Suggest removing unused tuple fields if they are the last fields

compiler/rustc_passes/messages.ftl

@@ -597,6 +597,15 @@ passes_pass_by_value =
 
 passes_proc_macro_bad_sig = {$kind} has incorrect signature
 
+passes_remove_fields =
+ consider removing { $num ->
+ [one] this
+ *[other] these
+ } { $num ->
+ [one] field
+ *[other] fields
+ }
+
 passes_repr_conflicting =
  conflicting representation hints
 

compiler/rustc_passes/src/dead.rs

@@ -23,8 +23,7 @@ use rustc_target::abi::FieldIdx;
 use std::mem;
 
 use crate::errors::{
- ChangeFieldsToBeOfUnitType, IgnoredDerivedImpls, MultipleDeadCodes, ParentInfo,
- UselessAssignment,
+ ChangeFields, IgnoredDerivedImpls, MultipleDeadCodes, ParentInfo, UselessAssignment,
 };
 
 // Any local node that may call something in its body block should be
@@ -993,17 +992,50 @@ impl<'tcx> DeadVisitor<'tcx> {
  };
 
  let diag = match report_on {
- ReportOn::TupleField => MultipleDeadCodes::UnusedTupleStructFields {
- multiple,
- num,
- descr,
- participle,
- name_list,
- change_fields_suggestion: ChangeFieldsToBeOfUnitType { num, spans: spans.clone() },
- parent_info,
- ignored_derived_impls,
- },
-
+ ReportOn::TupleField => {
+ let tuple_fields = if let Some(parent_id) = parent_item
+ && let node = tcx.hir_node_by_def_id(parent_id)
+ && let hir::Node::Item(hir::Item {
+ kind: hir::ItemKind::Struct(hir::VariantData::Tuple(fields, _, _), _),
+ ..
+ }) = node
+ {
+ *fields
+ } else {
+ &[]
+ };
+
+ let trailing_tuple_fields = if tuple_fields.len() >= dead_codes.len() {
+ LocalDefIdSet::from_iter(
+ tuple_fields
+ .iter()
+ .skip(tuple_fields.len() - dead_codes.len())
+ .map(|f| f.def_id),
+ )
+ } else {
+ LocalDefIdSet::default()
+ };
+
+ let fields_suggestion =
+ // Suggest removal if all tuple fields are at the end.
+ // Otherwise suggest removal or changing to unit type
+ if dead_codes.iter().all(|dc| trailing_tuple_fields.contains(&dc.def_id)) {
+ ChangeFields::Remove { num }
+ } else {
+ ChangeFields::ChangeToUnitTypeOrRemove { num, spans: spans.clone() }
+ };
+
+ MultipleDeadCodes::UnusedTupleStructFields {
+ multiple,
+ num,
+ descr,
+ participle,
+ name_list,
+ change_fields_suggestion: fields_suggestion,
+ parent_info,
+ ignored_derived_impls,
+ }
+ }
  ReportOn::NamedField => MultipleDeadCodes::DeadCodes {
  multiple,
  num,

compiler/rustc_passes/src/errors.rs

@@ -1589,7 +1589,7 @@ pub enum MultipleDeadCodes<'tcx> {
  participle: &'tcx str,
  name_list: DiagSymbolList,
  #[subdiagnostic]
- change_fields_suggestion: ChangeFieldsToBeOfUnitType,
+ change_fields_suggestion: ChangeFields,
  #[subdiagnostic]
  parent_info: Option<ParentInfo<'tcx>>,
  #[subdiagnostic]
@@ -1616,11 +1616,18 @@ pub struct IgnoredDerivedImpls {
 }
 
 #[derive(Subdiagnostic)]
-#[multipart_suggestion(passes_change_fields_to_be_of_unit_type, applicability = "has-placeholders")]
-pub struct ChangeFieldsToBeOfUnitType {
- pub num: usize,
- #[suggestion_part(code = "()")]
- pub spans: Vec<Span>,
+pub enum ChangeFields {
+ #[multipart_suggestion(
+ passes_change_fields_to_be_of_unit_type,
+ applicability = "has-placeholders"
+ )]
+ ChangeToUnitTypeOrRemove {
+ num: usize,
+ #[suggestion_part(code = "()")]
+ spans: Vec<Span>,
+ },
+ #[help(passes_remove_fields)]
+ Remove { num: usize },
 }
 
 #[derive(Diagnostic)]

tests/ui/lint/dead-code/tuple-struct-field.rs

@@ -5,15 +5,20 @@ use std::marker::PhantomData;
 
 const LEN: usize = 4;
 
