Auto merge of rust-lang#95977 - FabianWolff:issue-92790-dead-tuple, r=estebank

Warn about dead tuple struct fields

Continuation of rust-lang#92972. Fixes rust-lang#92790.

The language team has already commented on this in rust-lang#92972 (comment); I have incorporated their requests here. Specifically, there is now a new allow-by-default `unused_tuple_struct_fields` lint (name bikesheddable), and fields of unit type are ignored (rust-lang#92972 (comment)), so error messages look like this:
```
error: field is never read: `1`
  --> $DIR/tuple-struct-field.rs:6:21
   |
LL | struct Wrapper(i32, [u8; LEN], String);
   |                     ^^^^^^^^^
   |
help: change the field to unit type to suppress this warning while preserving the field numbering
   |
LL | struct Wrapper(i32, (), String);
   |                     ~~
```
r? `@joshtriplett`

Author: bors

compiler/rustc_apfloat/src/ppc.rs

@@ -30,7 +30,7 @@ pub type DoubleDouble = DoubleFloat<ieee::Double>;
 // FIXME: Implement all operations in DoubleDouble, and delete these
 // semantics.
 // FIXME(eddyb) This shouldn't need to be `pub`, it's only used in bounds.
-pub struct FallbackS<F>(F);
+pub struct FallbackS<F>(#[allow(unused)] F);
 type Fallback<F> = ieee::IeeeFloat<FallbackS<F>>;
 impl<F: Float> ieee::Semantics for FallbackS<F> {
     // Forbid any conversion to/from bits.
@@ -45,7 +45,7 @@ impl<F: Float> ieee::Semantics for FallbackS<F> {
 // truncate the mantissa. The result of that second conversion
 // may be inexact, but should never underflow.
 // FIXME(eddyb) This shouldn't need to be `pub`, it's only used in bounds.
-pub struct FallbackExtendedS<F>(F);
+pub struct FallbackExtendedS<F>(#[allow(unused)] F);
 type FallbackExtended<F> = ieee::IeeeFloat<FallbackExtendedS<F>>;
 impl<F: Float> ieee::Semantics for FallbackExtendedS<F> {
     // Forbid any conversion to/from bits.

compiler/rustc_lint_defs/src/builtin.rs

@@ -630,6 +630,32 @@ declare_lint! {
     "detects attributes that were not used by the compiler"
 }
 
+declare_lint! {
+    /// The `unused_tuple_struct_fields` lint detects fields of tuple structs
+    /// that are never read.
+    ///
+    /// ### Example
+    ///
+    /// ```
+    /// #[warn(unused_tuple_struct_fields)]
+    /// struct S(i32, i32, i32);
+    /// let s = S(1, 2, 3);
+    /// let _ = (s.0, s.2);
+    /// ```
+    ///
+    /// {{produces}}
+    ///
+    /// ### Explanation
+    ///
+    /// Tuple struct fields that are never read anywhere may indicate a
+    /// mistake or unfinished code. To silence this warning, consider
+    /// removing the unused field(s) or, to preserve the numbering of the
+    /// remaining fields, change the unused field(s) to have unit type.
+    pub UNUSED_TUPLE_STRUCT_FIELDS,
+    Allow,
+    "detects tuple struct fields that are never read"
+}
+
 declare_lint! {
     /// The `unreachable_code` lint detects unreachable code paths.
     ///
@@ -3281,6 +3307,7 @@ declare_lint_pass! {
         UNSUPPORTED_CALLING_CONVENTIONS,
         BREAK_WITH_LABEL_AND_LOOP,
         UNUSED_ATTRIBUTES,
+        UNUSED_TUPLE_STRUCT_FIELDS,
         NON_EXHAUSTIVE_OMITTED_PATTERNS,
         TEXT_DIRECTION_CODEPOINT_IN_COMMENT,
         DEREF_INTO_DYN_SUPERTRAIT,