Support non-literal discriminant values

While preserving favouring literal values where possible.

num_enum/tests/try_build/compile_fail/alternative_exprs.rs

@@ -0,0 +1,13 @@
+const THREE: u8 = 3;
+
+#[derive(num_enum::TryFromPrimitive)]
+#[repr(i8)]
+enum Numbers {
+    Zero = 0,
+    #[num_enum(alternatives = [-1, 2, THREE])]
+    One = 1,
+}
+
+fn main() {
+
+}

num_enum/tests/try_build/compile_fail/alternative_exprs.stderr

@@ -0,0 +1,5 @@
+error: Only literals are allowed as num_enum alternate values
+ --> tests/try_build/compile_fail/alternative_exprs.rs:7:39
+  |
+7 |     #[num_enum(alternatives = [-1, 2, THREE])]
+  |                                       ^^^^^

num_enum/tests/try_from_primitive.rs

@@ -120,8 +120,79 @@ fn wrong_order() {
     assert_eq!(four, Ok(Enum::Four));
 }
 
-#[cfg(feature = "complex-expression")]
+#[test]
+fn negative_values() {
+    #[derive(Debug, Eq, PartialEq, TryFromPrimitive)]
+    #[repr(i8)]
+    enum Enum {
+        MinusTwo = -2,
+        MinusOne = -1,
+        Zero = 0,
+        One = 1,
+        Two = 2,
+    }
+
+    let minus_two: Result<Enum, _> = (-2i8).try_into();
+    assert_eq!(minus_two, Ok(Enum::MinusTwo));
+
+    let minus_one: Result<Enum, _> = (-1i8).try_into();
+    assert_eq!(minus_one, Ok(Enum::MinusOne));
+
+    let zero: Result<Enum, _> = 0i8.try_into();
+    assert_eq!(zero, Ok(Enum::Zero));
+
+    let one: Result<Enum, _> = 1i8.try_into();
+    assert_eq!(one, Ok(Enum::One));
+
+    let two: Result<Enum, _> = 2i8.try_into();
+    assert_eq!(two, Ok(Enum::Two));
+}
+
+#[test]
+fn discriminant_expressions() {
+    const ONE: u8 = 1;
+
+    #[derive(Debug, Eq, PartialEq, TryFromPrimitive)]
+    #[repr(u8)]
+    enum Enum {
+        Zero,
+        One = ONE,
+        Two,
+        Four = 4u8,
+        Five,
+        Six = ONE + ONE + 2u8 + 2,
+    }
+
+    let zero: Result<Enum, _> = 0u8.try_into();
+    assert_eq!(zero, Ok(Enum::Zero));
+
+    let one: Result<Enum, _> = 1u8.try_into();
+    assert_eq!(one, Ok(Enum::One));
+
+    let two: Result<Enum, _> = 2u8.try_into();
+    assert_eq!(two, Ok(Enum::Two));
+
+    let three: Result<Enum, _> = 3u8.try_into();
+    assert_eq!(
+        three.unwrap_err().to_string(),
+        "No discriminant in enum `Enum` matches the value `3`",
+    );
+
+    let four: Result<Enum, _> = 4u8.try_into();
+    assert_eq!(four, Ok(Enum::Four));
+
+    let five: Result<Enum, _> = 5u8.try_into();
+    assert_eq!(five, Ok(Enum::Five));
+
+    let six: Result<Enum, _> = 6u8.try_into();
+    assert_eq!(six, Ok(Enum::Six));
+}
+
+#[cfg(feature = "complex-expressions")]
 mod complex {
+    use num_enum::TryFromPrimitive;
+    use std::convert::TryInto;
+
     const ONE: u8 = 1;
 
