Using index's span when inference of recv failed when calling method on result of index op

Author: mu001999

compiler/rustc_hir_typeck/src/expr.rs

@@ -12,6 +12,7 @@ use crate::errors::{
     FieldMultiplySpecifiedInInitializer, FunctionalRecordUpdateOnNonStruct, HelpUseLatestEdition,
     YieldExprOutsideOfCoroutine,
 };
+use crate::expr_use_visitor::TypeInformationCtxt;
 use crate::fatally_break_rust;
 use crate::type_error_struct;
 use crate::CoroutineTypes;
@@ -1329,7 +1330,13 @@ impl<'a, 'tcx> FnCtxt<'a, 'tcx> {
     ) -> Ty<'tcx> {
         let rcvr_t = self.check_expr(rcvr);
         // no need to check for bot/err -- callee does that
-        let rcvr_t = self.structurally_resolve_type(rcvr.span, rcvr_t);
+        let rcvr_t = if let ExprKind::Index(_, index, _) = rcvr.kind
+            && self.typeck_results().expr_ty(index).is_ty_var()
+        {
+            self.structurally_resolve_type(index.span, rcvr_t)
+        } else {
+            self.structurally_resolve_type(rcvr.span, rcvr_t)
+        };
 
         let method = match self.lookup_method(rcvr_t, segment, segment.ident.span, expr, rcvr, args)
         {

tests/ui/inference/need_type_info/calling-method-on-result-of-index-op-issue-125924.rs

@@ -0,0 +1,18 @@
+struct Foo;
+
+impl Foo {
+    fn foo(&self) {}
+}
+
+fn _bar() {
+    let x = vec![]; //~ ERROR type annotations needed for `Vec<_>`
+    x[0usize].foo();
+}
+
+fn main() {
+    let thing: Vec<Foo> = vec![];
+
+    let foo = |i| {
+        thing[i].foo(); //~ ERROR type annotations needed
+    };
+}

tests/ui/inference/need_type_info/calling-method-on-result-of-index-op-issue-125924.stderr

@@ -0,0 +1,22 @@
+error[E0282]: type annotations needed for `Vec<_>`
+  --> $DIR/calling-method-on-result-of-index-op-issue-125924.rs:8:9
+   |
+LL |     let x = vec![];
+   |         ^
+LL |     x[0usize].foo();
+   |     --------- type must be known at this point
+   |
+help: consider giving `x` an explicit type, where the placeholders `_` are specified
+   |
+LL |     let x: Vec<_> = vec![];
+   |          ++++++++
+
+error[E0282]: type annotations needed
+  --> $DIR/calling-method-on-result-of-index-op-issue-125924.rs:16:15
+   |
+LL |         thing[i].foo();
+   |               ^ cannot infer type
+
+error: aborting due to 2 previous errors
+
+For more information about this error, try `rustc --explain E0282`.