lint against repeated repr attributes

compiler/rustc_lint_defs/src/builtin.rs

@@ -271,6 +271,30 @@ 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 +40,7 @@ use rustc_session::errors::feature_err;
 use rustc_session::lint;
 use rustc_session::lint::builtin::{
     CONFLICTING_REPR_HINTS, INVALID_DOC_ATTRIBUTES, MALFORMED_DIAGNOSTIC_FORMAT_LITERALS,
-    MISPLACED_DIAGNOSTIC_ATTRIBUTES, UNUSED_ATTRIBUTES,
+    MISPLACED_DIAGNOSTIC_ATTRIBUTES, REPEATED_REPRS, UNUSED_ATTRIBUTES,
 };
 use rustc_span::edition::Edition;
 use rustc_span::{DUMMY_SP, Ident, Span, Symbol, sym};
@@ -1212,24 +1212,59 @@ impl<'tcx> CheckAttrVisitor<'tcx> {
         let mut is_c = false;
         let mut is_simd = false;
         let mut is_transparent = false;
+        let mut is_align = false;
+        let mut is_packed = false;
+        let mut repeated_repr = false;
+        let mut maybe_last_int_type = None;
 
         for (repr, _repr_span) in reprs {
             match repr {
                 ReprAttr::ReprRust => {
+                    if is_explicit_rust {
+                        repeated_repr = true;
+                    }
                     is_explicit_rust = true;
                 }
                 ReprAttr::ReprC => {
+                    if is_c {
+                        repeated_repr = true;
+                    }
                     is_c = true;
                 }
-                ReprAttr::ReprAlign(..) => {}
-                ReprAttr::ReprPacked(_) => {}
+                ReprAttr::ReprAlign(..) => {
+                    if is_align {
+                        repeated_repr = true;
+                    }
+                    is_align = true;
+                }
+                ReprAttr::ReprPacked(..) => {
+                    if is_packed {
+                        repeated_repr = true;
+                    }
+                    is_packed = true;
+                }
                 ReprAttr::ReprSimd => {
+                    if is_simd {
+                        repeated_repr = true;
+                    }
                     is_simd = true;
                 }
                 ReprAttr::ReprTransparent => {
+                    // No need to check for repeated transparent because that is already checked
+                    // when checking for any other attribute together with transparent.
                     is_transparent = true;
                 }
-                ReprAttr::ReprInt(_) => {
+                ReprAttr::ReprInt(int_type) => {
+                    if let Some(last_int_type) = maybe_last_int_type
+                        && last_int_type == int_type
+                    {
+                        // We'll "miss" detecting repeated int reprs if the user specifies
+                        // #[repr(u8, u64, u8)] for example. But that's okay because we've got
+                        // conflicting reprs anyway so it's not worth the effort to do more precise
+                        // tracking.
+                        repeated_repr = true;
+                    }
+                    maybe_last_int_type = Some(int_type);
                     int_reprs += 1;
                 }
             };
@@ -1274,10 +1309,19 @@ impl<'tcx> CheckAttrVisitor<'tcx> {
             self.tcx.emit_node_span_lint(
                 CONFLICTING_REPR_HINTS,
                 hir_id,
-                hint_spans.collect::<Vec<Span>>(),
+                hint_spans.clone().collect::<Vec<Span>>(),
                 errors::ReprConflictingLint,
             );
         }
+
+        if repeated_repr {
+            self.tcx.emit_node_span_lint(
+                REPEATED_REPRS,
+                hir_id,
+                hint_spans.collect::<Vec<Span>>(),
+                errors::RepeatedRepr,
+            );
+        }
     }
 
     /// Outputs an error for attributes that can only be applied to macros, such as

compiler/rustc_passes/src/errors.rs

@@ -703,6 +703,11 @@ pub(crate) struct TransparentIncompatible {
     pub target: String,
 }
 
+#[derive(Diagnostic)]
+#[diag("representation 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 representation attribute is specified more than once
 #[repr(C)]
 enum Foo {}
 

tests/ui/attributes/issue-100631.stderr

@@ -1,12 +1,24 @@
-error[E0084]: unsupported representation for zero-variant enum
+warning: representation 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 representation attribute is specified more than once
 #[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: representation 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:37:1
    |
@@ -104,124 +115,124 @@ 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: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: 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"]
    | ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
@@ -316,5 +327,5 @@ note: attribute also specified here
 LL | #![no_builtins]
    | ^^^^^^^^^^^^^^^
 
-error: aborting due to 25 previous errors
+error: aborting due to 25 previous errors; 1 warning emitted
 

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

@@ -36,14 +36,14 @@ struct G(i32); //~ ERROR type has conflicting packed and align representation hi
 #[repr(packed)]
 struct H(i32); //~ ERROR type has conflicting packed and align representation hints
 
-#[repr(packed, packed(2))]
+#[repr(packed, packed(2))] //~ WARN representation attribute is specified more than once
 struct I(i32); //~ ERROR type has conflicting packed representation hints
 
-#[repr(packed(2))]
+#[repr(packed(2))] //~ WARN representation attribute is specified more than once
 #[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))]