Be conservative about deriving Debug/Default with large alignment

When there is large enough alignment that we might generate padding which has
more members that `RUST_DERIVE_IN_ARRAY_LIMIT`, we can break our ability to
derive traits. This commit solves this issue conservatively: there are cases
where we leave a derive on the table, because in order to know that we could add
that derive, we would need to compute padding before we determine whether we can
derive.

Fixes rust-lang#648

src/ir/analysis/derive_debug.rs

@@ -141,6 +141,15 @@ impl<'ctx, 'gen> MonotoneFramework for CannotDeriveDebug<'ctx, 'gen> {
             };
         }
 
+        if ty.layout(self.ctx).map_or(false, |l| l.align > RUST_DERIVE_IN_ARRAY_LIMIT) {
+            // We have to be conservative: the struct *could* have enough
+            // padding that we emit an array that is longer than
+            // `RUST_DERIVE_IN_ARRAY_LIMIT`. If we moved padding calculations
+            // into the IR and computed them before this analysis, then we could
+            // be precise rather than conservative here.
+            return self.insert(id);
+        }
+
         match *ty.kind() {
             // Handle the simple cases. These can derive debug without further
             // information.

src/ir/comp.rs

@@ -7,6 +7,7 @@ use super::dot::DotAttributes;
 use super::item::{IsOpaque, Item};
 use super::layout::Layout;
 use super::traversal::{EdgeKind, Trace, Tracer};
+use super::ty::RUST_DERIVE_IN_ARRAY_LIMIT;
 use super::template::TemplateParameters;
 use clang;
 use codegen::struct_layout::{align_to, bytes_from_bits_pow2};
@@ -1435,6 +1436,10 @@ impl<'a> CanDeriveDefault<'a> for CompInfo {
             return true;
         }
 
+        if layout.map_or(false, |l| l.align > RUST_DERIVE_IN_ARRAY_LIMIT) {
+            return false;
+        }
+
         if self.kind == CompKind::Union {
             if ctx.options().unstable_rust {
                 return false;

tests/expectations/tests/issue-648-derive-debug-with-padding.rs

@@ -0,0 +1,65 @@
+/* automatically generated by rust-bindgen */
+
+
+#![allow(dead_code, non_snake_case, non_camel_case_types, non_upper_case_globals)]
+
+
+/// We emit a `[u8; 63usize]` padding field for this struct, which cannot derive
+/// Debug because 63 is over the hard coded limit. (Yes, this struct doesn't end
+/// up with the reight alignment, we're waiting on `#[repr(align="N")]` to land
+/// in rustc).
+#[repr(C)]
+#[derive(Copy)]
+pub struct NoDebug {
+    pub c: ::std::os::raw::c_char,
+    pub __bindgen_padding_0: [u8; 63usize],
+}
+#[test]
+fn bindgen_test_layout_NoDebug() {
+    assert_eq!(::std::mem::size_of::<NoDebug>() , 64usize , concat ! (
+               "Size of: " , stringify ! ( NoDebug ) ));
+    assert_eq! (unsafe {
+                & ( * ( 0 as * const NoDebug ) ) . c as * const _ as usize } ,
+                0usize , concat ! (
+                "Alignment of field: " , stringify ! ( NoDebug ) , "::" ,
+                stringify ! ( c ) ));
+}
+impl Clone for NoDebug {
+    fn clone(&self) -> Self { *self }
+}
+impl Default for NoDebug {
+    fn default() -> Self { unsafe { ::std::mem::zeroed() } }
+}
+/// This should derive Debug because the padding size is less than the max derive
+/// Debug impl for arrays. However, we conservatively don't derive Debug because
+/// we determine Debug derive-ability before we compute padding, which happens at
+/// codegen. (Again, we expect to get the alignment wrong for similar reasons.)
+#[repr(C)]
+#[derive(Copy)]
+pub struct ShouldDeriveDebugButDoesNot {
+    pub c: [::std::os::raw::c_char; 32usize],
+    pub d: ::std::os::raw::c_char,
+    pub __bindgen_padding_0: [u8; 31usize],
+}
+#[test]
+fn bindgen_test_layout_ShouldDeriveDebugButDoesNot() {
+    assert_eq!(::std::mem::size_of::<ShouldDeriveDebugButDoesNot>() , 64usize
+               , concat ! (
+               "Size of: " , stringify ! ( ShouldDeriveDebugButDoesNot ) ));
+    assert_eq! (unsafe {
+                & ( * ( 0 as * const ShouldDeriveDebugButDoesNot ) ) . c as *
+                const _ as usize } , 0usize , concat ! (
+                "Alignment of field: " , stringify ! (
+                ShouldDeriveDebugButDoesNot ) , "::" , stringify ! ( c ) ));
+    assert_eq! (unsafe {
+                & ( * ( 0 as * const ShouldDeriveDebugButDoesNot ) ) . d as *
+                const _ as usize } , 32usize , concat ! (
+                "Alignment of field: " , stringify ! (
+                ShouldDeriveDebugButDoesNot ) , "::" , stringify ! ( d ) ));
+}
+impl Clone for ShouldDeriveDebugButDoesNot {
+    fn clone(&self) -> Self { *self }
+}
+impl Default for ShouldDeriveDebugButDoesNot {
+    fn default() -> Self { unsafe { ::std::mem::zeroed() } }
+}

