lint against repeated repr attributes

compiler/rustc_hir/src/attrs/data_structures.rs

@@ -171,7 +171,19 @@ impl OptimizeAttr {
     }
 }
 
-#[derive(PartialEq, Debug, Encodable, Decodable, Copy, Clone, StableHash, PrintAttribute)]
+#[derive(
+    PartialEq,
+    Eq,
+    Debug,
+    PartialOrd,
+    Ord,
+    Encodable,
+    Decodable,
+    Copy,
+    Clone,
+    StableHash,
+    PrintAttribute
+)]
 pub enum ReprAttr {
     ReprInt(IntType),
     ReprRust,
@@ -187,7 +199,7 @@ pub enum TransparencyError {
     MultipleTransparencyAttrs(Span, Span),
 }
 
-#[derive(Eq, PartialEq, Debug, Copy, Clone)]
+#[derive(Eq, PartialEq, Debug, Copy, Clone, PartialOrd, Ord)]
 #[derive(Encodable, Decodable, StableHash, PrintAttribute)]
 pub enum IntType {
     SignedInt(ast::IntTy),

compiler/rustc_lint_defs/src/builtin.rs

@@ -272,6 +272,30 @@ declare_lint! {
     };
 }
 
+declare_lint! {
+    /// The `repeated_reprs` lint detects when the same representation is
+    /// specified more than once in a `#[repr(..)]` attribute.
+    ///
+    /// ### Example
+    ///
+    /// ```rust
+    /// #[repr(C)]
+    /// #[repr(C)]
+    /// enum Foo { A }
+    /// ```
+    ///
+    /// {{produces}}
+    ///
+    /// ### Explanation
+    ///
+    /// While some representations may be specified more than once, the compiler
+    /// will reject repeated uses of some others. For consistency, prefer to
+    /// only specify the representation once.
+    pub REPEATED_REPRS,
+    Warn,
+    "detects repeated representations in `#[repr(..)]` attributes",
+}
+
 declare_lint! {
     /// The `meta_variable_misuse` lint detects possible meta-variable misuse
     /// in macro definitions.

compiler/rustc_passes/src/check_attr.rs

@@ -40,7 +40,8 @@ use rustc_session::errors::feature_err;
 use rustc_session::lint;
 use rustc_session::lint::builtin::{
     CONFLICTING_REPR_HINTS, INVALID_DOC_ATTRIBUTES, MALFORMED_DIAGNOSTIC_ATTRIBUTES,
-    MALFORMED_DIAGNOSTIC_FORMAT_LITERALS, MISPLACED_DIAGNOSTIC_ATTRIBUTES, UNUSED_ATTRIBUTES,
+    MALFORMED_DIAGNOSTIC_FORMAT_LITERALS, MISPLACED_DIAGNOSTIC_ATTRIBUTES, REPEATED_REPRS,
+    UNUSED_ATTRIBUTES,
 };
 use rustc_span::edition::Edition;
 use rustc_span::{DUMMY_SP, Ident, Span, Symbol, sym};
@@ -1236,20 +1237,49 @@ impl<'tcx> CheckAttrVisitor<'tcx> {
                 ReprAttr::ReprC => {
                     is_c = true;
                 }
-                ReprAttr::ReprAlign(..) => {}
-                ReprAttr::ReprPacked(_) => {}
+                ReprAttr::ReprAlign(..) => (),
+                ReprAttr::ReprPacked(..) => (),
                 ReprAttr::ReprSimd => {
                     is_simd = true;
                 }
                 ReprAttr::ReprTransparent => {
                     is_transparent = true;
                 }
-                ReprAttr::ReprInt(_) => {
+                ReprAttr::ReprInt(..) => {
                     int_reprs += 1;
                 }
             };
         }
 
