panic if libc constants have the wrong size

Author: pvdrz

src/shims/fs.rs

@@ -356,22 +356,22 @@ pub trait EvalContextExt<'mir, 'tcx: 'mir>: crate::MiriEvalContextExt<'mir, 'tcx
 
         let file_type = metadata.file_type();
 
+        let mode_name = if file_type.is_file() {
+            "S_IFREG"
+        } else if file_type.is_dir() {
+            "S_IFDIR"
+        } else {
+            "S_IFLNK"
+        };
+
         // The `mode` field specifies the type of the file and the permissions over the file for
         // the owner, its group and other users. Given that we can only provide the file type
         // without using platform specific methods, we only set the bits corresponding to the file
         // type. This should be an `__u16` but `libc` provides its values as `u32`.
-        let mode = if file_type.is_file() {
-            this.eval_libc("S_IFREG")?
-        } else if file_type.is_dir() {
-            this.eval_libc("S_IFDIR")?
-        } else {
-            this.eval_libc("S_IFLNK")?
-        }.to_u32()?;
-
-        let mode: u16 = mode.try_into().map_err(|e|
-            err_unsup_format!("Failed to convert 32-bit value to u16: {}",
-            e
-        ))?;
+        let mode: u16 = this.eval_libc(mode_name)?
+                .to_u32()?
+                .try_into()
+                .unwrap_or_else(|_| bug!("libc contains bad value for `{}` constant", mode_name));
 
         let size = metadata.len();