     #[derive(Debug, Eq, PartialEq, TryFromPrimitive)]
@@ -261,26 +332,29 @@ fn error_variant_is_allowed() {
 #[test]
 fn alternative_values() {
     #[derive(Debug, Eq, PartialEq, TryFromPrimitive)]
-    #[repr(u8)]
+    #[repr(i8)]
     enum Enum {
         Zero = 0,
-        #[num_enum(alternatives = [2, 3])]
-        OneTwoOrThree = 1,
+        #[num_enum(alternatives = [-1, 2, 3])]
+        OneTwoThreeOrMinusOne = 1,
     }
 
-    let zero: Result<Enum, _> = 0u8.try_into();
+    let minus_one: Result<Enum, _> = (-1i8).try_into();
+    assert_eq!(minus_one, Ok(Enum::OneTwoThreeOrMinusOne));
+
+    let zero: Result<Enum, _> = 0i8.try_into();
     assert_eq!(zero, Ok(Enum::Zero));
 
-    let one: Result<Enum, _> = 1u8.try_into();
-    assert_eq!(one, Ok(Enum::OneTwoOrThree));
+    let one: Result<Enum, _> = 1i8.try_into();
+    assert_eq!(one, Ok(Enum::OneTwoThreeOrMinusOne));
 
-    let two: Result<Enum, _> = 2u8.try_into();
-    assert_eq!(two, Ok(Enum::OneTwoOrThree));
+    let two: Result<Enum, _> = 2i8.try_into();
+    assert_eq!(two, Ok(Enum::OneTwoThreeOrMinusOne));
 
-    let three: Result<Enum, _> = 3u8.try_into();
-    assert_eq!(three, Ok(Enum::OneTwoOrThree));
+    let three: Result<Enum, _> = 3i8.try_into();
+    assert_eq!(three, Ok(Enum::OneTwoThreeOrMinusOne));
 
-    let four: Result<Enum, _> = 4u8.try_into();
+    let four: Result<Enum, _> = 4i8.try_into();
     assert_eq!(
         four.unwrap_err().to_string(),
         "No discriminant in enum `Enum` matches the value `4`"

num_enum_derive/src/lib.rs

@@ -8,7 +8,8 @@ use syn::{
     parse::{Parse, ParseStream},
     parse_macro_input, parse_quote,
     spanned::Spanned,
-    Attribute, Data, DeriveInput, Error, Expr, Fields, Ident, Lit, LitInt, LitStr, Meta, Result,
+    Attribute, Data, DeriveInput, Error, Expr, ExprUnary, Fields, Ident, Lit, LitInt, LitStr, Meta,
+    Result, UnOp,
 };
 
 macro_rules! die {
@@ -32,14 +33,32 @@ fn literal(i: i128) -> Expr {
     }
 }
 
-fn expr_to_int(val_exp: &Expr) -> Result<i128> {
-    Ok(match val_exp {
-        Expr::Lit(ref val_exp_lit) => match val_exp_lit.lit {
-            Lit::Int(ref lit_int) => lit_int.base10_parse()?,
-            _ => die!(val_exp => "Expected integer"),
-        },
-        _ => die!(val_exp => "Expected literal"),
-    })
+enum DiscriminantValue {
+    Literal(i128),
+    Expr(Expr),
+}
+
+fn parse_discriminant(val_exp: &Expr) -> Result<DiscriminantValue> {
+    match val_exp {
+        Expr::Lit(ref val_exp_lit) => {
+            if let Lit::Int(ref lit_int) = val_exp_lit.lit {
+                return Ok(DiscriminantValue::Literal(lit_int.base10_parse()?));
+            }
+        }
+        Expr::Unary(ExprUnary {
+            op: UnOp::Neg(..),
+            expr,
+            ..
+        }) => {
+            if let Expr::Lit(ref val_exp_lit) = **expr {
+                if let Lit::Int(ref lit_int) = val_exp_lit.lit {
+                    return Ok(DiscriminantValue::Literal(-lit_int.base10_parse()?));
+                }
+            }
+        }
+        _ => {}
+    }
+    Ok(DiscriminantValue::Expr(val_exp.clone()))
 }
 
 mod kw {
@@ -307,7 +326,7 @@ impl Parse for EnumInfo {
             let mut has_catch_all_variant: bool = false;
 