+        if !reprs.is_empty() {
+            let sorted_reprs = {
+                let mut to_sort = reprs.to_owned();
+                to_sort.sort();
+                to_sort
+            };
+
+            // To collect all duplicates, get subslices where all of the elements of the subslice
+            // are equal, then filter out all those whose length is not 1. We could return warnings
+            // for each of them, but that's annoyingly excessive. So we instead collect all spans in
+            // one big Vec.
+            let spans: Vec<Span> = sorted_reprs
+                .chunk_by(|(a, _), (b, _)| a == b)
+                .map(ToOwned::to_owned)
+                .filter(|slice| slice.len() != 1)
+                .flatten()
+                .map(|(_, span)| span)
+                .collect();
+
+            if !spans.is_empty() {
+                self.tcx.emit_node_span_lint(
+                    REPEATED_REPRS,
+                    hir_id,
+                    spans,
+                    diagnostics::RepeatedRepr,
+                );
+            }
+        }
+
         // Just point at all repr hints if there are any incompatibilities.
         // This is not ideal, but tracking precisely which ones are at fault is a huge hassle.
         let hint_spans = reprs.iter().map(|(_, span)| *span);

compiler/rustc_passes/src/diagnostics.rs

@@ -695,6 +695,11 @@ pub(crate) struct TransparentIncompatible {
     pub target: String,
 }
 
+#[derive(Diagnostic)]
+#[diag("`#[repr(..)]` attribute is specified more than once")]
+#[note("for consistency, only specify the representation once")]
+pub(crate) struct RepeatedRepr;
+
 #[derive(Diagnostic)]
 #[diag("deprecated attribute must be paired with either stable or unstable attribute", code = E0549)]
 pub(crate) struct DeprecatedAttribute {

tests/ui/attributes/issue-100631.rs

@@ -2,6 +2,7 @@
 // can reasonably deal with multiple attributes.
 // `repr` will use `TyCtxt::get_attrs` since it's `DuplicatesOk`.
 #[repr(C)] //~ ERROR: unsupported representation for zero-variant enum [E0084]
+//~^ WARN `#[repr(..)]` attribute is specified more than once [repeated_reprs]
 #[repr(C)]
 enum Foo {}
 

tests/ui/attributes/issue-100631.stderr

@@ -1,12 +1,24 @@
-error[E0084]: unsupported representation for zero-variant enum
+warning: `#[repr(..)]` attribute is specified more than once
   --> $DIR/issue-100631.rs:4:8
    |
 LL | #[repr(C)]
    |        ^
+LL |
 LL | #[repr(C)]
+   |        ^
+   |
+   = note: for consistency, only specify the representation once
+   = note: `#[warn(repeated_reprs)]` on by default
+
+error[E0084]: unsupported representation for zero-variant enum
+  --> $DIR/issue-100631.rs:4:8
+   |
+LL | #[repr(C)]
+   |        ^
+...
 LL | enum Foo {}
    | -------- zero-variant enum
 
-error: aborting due to 1 previous error
+error: aborting due to 1 previous error; 1 warning emitted
 
 For more information about this error, try `rustc --explain E0084`.

tests/ui/lint/unused/unused-attr-duplicate.rs

@@ -64,8 +64,7 @@ fn t1() {}
 #[must_use = "some message"]
 //~^ ERROR unused attribute
 //~| WARN this was previously accepted
-// No warnings for #[repr], would require more logic.
-#[repr(C)]
+#[repr(C)] //~ WARN `#[repr(..)]` attribute is specified more than once [repeated_reprs]
 #[repr(C)]
 #[non_exhaustive]
 #[non_exhaustive] //~ ERROR unused attribute

tests/ui/lint/unused/unused-attr-duplicate.stderr

@@ -16,6 +16,17 @@ note: the lint level is defined here
 LL | #![deny(unused_attributes)]
    |         ^^^^^^^^^^^^^^^^^
 
