make error codes reflect reality better

compiler/rustc_error_codes/src/error_codes/E0534.md

@@ -1,8 +1,14 @@
+#### Note: this error code is no longer emitted by the compiler
+
+This is because it was too specific to the `inline` attribute.
+Similar diagnostics occur for other attributes too.
+The example here will now emit `E0805`
+
 The `inline` attribute was malformed.
 
 Erroneous code example:
 
-```compile_fail,E0534
+```compile_fail,E0805
 #[inline()] // error: expected one argument
 pub fn something() {}
 

compiler/rustc_error_codes/src/error_codes/E0535.md

@@ -1,8 +1,13 @@
-An unknown argument was given to the `inline` attribute.
+#### Note: this error code is no longer emitted by the compiler
+
+This is because it was too specific to the `inline` attribute.
+Similar diagnostics occur for other attributes too.
+The example here will now emit `E0539`
+
 
 Erroneous code example:
 
-```compile_fail,E0535
+```compile_fail,E0539
 #[inline(unknown)] // error: invalid argument
 pub fn something() {}
 

compiler/rustc_error_codes/src/error_codes/E0539.md

@@ -24,8 +24,7 @@ struct Stable;
 const fn stable_fn() {}
 ```
 
-Meta items are the key-value pairs inside of an attribute.
-To fix these issues you need to give required key-value pairs.
+To fix the above example, you can write the following:
 
 ```
 #![feature(staged_api)]
@@ -49,3 +48,29 @@ struct Stable;
 #[rustc_const_stable(feature = "stable_fn", since = "1.39.0")] // ok!
 const fn stable_fn() {}
 ```
+
+Several causes of this are,
+an attribute may have expected you to give a list but you gave a
+`name = value` pair:
+
+```compile_fail,E0539
+// wrong, should be `#[repr(C)]`
+#[repr = "C"]
+struct Foo {}
+```
+
+Or a `name = value` pair, but you gave a list:
+
+```compile_fail,E0539
+// wrong, should be `note = "reason"`
+#[deprecated(since = "1.0.0", note("reason"))]
+struct Foo {}
+```
+
+Or it expected some specific word but you gave an unexpected one:
+
+```compile_fail,E0539
+// should be `always` or `never`
+#[inline(maybe_if_you_feel_like_it)]
+fn foo() {}
+```

compiler/rustc_error_codes/src/error_codes/E0565.md

@@ -9,10 +9,9 @@ struct Repr {}
 fn main() {}
 ```
 
-Literals in attributes are new and largely unsupported in built-in attributes.
-Work to support literals where appropriate is ongoing. Try using an unquoted
-name instead:
-
+Not all attributes support literals in their input,
+and in some cases they expect an identifier instead.
+That would be the solution in the case of `repr`:
 ```
 #[repr(C)] // ok!
 struct Repr {}

compiler/rustc_error_codes/src/error_codes/E0805.md

@@ -0,0 +1,26 @@
+An attribute was given an invalid number of arguments
+
+Erroneous code example:
+
+```compile_fail,E0805
+#[inline()] // error! should either have a single argument, or no parentheses
+fn foo() {}
+
+#[inline(always, never)] // error! should have only one argument, not two
+fn bar() {}
+```
+
+To fix this, either give the right number of arguments the attribute needs.
+In the case of inline, this could be none at all:
+
+```
+#[inline]
+fn foo() {}
+```
+
+or only one:
+
+```
+#[inline(always)]
+fn foo() {}
+```