Improve overall quality of compile_error! errors

Also updates UI tests.

test_suite/tests/ui/identifier/both.stderr

@@ -1,8 +1,14 @@
-error: `field_identifier` and `variant_identifier` cannot both be set
- --> $DIR/both.rs:4:10
+error: #[serde(field_identifier)] and #[serde(variant_identifier)] cannot both be set
+ --> $DIR/both.rs:5:9
   |
-4 | #[derive(Deserialize)]
-  |          ^^^^^^^^^^^
+5 | #[serde(field_identifier, variant_identifier)]
+  |         ^^^^^^^^^^^^^^^^
 
-error: aborting due to previous error
+error: #[serde(field_identifier)] and #[serde(variant_identifier)] cannot both be set
+ --> $DIR/both.rs:5:27
+  |
+5 | #[serde(field_identifier, variant_identifier)]
+  |                           ^^^^^^^^^^^^^^^^^^
+
+error: aborting due to 2 previous errors
 

test_suite/tests/ui/identifier/field_struct.stderr

@@ -1,8 +1,8 @@
-error: `field_identifier` can only be used on an enum
- --> $DIR/field_struct.rs:4:10
+error: #[serde(field_identifier)] can only be used on an enum
+ --> $DIR/field_struct.rs:6:1
   |
-4 | #[derive(Deserialize)]
-  |          ^^^^^^^^^^^
+6 | struct S;
+  | ^^^^^^
 
 error: aborting due to previous error
 

test_suite/tests/ui/identifier/field_tuple.stderr

@@ -1,8 +1,8 @@
-error: field_identifier may only contain unit variants
- --> $DIR/field_tuple.rs:4:10
+error: #[serde(field_identifier)] may only contain unit variants
+ --> $DIR/field_tuple.rs:8:5
   |
-4 | #[derive(Deserialize)]
-  |          ^^^^^^^^^^^
+8 |     B(u8, u8),
+  |     ^^^^^^^^^
 
 error: aborting due to previous error
 

test_suite/tests/ui/identifier/newtype_not_last.stderr

@@ -1,8 +1,8 @@
 error: `Other` must be the last variant
- --> $DIR/newtype_not_last.rs:4:10
+ --> $DIR/newtype_not_last.rs:8:5
   |
-4 | #[derive(Deserialize)]
-  |          ^^^^^^^^^^^
+8 |     Other(String),
+  |     ^^^^^^^^^^^^^
 
 error: aborting due to previous error
 

test_suite/tests/ui/identifier/not_unit.stderr

@@ -1,8 +1,9 @@
 error: #[serde(other)] must be on a unit variant
- --> $DIR/not_unit.rs:4:10
+ --> $DIR/not_unit.rs:8:5
   |
-4 | #[derive(Deserialize)]
-  |          ^^^^^^^^^^^
+8 | /     #[serde(other)]
+9 | |     Other(u8, u8),
+  | |_________________^
 
 error: aborting due to previous error
 

test_suite/tests/ui/identifier/other_not_last.stderr

@@ -1,8 +1,9 @@
-error: #[serde(other)] must be the last variant
- --> $DIR/other_not_last.rs:4:10
+error: #[serde(other)] must be on the last variant
+ --> $DIR/other_not_last.rs:8:5
   |
-4 | #[derive(Deserialize)]
-  |          ^^^^^^^^^^^
+8 | /     #[serde(other)]
+9 | |     Other,
+  | |_________^
 
 error: aborting due to previous error
 

test_suite/tests/ui/identifier/serialize.stderr

@@ -1,8 +1,12 @@
 error: field identifiers cannot be serialized
- --> $DIR/serialize.rs:4:10
+ --> $DIR/serialize.rs:5:1
   |
-4 | #[derive(Serialize)]
-  |          ^^^^^^^^^
+5 | / #[serde(field_identifier)]
+6 | | enum F {
+7 | |     A,
+8 | |     B,
+9 | | }
+  | |_^
 
 error: aborting due to previous error
 

test_suite/tests/ui/identifier/variant_struct.stderr

@@ -1,8 +1,8 @@
-error: `variant_identifier` can only be used on an enum
- --> $DIR/variant_struct.rs:4:10
+error: #[serde(variant_identifier)] can only be used on an enum
+ --> $DIR/variant_struct.rs:6:1
   |
-4 | #[derive(Deserialize)]
-  |          ^^^^^^^^^^^
+6 | struct S;
+  | ^^^^^^
 
 error: aborting due to previous error
 

test_suite/tests/ui/identifier/variant_tuple.stderr

@@ -1,8 +1,8 @@
-error: variant_identifier may only contain unit variants
- --> $DIR/variant_tuple.rs:4:10
+error: #[serde(variant_identifier)] may only contain unit variants
+ --> $DIR/variant_tuple.rs:8:5
   |
-4 | #[derive(Deserialize)]
-  |          ^^^^^^^^^^^
+8 |     B(u8, u8),
+  |     ^^^^^^^^^
 
 error: aborting due to previous error