             // Vec to keep track of the used discriminants and alt values.
-            let mut val_set: BTreeSet<i128> = BTreeSet::new();
+            let mut discriminant_int_val_set = BTreeSet::new();
 
             let mut next_discriminant = literal(0);
             for variant in data.variants.into_iter() {
@@ -319,7 +338,7 @@ impl Parse for EnumInfo {
                 };
 
                 let mut attr_spans: AttributeSpans = Default::default();
-                let mut alternative_values: Vec<Expr> = vec![];
+                let mut raw_alternative_values: Vec<Expr> = vec![];
                 // Keep the attribute around for better error reporting.
                 let mut alt_attr_ref: Vec<&Attribute> = vec![];
 
@@ -398,7 +417,7 @@ impl Parse for EnumInfo {
                                         }
                                         NumEnumVariantAttributeItem::Alternatives(alternatives) => {
                                             attr_spans.alternatives.push(alternatives.span());
-                                            alternative_values.extend(alternatives.expressions);
+                                            raw_alternative_values.extend(alternatives.expressions);
                                             alt_attr_ref.push(attribute);
                                         }
                                     }
@@ -422,75 +441,116 @@ impl Parse for EnumInfo {
                     }
                 }
 
-                let canonical_value = discriminant;
-                let canonical_value_int = expr_to_int(&canonical_value)?;
+                let discriminant_value = parse_discriminant(&discriminant)?;
 
                 // Check for collision.
-                if val_set.contains(&canonical_value_int) {
-                    die!(ident => format!("The discriminant '{}' collides with a value attributed to a previous variant", canonical_value_int))
+                // We can't do const evaluation, or even compare arbitrary Exprs,
+                // so unfortunately we can't check for duplicates.
+                // That's not the end of the world, just we'll end up with compile errors for
+                // matches with duplicate branches in generated code instead of nice friendly error messages.
+                if let DiscriminantValue::Literal(canonical_value_int) = discriminant_value {
+                    if discriminant_int_val_set.contains(&canonical_value_int) {
+                        die!(ident => format!("The discriminant '{}' collides with a value attributed to a previous variant", canonical_value_int))
+                    }
                 }
 
                 // Deal with the alternative values.
-                let alt_val = alternative_values
+                let alternate_values = raw_alternative_values
                     .iter()
-                    .map(expr_to_int)
+                    .map(parse_discriminant)
                     .collect::<Result<Vec<_>>>()?;
 
-                debug_assert_eq!(alt_val.len(), alternative_values.len());
-
-                if !alt_val.is_empty() {
-                    let mut alt_val_sorted = alt_val.clone();
-                    alt_val_sorted.sort_unstable();
-                    let alt_val_sorted = alt_val_sorted;
-
-                    // check if the current discriminant is not in the alternative values.
-                    if let Some(i) = alt_val.iter().position(|&x| x == canonical_value_int) {
-                        die!(&alternative_values[i] => format!("'{}' in the alternative values is already attributed as the discriminant of this variant", canonical_value_int));
+                debug_assert_eq!(alternate_values.len(), raw_alternative_values.len());
+
+                if !alternate_values.is_empty() {
+                    let mut sorted_alternate_int_values = alternate_values
+                        .into_iter()
+                        .map(|v| {
+                            match v {
+                                DiscriminantValue::Literal(value) => Ok(value),
+                                DiscriminantValue::Expr(expr) => {
+                                    // We can't do uniqueness checking on non-literals, so we don't allow them as alternate values.
+                                    // We could probably allow them, but there doesn't seem to be much of a use-case,
+                                    // and it's easier to give good error messages about duplicate values this way,
+                                    // rather than rustc errors on conflicting match branches.
+                                    die!(expr => format!("Only literals are allowed as num_enum alternate values"))
+                                },
+                            }
+                        })
+                        .collect::<Result<Vec<i128>>>()?;
+                    sorted_alternate_int_values.sort_unstable();
+                    let sorted_alternate_int_values = sorted_alternate_int_values;
+
+                    // Check if the current discriminant is not in the alternative values.
+                    if let DiscriminantValue::Literal(canonical_value_int) = discriminant_value {
+                        if let Ok(index) = sorted_alternate_int_values
+                            .binary_search(&canonical_value_int)
+                        {
+                            die!(&raw_alternative_values[index] => format!("'{}' in the alternative values is already attributed as the discriminant of this variant", canonical_value_int));
+                        }
                     }
 
