Implement tuple array diagnostic

Co-authored-by: David Wood <Q0KPU0H1YOEPHRY1R2SN5B5RL@david.davidtw.co>

compiler/rustc_typeck/src/check/callee.rs

@@ -307,6 +307,36 @@ impl<'a, 'tcx> FnCtxt<'a, 'tcx> {
         }
     }
 
+    /// Give appropriate suggestion when encountering `[("a", 0) ("b", 1)]`, where the
+    /// likely intention is to create an array containing tuples.
+    fn maybe_suggest_bad_array_definition(
+        &self,
+        err: &mut DiagnosticBuilder<'a>,
+        call_expr: &'tcx hir::Expr<'tcx>,
+        callee_expr: &'tcx hir::Expr<'tcx>,
+    ) -> bool {
+        let hir_id = self.tcx.hir().get_parent_node(call_expr.hir_id);
+        let parent_node = self.tcx.hir().get(hir_id);
+        if let (
+            hir::Node::Expr(hir::Expr { kind: hir::ExprKind::Array(_), .. }),
+            hir::ExprKind::Tup(exp),
+            hir::ExprKind::Call(_, args),
+        ) = (parent_node, &callee_expr.kind, &call_expr.kind)
+        {
+            if args.len() == exp.len() {
+                let start = callee_expr.span.shrink_to_hi();
+                err.span_suggestion(
+                    start,
+                    "consider separating array elements with a comma",
+                    ",".to_string(),
+                    Applicability::MaybeIncorrect,
+                );
+                return true;
+            }
+        }
+        false
+    }
+
     fn confirm_builtin_call(
         &self,
         call_expr: &'tcx hir::Expr<'tcx>,
@@ -422,7 +452,9 @@ impl<'a, 'tcx> FnCtxt<'a, 'tcx> {
                     _ => Res::Err,
                 };
 
-                err.span_label(call_expr.span, "call expression requires function");
+                if !self.maybe_suggest_bad_array_definition(&mut err, call_expr, callee_expr) {
+                    err.span_label(call_expr.span, "call expression requires function");
+                }
 
                 if let Some(span) = self.tcx.hir().res_span(def) {
                     let callee_ty = callee_ty.to_string();

src/test/ui/issues/issue-5100.stderr

@@ -59,10 +59,8 @@ LL |         &(true, false) => ()
 error[E0618]: expected function, found `(char, char)`
   --> $DIR/issue-5100.rs:48:14
    |
-LL |       let v = [('a', 'b')
+LL |     let v = [('a', 'b')
-   |  ______________-^^^^^^^^^
+   |              ^^^^^^^^^^- help: consider separating array elements with a comma: `,`
-LL | |              ('c', 'd'),
-   | |_______________________- call expression requires function
 
 error[E0308]: mismatched types
   --> $DIR/issue-5100.rs:55:19

src/test/ui/tuple/array-diagnostics.rs

@@ -0,0 +1,7 @@
+fn main() {
+    let _tmp = [
+        ("C200B40A82", 3),
+        ("C200B40A83", 4) //~ ERROR: expected function, found `(&'static str, {integer})` [E0618]
+        ("C200B40A8537", 5),
+    ];
+}

src/test/ui/tuple/array-diagnostics.stderr

@@ -0,0 +1,9 @@
+error[E0618]: expected function, found `(&'static str, {integer})`
+  --> $DIR/array-diagnostics.rs:4:9
+   |
+LL |         ("C200B40A83", 4)
+   |         ^^^^^^^^^^^^^^^^^- help: consider separating array elements with a comma: `,`
+
+error: aborting due to previous error
+
+For more information about this error, try `rustc --explain E0618`.