Add checks for conflicts for aliases

- Check that alias is not the same as name of other field (it still can be the name of owning field/variant)
- Check that aliases are unique, i. e. two different fields does not use the same alias

test_suite/tests/ui/conflict/alias-enum.rs

@@ -0,0 +1,79 @@
+#![allow(non_camel_case_types)]
+
+use serde_derive::Deserialize;
+
+#[derive(Deserialize)]
+enum E {
+    S1 {
+        /// Expected error on "alias b", because this is a name of other field
+        /// Error on "alias a" is not expected because this is a name of this field
+        /// Error on "alias c" is not expected because field `c` is skipped
+        #[serde(alias = "a", alias = "b", alias = "c")]
+        a: (),
+
+        /// Expected error on "alias c", because it is already used as alias of `a`
+        #[serde(alias = "c")]
+        b: (),
+
+        #[serde(skip_deserializing)]
+        c: (),
+    },
+
+    S2 {
+        /// Expected error on "alias c", because this is a name of other field after
+        /// applying rename rules
+        #[serde(alias = "b", alias = "c")]
+        a: (),
+
+        #[serde(rename = "c")]
+        b: (),
+    },
+
+    #[serde(rename_all = "UPPERCASE")]
+    S3 {
+        /// Expected error on "alias B", because this is a name of field after
+        /// applying rename rules
+        #[serde(alias = "B", alias = "c")]
+        a: (),
+        b: (),
+    },
+}
+
+#[derive(Deserialize)]
+enum E1 {
+    /// Expected error on "alias b", because this is a name of other variant
+    /// Error on "alias a" is not expected because this is a name of this variant
+    /// Error on "alias c" is not expected because variant `c` is skipped
+    #[serde(alias = "a", alias = "b", alias = "c")]
+    a,
+
+    /// Expected error on "alias c", because it is already used as alias of `a`
+    #[serde(alias = "c")]
+    b,
+
+    #[serde(skip_deserializing)]
+    c,
+}
+
+#[derive(Deserialize)]
+enum E2 {
+    /// Expected error on "alias c", because this is a name of other variant after
+    /// applying rename rules
+    #[serde(alias = "b", alias = "c")]
+    a,
+
+    #[serde(rename = "c")]
+    b,
+}
+
+#[derive(Deserialize)]
+#[serde(rename_all = "UPPERCASE")]
+enum E3 {
+    /// Expected error on "alias B", because this is a name of variant after
+    /// applying rename rules
+    #[serde(alias = "B", alias = "c")]
+    a,
+    b,
+}
+
+fn main() {}

test_suite/tests/ui/conflict/alias-enum.stderr

@@ -0,0 +1,71 @@
+error: alias `b` conflicts with deserialization name of other field
+  --> tests/ui/conflict/alias-enum.rs:8:9
+   |
+8  | /         /// Expected error on "alias b", because this is a name of other field
+9  | |         /// Error on "alias a" is not expected because this is a name of this field
+10 | |         /// Error on "alias c" is not expected because field `c` is skipped
+11 | |         #[serde(alias = "a", alias = "b", alias = "c")]
+12 | |         a: (),
+   | |_____________^
+
+error: alias `c` already used by field a
+  --> tests/ui/conflict/alias-enum.rs:14:9
+   |
+14 | /         /// Expected error on "alias c", because it is already used as alias of `a`
+15 | |         #[serde(alias = "c")]
+16 | |         b: (),
+   | |_____________^
+
+error: alias `c` conflicts with deserialization name of other field
+  --> tests/ui/conflict/alias-enum.rs:23:9
+   |
+23 | /         /// Expected error on "alias c", because this is a name of other field after
+24 | |         /// applying rename rules
+25 | |         #[serde(alias = "b", alias = "c")]
+26 | |         a: (),
+   | |_____________^
+
+error: alias `B` conflicts with deserialization name of other field
+  --> tests/ui/conflict/alias-enum.rs:34:9
+   |
+34 | /         /// Expected error on "alias B", because this is a name of field after
+35 | |         /// applying rename rules
+36 | |         #[serde(alias = "B", alias = "c")]
+37 | |         a: (),
+   | |_____________^
+
+error: alias `b` conflicts with deserialization name of other variant
+  --> tests/ui/conflict/alias-enum.rs:44:5
+   |
+44 | /     /// Expected error on "alias b", because this is a name of other variant
+45 | |     /// Error on "alias a" is not expected because this is a name of this variant
+46 | |     /// Error on "alias c" is not expected because variant `c` is skipped
+47 | |     #[serde(alias = "a", alias = "b", alias = "c")]
+48 | |     a,
+   | |_____^
+
+error: alias `c` already used by variant a
+  --> tests/ui/conflict/alias-enum.rs:50:5
+   |
+50 | /     /// Expected error on "alias c", because it is already used as alias of `a`
+51 | |     #[serde(alias = "c")]
+52 | |     b,
+   | |_____^
+
+error: alias `c` conflicts with deserialization name of other variant
+  --> tests/ui/conflict/alias-enum.rs:60:5
+   |
+60 | /     /// Expected error on "alias c", because this is a name of other variant after
+61 | |     /// applying rename rules
+62 | |     #[serde(alias = "b", alias = "c")]
+63 | |     a,
+   | |_____^
+
+error: alias `B` conflicts with deserialization name of other variant
+  --> tests/ui/conflict/alias-enum.rs:72:5
+   |
+72 | /     /// Expected error on "alias B", because this is a name of variant after
+73 | |     /// applying rename rules
+74 | |     #[serde(alias = "B", alias = "c")]
+75 | |     a,
+   | |_____^

