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Improve diag when calling method on result of index op #125985

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Improve diag when calling method on result of index op #125985

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9 changes: 8 additions & 1 deletion compiler/rustc_hir_typeck/src/expr.rs
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Original file line number Diff line number Diff line change
Expand Up @@ -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;
Expand Down Expand Up @@ -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)
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@compiler-errors compiler-errors Jun 5, 2024 •
edited
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This is not correct in case the ambiguity is not due to the index expression:

fn main() {
    let x = vec![];
    x[0usize].method();
}
error[E0282]: type annotations needed for `Vec<_>`
 --> /home/michael/test.rs:2:9
  |
2 |     let x = vec![];
  |         ^
3 |     x[0usize].method();
  |       ------ type must be known at this point

See above how after this PR, we now underline the 0usize even though it is not ambiguous, and there is a single implementation that satisfies the index operator.

Again, I really don't think we should make this change in such a special case.

Systematically determining why the method receiver is ambiguous due to inference is difficult, and I'd rather not patch a single case since it doesn't seem to me more special than any other example.

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How about the new change?

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This is not correct for cases where the key's ambiguity doesn't affect the value's ambiguity. Consider HashMap<i32, _>.

} else {
self.structurally_resolve_type(rcvr.span, rcvr_t)
};

let method = match self.lookup_method(rcvr_t, segment, segment.ident.span, expr, rcvr, args)
{
Expand Down
18 changes: 18 additions & 0 deletions tests/ui/inference/need_type_info/calling-method-on-result-of-index-op-issue-125924.rs
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Original file line number Diff line number Diff line change
@@ -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
};
}
22 changes: 22 additions & 0 deletions tests/ui/inference/need_type_info/calling-method-on-result-of-index-op-issue-125924.stderr
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Original file line number Diff line number Diff line change
@@ -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`.
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