-struct SingleUnused(i32, [u8; LEN], String);
-//~^ ERROR: field `1` is never read
+struct UnusedAtTheEnd(i32, f32, [u8; LEN], String, u8);
+//~^ ERROR:fields `1`, `2`, `3`, and `4` are never read
+//~| NOTE: fields in this struct
+//~| HELP: consider removing these fields
+
+struct UnusedJustOneField(i32);
+//~^ ERROR: field `0` is never read
 //~| NOTE: field in this struct
-//~| HELP: consider changing the field to be of unit type
+//~| HELP: consider removing this field
 
-struct MultipleUnused(i32, f32, String, u8);
-//~^ ERROR: fields `0`, `1`, `2`, and `3` are never read
+struct UnusedInTheMiddle(i32, f32, String, u8, u32);
+//~^ ERROR: fields `1`, `2`, and `4` are never read
 //~| NOTE: fields in this struct
-//~| HELP: consider changing the fields to be of unit type
+//~| HELP: consider changing the fields to be of unit type to suppress this warning while preserving the field numbering, or remove the fields
 
 struct GoodUnit(());
 
@@ -23,15 +28,19 @@ struct Void;
 struct GoodVoid(Void);
 
 fn main() {
- let w = SingleUnused(42, [0, 1, 2, 3], "abc".to_string());
- let _ = w.0;
- let _ = w.2;
+ let u1 = UnusedAtTheEnd(42, 3.14, [0, 1, 2, 3], "def".to_string(), 4u8);
+ let _ = u1.0;
+
+ let _ = UnusedJustOneField(42);
+
+ let u2 = UnusedInTheMiddle(42, 3.14, "def".to_string(), 4u8, 5);
+ let _ = u2.0;
+ let _ = u2.3;
 
- let m = MultipleUnused(42, 3.14, "def".to_string(), 4u8);
 
  let gu = GoodUnit(());
  let gp = GoodPhantom(PhantomData);
  let gv = GoodVoid(Void);
 
- let _ = (gu, gp, gv, m);
+ let _ = (gu, gp, gv);
 }

tests/ui/lint/dead-code/tuple-struct-field.stderr

@@ -1,33 +1,40 @@
-error: field `1` is never read
- --> $DIR/tuple-struct-field.rs:8:26
+error: fields `1`, `2`, `3`, and `4` are never read
+ --> $DIR/tuple-struct-field.rs:8:28
  |
-LL | struct SingleUnused(i32, [u8; LEN], String);
- | ------------ ^^^^^^^^^
+LL | struct UnusedAtTheEnd(i32, f32, [u8; LEN], String, u8);
+ | -------------- ^^^ ^^^^^^^^^ ^^^^^^ ^^
  | |
- | field in this struct
+ | fields in this struct
  |
+ = help: consider removing these fields
 note: the lint level is defined here
  --> $DIR/tuple-struct-field.rs:1:9
  |
 LL | #![deny(dead_code)]
  | ^^^^^^^^^
-help: consider changing the field to be of unit type to suppress this warning while preserving the field numbering, or remove the field
+
+error: field `0` is never read
+ --> $DIR/tuple-struct-field.rs:13:27
+ |
+LL | struct UnusedJustOneField(i32);
+ | ------------------ ^^^
+ | |
+ | field in this struct
  |
-LL | struct SingleUnused(i32, (), String);
- | ~~
+ = help: consider removing this field
 
-error: fields `0`, `1`, `2`, and `3` are never read
- --> $DIR/tuple-struct-field.rs:13:23
+error: fields `1`, `2`, and `4` are never read
+ --> $DIR/tuple-struct-field.rs:18:31
  |
-LL | struct MultipleUnused(i32, f32, String, u8);
- | -------------- ^^^ ^^^ ^^^^^^ ^^
+LL | struct UnusedInTheMiddle(i32, f32, String, u8, u32);
+ | ----------------- ^^^ ^^^^^^  ^^^
  | |
  | fields in this struct
  |
 help: consider changing the fields to be of unit type to suppress this warning while preserving the field numbering, or remove the fields
  |
-LL | struct MultipleUnused((), (), (), ());
- | ~~ ~~ ~~ ~~
+LL | struct UnusedInTheMiddle(i32, (), (), u8, ());
+ |  ~~ ~~  ~~
 
-error: aborting due to 2 previous errors
+error: aborting due to 3 previous errors