test_suite/tests/ui/conflict/alias.rs

@@ -0,0 +1,40 @@
+use serde_derive::Deserialize;
+
+#[derive(Deserialize)]
+struct S1 {
+    /// Expected error on "alias b", because this is a name of other field
+    /// Error on "alias a" is not expected because this is a name of this field
+    /// Error on "alias c" is not expected because field `c` is skipped
+    #[serde(alias = "a", alias = "b", alias = "c")]
+    a: (),
+
+    /// Expected error on "alias c", because it is already used as alias of `a`
+    #[serde(alias = "c")]
+    b: (),
+
+    #[serde(skip_deserializing)]
+    c: (),
+}
+
+#[derive(Deserialize)]
+struct S2 {
+    /// Expected error on "alias c", because this is a name of other field after
+    /// applying rename rules
+    #[serde(alias = "b", alias = "c")]
+    a: (),
+
+    #[serde(rename = "c")]
+    b: (),
+}
+
+#[derive(Deserialize)]
+#[serde(rename_all = "UPPERCASE")]
+struct S3 {
+    /// Expected error on "alias B", because this is a name of field after
+    /// applying rename rules
+    #[serde(alias = "B", alias = "c")]
+    a: (),
+    b: (),
+}
+
+fn main() {}

test_suite/tests/ui/conflict/alias.stderr

@@ -0,0 +1,35 @@
+error: alias `b` conflicts with deserialization name of other field
+ --> tests/ui/conflict/alias.rs:5:5
+  |
+5 | /     /// Expected error on "alias b", because this is a name of other field
+6 | |     /// Error on "alias a" is not expected because this is a name of this field
+7 | |     /// Error on "alias c" is not expected because field `c` is skipped
+8 | |     #[serde(alias = "a", alias = "b", alias = "c")]
+9 | |     a: (),
+  | |_________^
+
+error: alias `c` already used by field a
+  --> tests/ui/conflict/alias.rs:11:5
+   |
+11 | /     /// Expected error on "alias c", because it is already used as alias of `a`
+12 | |     #[serde(alias = "c")]
+13 | |     b: (),
+   | |_________^
+
+error: alias `c` conflicts with deserialization name of other field
+  --> tests/ui/conflict/alias.rs:21:5
+   |
+21 | /     /// Expected error on "alias c", because this is a name of other field after
+22 | |     /// applying rename rules
+23 | |     #[serde(alias = "b", alias = "c")]
+24 | |     a: (),
+   | |_________^
+
+error: alias `B` conflicts with deserialization name of other field
+  --> tests/ui/conflict/alias.rs:33:5
+   |
+33 | /     /// Expected error on "alias B", because this is a name of field after
+34 | |     /// applying rename rules
+35 | |     #[serde(alias = "B", alias = "c")]
+36 | |     a: (),
+   | |_________^