                     // Search for duplicates, the vec is sorted. Warn about them.
-                    if (1..alt_val_sorted.len()).any(|i| alt_val_sorted[i] == alt_val_sorted[i - 1])
-                    {
+                    if (1..sorted_alternate_int_values.len()).any(|i| {
+                        sorted_alternate_int_values[i] == sorted_alternate_int_values[i - 1]
+                    }) {
                         let attr = *alt_attr_ref.last().unwrap();
                         die!(attr => "There is duplication in the alternative values");
                     }
-                    // Search if those alt_val where already attributed.
-                    // (The val_set is BTreeSet, and iter().next_back() is the is the maximum in the set.)
-                    if let Some(last_upper_val) = val_set.iter().next_back() {
-                        if alt_val_sorted.first().unwrap() <= last_upper_val {
-                            for (i, val) in alt_val_sorted.iter().enumerate() {
-                                if val_set.contains(val) {
-                                    die!(&alternative_values[i] => format!("'{}' in the alternative values is already attributed to a previous variant", val));
+                    // Search if those discriminant_int_val_set where already attributed.
+                    // (discriminant_int_val_set is BTreeSet, and iter().next_back() is the is the maximum in the set.)
+                    if let Some(last_upper_val) = discriminant_int_val_set.iter().next_back() {
+                        if sorted_alternate_int_values.first().unwrap() <= last_upper_val {
+                            for (i, val) in sorted_alternate_int_values.iter().enumerate() {
+                                if discriminant_int_val_set.contains(val) {
+                                    die!(&raw_alternative_values[i] => format!("'{}' in the alternative values is already attributed to a previous variant", val));
                                 }
                             }
                         }
                     }
 
                     // Reconstruct the alternative_values vec of Expr but sorted.
-                    alternative_values = alt_val_sorted
+                    raw_alternative_values = sorted_alternate_int_values
                         .iter()
                         .map(|val| literal(val.to_owned()))
                         .collect();
 
                     // Add the alternative values to the the set to keep track.
-                    val_set.extend(alt_val_sorted);
+                    discriminant_int_val_set.extend(sorted_alternate_int_values);
                 }
 
                 // Add the current discriminant to the the set to keep track.
-                let newly_inserted = val_set.insert(canonical_value_int);
-                debug_assert!(newly_inserted);
+                if let DiscriminantValue::Literal(canonical_value_int) = discriminant_value {
+                    discriminant_int_val_set.insert(canonical_value_int);
+                }
 
                 variants.push(VariantInfo {
                     ident,
                     attr_spans,
                     is_default,
                     is_catch_all,
-                    canonical_value,
-                    alternative_values,
+                    canonical_value: discriminant,
+                    alternative_values: raw_alternative_values,
                 });
 
                 // Get the next value for the discriminant.
-                next_discriminant = literal(canonical_value_int + 1);
+                next_discriminant = match discriminant_value {
+                    DiscriminantValue::Literal(int_value) => {
+                        if int_value >= -1 {
+                            literal(int_value + 1)
+                        } else {
+                            let value = literal((int_value + 1).abs());
+                            parse_quote! {
+                                -#value
+                            }
+                        }
+                    }
+                    DiscriminantValue::Expr(expr) => {
+                        parse_quote! {
+                            #repr::wrapping_add(#expr, 1)
+                        }
+                    }
+                }
             }
 
             EnumInfo {