tests/expectations/tests/layout_array.rs

@@ -47,7 +47,7 @@ pub type rte_mempool_get_count =
                               -> ::std::os::raw::c_uint>;
 /// Structure defining mempool operations structure
 #[repr(C)]
-#[derive(Debug, Copy)]
+#[derive(Copy)]
 pub struct rte_mempool_ops {
     /// < Name of mempool ops struct.
     pub name: [::std::os::raw::c_char; 32usize],
@@ -134,7 +134,7 @@ impl Clone for rte_spinlock_t {
 /// any function pointers stored directly in the mempool struct would not be.
 /// This results in us simply having "ops_index" in the mempool struct.
 #[repr(C)]
-#[derive(Debug, Copy)]
+#[derive(Copy)]
 pub struct rte_mempool_ops_table {
     /// < Spinlock for add/delete.
     pub sl: rte_spinlock_t,
@@ -173,7 +173,7 @@ impl Default for rte_mempool_ops_table {
 }
 /// Structure to hold malloc heap
 #[repr(C)]
-#[derive(Debug, Copy)]
+#[derive(Copy)]
 pub struct malloc_heap {
     pub lock: rte_spinlock_t,
     pub free_head: [malloc_heap__bindgen_ty_1; 13usize],

tests/expectations/tests/layout_array_too_long.rs

@@ -96,7 +96,7 @@ impl Clone for ip_frag_key {
 /// @internal Fragmented packet to reassemble.
 /// First two entries in the frags[] array are for the last and first fragments.
 #[repr(C)]
-#[derive(Debug, Copy)]
+#[derive(Copy)]
 pub struct ip_frag_pkt {
     /// < LRU list
     pub lru: ip_frag_pkt__bindgen_ty_1,

tests/expectations/tests/layout_kni_mbuf.rs

@@ -7,7 +7,7 @@
 pub const RTE_CACHE_LINE_MIN_SIZE: ::std::os::raw::c_uint = 64;
 pub const RTE_CACHE_LINE_SIZE: ::std::os::raw::c_uint = 64;
 #[repr(C)]
-#[derive(Debug, Copy)]
+#[derive(Copy)]
 pub struct rte_kni_mbuf {
     pub buf_addr: *mut ::std::os::raw::c_void,
     pub buf_physaddr: u64,

tests/expectations/tests/layout_large_align_field.rs

@@ -129,7 +129,7 @@ impl Clone for ip_frag_key {
 /// @internal Fragmented packet to reassemble.
 /// First two entries in the frags[] array are for the last and first fragments.
 #[repr(C)]
-#[derive(Debug, Copy)]
+#[derive(Copy)]
 pub struct ip_frag_pkt {
     /// < LRU list
     pub lru: ip_frag_pkt__bindgen_ty_1,
@@ -258,7 +258,7 @@ impl Default for ip_pkt_list {
 }
 /// fragmentation table statistics
 #[repr(C)]
-#[derive(Debug, Default, Copy)]
+#[derive(Copy)]
 pub struct ip_frag_tbl_stat {
     /// < total # of find/insert attempts.
     pub find_num: u64,
@@ -312,9 +312,12 @@ fn bindgen_test_layout_ip_frag_tbl_stat() {
 impl Clone for ip_frag_tbl_stat {
     fn clone(&self) -> Self { *self }
 }
+impl Default for ip_frag_tbl_stat {
+    fn default() -> Self { unsafe { ::std::mem::zeroed() } }
+}
 /// fragmentation table
 #[repr(C)]
-#[derive(Debug, Copy)]
+#[derive(Copy)]
 pub struct rte_ip_frag_tbl {
     /// < ttl for table entries.
     pub max_cycles: u64,

tests/expectations/tests/layout_mbuf.rs

@@ -58,7 +58,7 @@ impl Clone for rte_atomic16_t {
 }
 /// The generic rte_mbuf, containing a packet mbuf.
 #[repr(C)]
-#[derive(Debug, Copy)]
+#[derive(Copy)]
 pub struct rte_mbuf {
     pub cacheline0: MARKER,
     /// < Virtual address of segment buffer.

tests/headers/issue-648-derive-debug-with-padding.h

@@ -0,0 +1,22 @@
+/**
+ * We emit a `[u8; 63usize]` padding field for this struct, which cannot derive
+ * Debug because 63 is over the hard coded limit. (Yes, this struct doesn't end
+ * up with the reight alignment, we're waiting on `#[repr(align="N")]` to land
+ * in rustc).
+ */
+struct NoDebug {
+    char c;
+    // padding of 63 bytes
+} __attribute__((__aligned__(64)));
+
+/**
+ * This should derive Debug because the padding size is less than the max derive
+ * Debug impl for arrays. However, we conservatively don't derive Debug because
+ * we determine Debug derive-ability before we compute padding, which happens at
+ * codegen. (Again, we expect to get the alignment wrong for similar reasons.)
+ */
+struct ShouldDeriveDebugButDoesNot {
+    char c[32];
+    char d;
+    // padding of 31 bytes
+} __attribute__((__aligned__(64)));