
@@ -1357,7 +1370,7 @@ impl<'a, 'tcx> Visitor<'tcx> for Liveness<'a, 'tcx> {
     }
 
     fn visit_arm(&mut self, arm: &'tcx hir::Arm<'tcx>) {
-        self.check_unused_vars_in_pat(&arm.pat, None, |_, _, _, _| {});
+        self.check_unused_vars_in_pat(&arm.pat, None, None, |_, _, _, _| {});
         intravisit::walk_arm(self, arm);
     }
 }
@@ -1396,7 +1409,7 @@ fn check_expr<'tcx>(this: &mut Liveness<'_, 'tcx>, expr: &'tcx Expr<'tcx>) {
         }
 
         hir::ExprKind::Let(let_expr) => {
-            this.check_unused_vars_in_pat(let_expr.pat, None, |_, _, _, _| {});
+            this.check_unused_vars_in_pat(let_expr.pat, None, None, |_, _, _, _| {});
         }
 
         // no correctness conditions related to liveness
@@ -1517,13 +1530,18 @@ impl<'tcx> Liveness<'_, 'tcx> {
 
     fn warn_about_unused_args(&self, body: &hir::Body<'_>, entry_ln: LiveNode) {
         for p in body.params {
-            self.check_unused_vars_in_pat(&p.pat, Some(entry_ln), |spans, hir_id, ln, var| {
+            self.check_unused_vars_in_pat(
-                if !self.live_on_entry(ln, var) {
+                &p.pat,
-                    self.report_unused_assign(hir_id, spans, var, |name| {
+                Some(entry_ln),
-                        format!("value passed to `{}` is never read", name)
+                Some(body),
-                    });
+                |spans, hir_id, ln, var| {
-                }
+                    if !self.live_on_entry(ln, var) {
-            });
+                        self.report_unused_assign(hir_id, spans, var, |name| {
+                            format!("value passed to `{}` is never read", name)
+                        });
+                    }
+                },
+            );
         }
     }
 
@@ -1531,6 +1549,7 @@ impl<'tcx> Liveness<'_, 'tcx> {
         &self,
         pat: &hir::Pat<'_>,
         entry_ln: Option<LiveNode>,
+        opt_body: Option<&hir::Body<'_>>,
         on_used_on_entry: impl Fn(Vec<Span>, HirId, LiveNode, Variable),
     ) {
         // In an or-pattern, only consider the variable; any later patterns must have the same
@@ -1558,7 +1577,7 @@ impl<'tcx> Liveness<'_, 'tcx> {
                     hir_ids_and_spans.into_iter().map(|(_, _, ident_span)| ident_span).collect();
                 on_used_on_entry(spans, id, ln, var);
             } else {
-                self.report_unused(hir_ids_and_spans, ln, var, can_remove);
+                self.report_unused(hir_ids_and_spans, ln, var, can_remove, pat, opt_body);
             }
         }
     }
@@ -1570,6 +1589,8 @@ impl<'tcx> Liveness<'_, 'tcx> {
         ln: LiveNode,
         var: Variable,
         can_remove: bool,
+        pat: &hir::Pat<'_>,
+        opt_body: Option<&hir::Body<'_>>,
     ) {
         let first_hir_id = hir_ids_and_spans[0].0;
 
@@ -1673,6 +1694,9 @@ impl<'tcx> Liveness<'_, 'tcx> {
                             .collect::<Vec<_>>(),
                         |lint| {
                             let mut err = lint.build(&format!("unused variable: `{}`", name));
+                            if self.has_added_lit_match_name_span(&name, opt_body, &mut err) {
+                                err.span_label(pat.span, "unused variable");
+                            }
                             err.multipart_suggestion(
                                 "if this is intentional, prefix it with an underscore",
                                 non_shorthands,
@@ -1686,6 +1710,42 @@ impl<'tcx> Liveness<'_, 'tcx> {
         }
     }
 
+    fn has_added_lit_match_name_span(
+        &self,
+        name: &str,
+        opt_body: Option<&hir::Body<'_>>,
+        err: &mut rustc_errors::DiagnosticBuilder<'_, ()>,
+    ) -> bool {
+        let mut has_litstring = false;
+        let Some(opt_body) = opt_body else {return false;};
+        let mut visitor = CollectLitsVisitor { lit_exprs: vec![] };
+        intravisit::walk_body(&mut visitor, opt_body);
+        for lit_expr in visitor.lit_exprs {
+            let hir::ExprKind::Lit(litx) = &lit_expr.kind else { continue };
+            let rustc_ast::LitKind::Str(syb, _) = litx.node else{ continue; };
+            let name_str: &str = syb.as_str();
+            let mut name_pa = String::from("{");
+            name_pa.push_str(&name);
+            name_pa.push('}');
+            if name_str.contains(&name_pa) {
+                err.span_label(
+                    lit_expr.span,
+                    "you might have meant to use string interpolation in this string literal",
+                );
+                err.multipart_suggestion(
+                    "string interpolation only works in `format!` invocations",
+                    vec![
+                        (lit_expr.span.shrink_to_lo(), "format!(".to_string()),
+                        (lit_expr.span.shrink_to_hi(), ")".to_string()),
+                    ],
+                    Applicability::MachineApplicable,
+                );
+                has_litstring = true;
+            }
+        }
+        has_litstring
+    }
+
     fn warn_about_dead_assign(&self, spans: Vec<Span>, hir_id: HirId, ln: LiveNode, var: Variable) {
         if !self.live_on_exit(ln, var) {
             self.report_unused_assign(hir_id, spans, var, |name| {

src/test/ui/type/issue-100584.rs

@@ -0,0 +1,15 @@
+#![deny(unused)]
+fn foo(xyza: &str) {
+//~^ ERROR unused variable: `xyza`
+    let _ = "{xyza}";
+}
+
+fn foo3(xyza: &str) {
+//~^ ERROR unused variable: `xyza`
+    let _ = "aaa{xyza}bbb";
+}
+
+fn main() {
+  foo("x");
+  foo3("xx");
+}

src/test/ui/type/issue-100584.stderr

@@ -0,0 +1,44 @@
+error: unused variable: `xyza`
+  --> $DIR/issue-100584.rs:2:8
+   |
+LL | fn foo(xyza: &str) {
+   |        ^^^^ unused variable
+LL |
+LL |     let _ = "{xyza}";
+   |             -------- you might have meant to use string interpolation in this string literal
+   |
+note: the lint level is defined here
+  --> $DIR/issue-100584.rs:1:9
+   |
+LL | #![deny(unused)]
+   |         ^^^^^^
+   = note: `#[deny(unused_variables)]` implied by `#[deny(unused)]`
+help: string interpolation only works in `format!` invocations
+   |
+LL |     let _ = format!("{xyza}");
+   |             ++++++++        +
+help: if this is intentional, prefix it with an underscore
+   |
+LL | fn foo(_xyza: &str) {
+   |        ~~~~~
+
+error: unused variable: `xyza`
+  --> $DIR/issue-100584.rs:7:9
+   |
+LL | fn foo3(xyza: &str) {
+   |         ^^^^ unused variable
+LL |
+LL |     let _ = "aaa{xyza}bbb";
+   |             -------------- you might have meant to use string interpolation in this string literal
+   |
+help: string interpolation only works in `format!` invocations
+   |
+LL |     let _ = format!("aaa{xyza}bbb");
+   |             ++++++++              +
+help: if this is intentional, prefix it with an underscore
+   |
+LL | fn foo3(_xyza: &str) {
+   |         ~~~~~
+
+error: aborting due to 2 previous errors
+