+warning: `#[repr(..)]` attribute is specified more than once
+  --> $DIR/unused-attr-duplicate.rs:67:8
+   |
+LL | #[repr(C)]
+   |        ^
+LL | #[repr(C)]
+   |        ^
+   |
+   = note: for consistency, only specify the representation once
+   = note: `#[warn(repeated_reprs)]` on by default
+
 error: unused attribute
   --> $DIR/unused-attr-duplicate.rs:14:1
    |
@@ -193,128 +204,128 @@ LL | #[must_use]
    = warning: this was previously accepted by the compiler but is being phased out; it will become a hard error in a future release!
 
 error: unused attribute
-  --> $DIR/unused-attr-duplicate.rs:71:1
+  --> $DIR/unused-attr-duplicate.rs:70:1
    |
 LL | #[non_exhaustive]
    | ^^^^^^^^^^^^^^^^^ help: remove this attribute
    |
 note: attribute also specified here
-  --> $DIR/unused-attr-duplicate.rs:70:1
+  --> $DIR/unused-attr-duplicate.rs:69:1
    |
 LL | #[non_exhaustive]
    | ^^^^^^^^^^^^^^^^^
 
 error: unused attribute
-  --> $DIR/unused-attr-duplicate.rs:77:1
+  --> $DIR/unused-attr-duplicate.rs:76:1
    |
 LL | #[automatically_derived]
    | ^^^^^^^^^^^^^^^^^^^^^^^^ help: remove this attribute
    |
 note: attribute also specified here
-  --> $DIR/unused-attr-duplicate.rs:76:1
+  --> $DIR/unused-attr-duplicate.rs:75:1
    |
 LL | #[automatically_derived]
    | ^^^^^^^^^^^^^^^^^^^^^^^^
 
 error: unused attribute
-  --> $DIR/unused-attr-duplicate.rs:81:1
+  --> $DIR/unused-attr-duplicate.rs:80:1
    |
 LL | #[inline(never)]
    | ^^^^^^^^^^^^^^^^ help: remove this attribute
    |
 note: attribute also specified here
-  --> $DIR/unused-attr-duplicate.rs:80:1
+  --> $DIR/unused-attr-duplicate.rs:79:1
    |
 LL | #[inline(always)]
    | ^^^^^^^^^^^^^^^^^
    = warning: this was previously accepted by the compiler but is being phased out; it will become a hard error in a future release!
 
 error: unused attribute
-  --> $DIR/unused-attr-duplicate.rs:84:1
+  --> $DIR/unused-attr-duplicate.rs:83:1
    |
 LL | #[cold]
    | ^^^^^^^ help: remove this attribute
    |
 note: attribute also specified here
-  --> $DIR/unused-attr-duplicate.rs:83:1
+  --> $DIR/unused-attr-duplicate.rs:82:1
    |
 LL | #[cold]
    | ^^^^^^^
 
 error: unused attribute
-  --> $DIR/unused-attr-duplicate.rs:86:1
+  --> $DIR/unused-attr-duplicate.rs:85:1
    |
 LL | #[track_caller]
    | ^^^^^^^^^^^^^^^ help: remove this attribute
    |
 note: attribute also specified here
-  --> $DIR/unused-attr-duplicate.rs:85:1
+  --> $DIR/unused-attr-duplicate.rs:84:1
    |
 LL | #[track_caller]
    | ^^^^^^^^^^^^^^^
 
 error: unused attribute
-  --> $DIR/unused-attr-duplicate.rs:100:1
+  --> $DIR/unused-attr-duplicate.rs:99:1
    |
 LL | #[export_name = "exported_symbol_name2"]
    | ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^ help: remove this attribute
    |
 note: attribute also specified here
-  --> $DIR/unused-attr-duplicate.rs:99:1
+  --> $DIR/unused-attr-duplicate.rs:98:1
    |
 LL | #[export_name = "exported_symbol_name"]
    | ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
    = warning: this was previously accepted by the compiler but is being phased out; it will become a hard error in a future release!
 
