Rollup merge of #139261 - RalfJung:msvc-align-mitigation, r=oli-obk

mitigate MSVC alignment issue on x86-32

This implements mitigation for https://github.com/rust-lang/rust/issues/112480 by stopping to emit `align` attributes on loads and function arguments when building for a win32 MSVC target. MSVC is known to not properly align `u64` and similar types, and claiming to LLVM that everything is properly aligned increases the chance that this will cause problems.

Of course, the misalignment is still a bug, but we can't fix that bug, only MSVC can.

Also add an errata note to the platform support page warning users about this known problem.

try-job: `i686-msvc*`

compiler/rustc_target/src/callconv/mod.rs

@@ -144,6 +144,7 @@ pub struct ArgAttributes {
     /// (corresponding to LLVM's dereferenceable_or_null attributes, i.e., it is okay for this to be
     /// set on a null pointer, but all non-null pointers must be dereferenceable).
     pub pointee_size: Size,
+    /// The minimum alignment of the pointee, if any.
     pub pointee_align: Option<Align>,
 }
 

compiler/rustc_target/src/spec/mod.rs

@@ -42,7 +42,9 @@ use std::path::{Path, PathBuf};
 use std::str::FromStr;
 use std::{fmt, io};
 
-use rustc_abi::{Endian, ExternAbi, Integer, Size, TargetDataLayout, TargetDataLayoutErrors};
+use rustc_abi::{
+    Align, Endian, ExternAbi, Integer, Size, TargetDataLayout, TargetDataLayoutErrors,
+};
 use rustc_data_structures::fx::{FxHashSet, FxIndexSet};
 use rustc_fs_util::try_canonicalize;
 use rustc_macros::{Decodable, Encodable, HashStable_Generic};
@@ -3599,6 +3601,25 @@ impl Target {
             _ => return None,
         })
     }
+
+    /// Returns whether this target is known to have unreliable alignment:
+    /// native C code for the target fails to align some data to the degree
+    /// required by the C standard. We can't *really* do anything about that
+    /// since unsafe Rust code may assume alignment any time, but we can at least
+    /// inhibit some optimizations, and we suppress the alignment checks that
+    /// would detect this unsoundness.
+    ///
+    /// Every target that returns less than `Align::MAX` here is still has a soundness bug.
+    pub fn max_reliable_alignment(&self) -> Align {
+        // FIXME(#112480) MSVC on x86-32 is unsound and fails to properly align many types with
+        // more-than-4-byte-alignment on the stack. This makes alignments larger than 4 generally
+        // unreliable on 32bit Windows.
+        if self.is_like_windows && self.arch == "x86" {
+            Align::from_bytes(4).unwrap()
+        } else {
+            Align::MAX
+        }
+    }
 }
 
 /// Either a target tuple string or a path to a JSON file.