Add long error explanation for E0693 #61137

Author: Ayush Kumar Mishra

src/librustc_error_codes/error_codes.rs

@@ -380,6 +380,7 @@ E0689: include_str!("./error_codes/E0689.md"),
 E0690: include_str!("./error_codes/E0690.md"),
 E0691: include_str!("./error_codes/E0691.md"),
 E0692: include_str!("./error_codes/E0692.md"),
+E0693: include_str!("./error_codes/E0693.md"),
 E0695: include_str!("./error_codes/E0695.md"),
 E0697: include_str!("./error_codes/E0697.md"),
 E0698: include_str!("./error_codes/E0698.md"),
@@ -594,7 +595,6 @@ E0748: include_str!("./error_codes/E0748.md"),
     E0667, // `impl Trait` in projections
     E0687, // in-band lifetimes cannot be used in `fn`/`Fn` syntax
     E0688, // in-band lifetimes cannot be mixed with explicit lifetime binders
-    E0693, // incorrect `repr(align)` attribute format
 //  E0694, // an unknown tool name found in scoped attributes
     E0696, // `continue` pointing to a labeled block
 //  E0702, // replaced with a generic attribute input check

src/librustc_error_codes/error_codes/E0693.md

@@ -0,0 +1,19 @@
+`align` representation hint was incorrectly declared.
+
+Erroneous code examples:
+
+```compile_fail,E0693
+#[repr(align=8)] // error!
+struct Align8(i8);
+
+#[repr(align="8")] // error!
+struct Align8(i8);
+```
+
+This is a syntax error at the level of attribute declarations. The proper
+syntax for `align` representation hint is the following:
+
+```
+#[repr(align(8))] // ok!
+struct Align8(i8);
+```

src/test/ui/repr/repr-align-assign.stderr

@@ -24,3 +24,4 @@ LL | #[repr(align="8")]
 
 error: aborting due to 4 previous errors
 
+For more information about this error, try `rustc --explain E0693`.