 error: unused attribute
-  --> $DIR/unused-attr-duplicate.rs:105:1
+  --> $DIR/unused-attr-duplicate.rs:104:1
    |
 LL | #[no_mangle]
    | ^^^^^^^^^^^^ help: remove this attribute
    |
 note: attribute also specified here
-  --> $DIR/unused-attr-duplicate.rs:104:1
+  --> $DIR/unused-attr-duplicate.rs:103:1
    |
 LL | #[no_mangle]
    | ^^^^^^^^^^^^
 
 error: unused attribute
-  --> $DIR/unused-attr-duplicate.rs:109:1
+  --> $DIR/unused-attr-duplicate.rs:108:1
    |
 LL | #[used]
    | ^^^^^^^ help: remove this attribute
    |
 note: attribute also specified here
-  --> $DIR/unused-attr-duplicate.rs:108:1
+  --> $DIR/unused-attr-duplicate.rs:107:1
    |
 LL | #[used]
    | ^^^^^^^
 
 error: unused attribute
-  --> $DIR/unused-attr-duplicate.rs:113:1
+  --> $DIR/unused-attr-duplicate.rs:112:1
    |
 LL | #[link_section = "__DATA,__mod_init_func"]
    | ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^ help: remove this attribute
    |
 note: attribute also specified here
-  --> $DIR/unused-attr-duplicate.rs:112:1
+  --> $DIR/unused-attr-duplicate.rs:111:1
    |
 LL | #[link_section = "__TEXT,__text"]
    | ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
    = warning: this was previously accepted by the compiler but is being phased out; it will become a hard error in a future release!
 
 error: unused attribute
-  --> $DIR/unused-attr-duplicate.rs:94:5
+  --> $DIR/unused-attr-duplicate.rs:93:5
    |
 LL |     #[link_name = "rust_dbg_extern_identity_u32"]
    |     ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^ help: remove this attribute
    |
 note: attribute also specified here
-  --> $DIR/unused-attr-duplicate.rs:93:5
+  --> $DIR/unused-attr-duplicate.rs:92:5
    |
 LL |     #[link_name = "this_does_not_exist"]
    |     ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
    = warning: this was previously accepted by the compiler but is being phased out; it will become a hard error in a future release!
 
-error: aborting due to 25 previous errors
+error: aborting due to 25 previous errors; 1 warning emitted
 

tests/ui/repr/conflicting-repr-hints.rs

@@ -43,7 +43,7 @@ struct I(i32); //~ ERROR type has conflicting packed representation hints
 #[repr(packed)]
 struct J(i32); //~ ERROR type has conflicting packed representation hints
 
-#[repr(packed, packed(1))]
+#[repr(packed, packed(1))] //~ WARN attribute is specified more than once
 struct K(i32);
 
 #[repr(packed, align(8))]

tests/ui/repr/conflicting-repr-hints.stderr

@@ -17,6 +17,15 @@ LL | #[repr(u32, u64)]
    = warning: this was previously accepted by the compiler but is being phased out; it will become a hard error in a future release!
    = note: for more information, see issue #68585 <https://github.com/rust-lang/rust/issues/68585>
 
+warning: `#[repr(..)]` attribute is specified more than once
+  --> $DIR/conflicting-repr-hints.rs:46:8
+   |
+LL | #[repr(packed, packed(1))]
+   |        ^^^^^^  ^^^^^^^^^
+   |
+   = note: for consistency, only specify the representation once
+   = note: `#[warn(repeated_reprs)]` on by default
+
 error[E0587]: type has conflicting packed and align representation hints
   --> $DIR/conflicting-repr-hints.rs:29:1
    |
@@ -77,7 +86,7 @@ error[E0587]: type has conflicting packed and align representation hints
 LL | pub union U {
    | ^^^^^^^^^^^
 
-error: aborting due to 12 previous errors
+error: aborting due to 12 previous errors; 1 warning emitted
 
 Some errors have detailed explanations: E0566, E0587, E0634.
 For more information about an error, try `rustc --explain E0566`.