feat(interface-types) Add the Vec1 type to represent a non-empty vector.

`Vec1` is used by `RecordType` to ensure that a record have at least 1
field. Then an `InterfaceValue::Record` is ensured to get at least one
value with the type-checking pass.

src/ast.rs

@@ -1,7 +1,7 @@
 //! Represents the WIT language as a tree. This is the central
 //! representation of the language.
 
-use crate::interpreter::Instruction;
+use crate::{interpreter::Instruction, vec1::Vec1};
 use std::str;
 
 /// Represents the types supported by WIT.
@@ -57,7 +57,7 @@ pub enum InterfaceType {
 #[derive(PartialEq, Debug, Clone)]
 pub struct RecordType {
     /// Types representing the fields.
-    pub fields: Vec<InterfaceType>,
+    pub fields: Vec1<InterfaceType>,
 }
 
 /// Represents the kind of type.

src/decoders/binary.rs

@@ -1,6 +1,6 @@
 //! Parse the WIT binary representation into an [AST](crate::ast).
 
-use crate::{ast::*, interpreter::Instruction};
+use crate::{ast::*, interpreter::Instruction, vec1::Vec1};
 use nom::{
     error::{make_error, ErrorKind, ParseError},
     Err, IResult,
@@ -112,7 +112,12 @@ fn record_type<'input, E: ParseError<&'input [u8]>>(
 ) -> IResult<&'input [u8], RecordType, E> {
     let (output, fields) = list(input, ty)?;
 
-    Ok((output, RecordType { fields }))
+    Ok((
+        output,
+        RecordType {
+            fields: Vec1::new(fields).expect("Record must have at least one field, zero given."),
+        },
+    ))
 }
 
 /// Parse a UTF-8 string.
@@ -640,7 +645,7 @@ mod tests {
                 InterfaceType::I32,
                 InterfaceType::I64,
                 InterfaceType::Record(RecordType {
-                    fields: vec![InterfaceType::S32],
+                    fields: vec1![InterfaceType::S32],
                 }),
             ],
         ));
@@ -670,16 +675,16 @@ mod tests {
             &[0x01][..],
             vec![
                 RecordType {
-                    fields: vec![InterfaceType::String],
+                    fields: vec1![InterfaceType::String],
                 },
                 RecordType {
-                    fields: vec![InterfaceType::String, InterfaceType::I32],
+                    fields: vec1![InterfaceType::String, InterfaceType::I32],
                 },
                 RecordType {
-                    fields: vec![
+                    fields: vec1![
                         InterfaceType::String,
                         InterfaceType::Record(RecordType {
-                            fields: vec![InterfaceType::I32, InterfaceType::I32],
+                            fields: vec1![InterfaceType::I32, InterfaceType::I32],
                         }),
                         InterfaceType::F64,
                     ],
@@ -864,7 +869,7 @@ mod tests {
                     outputs: vec![InterfaceType::S32],
                 },
                 Type::Record(RecordType {
-                    fields: vec![InterfaceType::S32, InterfaceType::S32],
+                    fields: vec1![InterfaceType::S32, InterfaceType::S32],
                 }),
             ],
         ));

src/decoders/wat.rs

@@ -1,6 +1,6 @@
 //! Parse the WIT textual representation into an [AST](crate::ast).
 
-use crate::{ast::*, interpreter::Instruction};
+use crate::{ast::*, interpreter::Instruction, vec1::Vec1};
 pub use wast::parser::ParseBuffer as Buffer;
 use wast::parser::{self, Cursor, Parse, Parser, Peek, Result};
 
@@ -151,7 +151,9 @@ impl Parse<'_> for RecordType {
             })?);
         }
 
-        Ok(RecordType { fields })
+        Ok(RecordType {
+            fields: Vec1::new(fields).expect("Record must have at least one field, zero given."),
+        })
     }
 }
 
@@ -681,7 +683,7 @@ mod tests {
             InterfaceType::I32,
             InterfaceType::I64,
             InterfaceType::Record(RecordType {
-                fields: vec![InterfaceType::String],
+                fields: vec1![InterfaceType::String],
             }),
         ];
 
@@ -704,16 +706,16 @@ mod tests {
         ];
         let outputs = vec![
             RecordType {
-                fields: vec![InterfaceType::String],
+                fields: vec1![InterfaceType::String],
             },
             RecordType {
-                fields: vec![InterfaceType::String, InterfaceType::I32],
+                fields: vec1![InterfaceType::String, InterfaceType::I32],
             },
             RecordType {
-                fields: vec![
+                fields: vec1![
                     InterfaceType::String,
                     InterfaceType::Record(RecordType {
-                        fields: vec![InterfaceType::I32, InterfaceType::I32],
+                        fields: vec1![InterfaceType::I32, InterfaceType::I32],
                     }),
                     InterfaceType::F64,
                 ],
@@ -874,7 +876,7 @@ mod tests {
     fn test_type_record() {
         let input = buffer(r#"(@interface type (record (field string) (field i32)))"#);
         let output = Interface::Type(Type::Record(RecordType {
-            fields: vec![InterfaceType::String, InterfaceType::I32],
+            fields: vec1![InterfaceType::String, InterfaceType::I32],
         }));
 
         assert_eq!(parser::parse::<Interface>(&input).unwrap(), output);

src/encoders/binary.rs

@@ -445,7 +445,7 @@ mod tests {
         assert_to_bytes!(InterfaceType::I64, &[0x0d]);
         assert_to_bytes!(
             InterfaceType::Record(RecordType {
-                fields: vec![InterfaceType::String]
+                fields: vec1![InterfaceType::String]
             }),
             &[0x0e, 0x01, 0x0a]
         );
@@ -455,7 +455,7 @@ mod tests {
     fn test_record_type() {
         assert_to_bytes!(
             RecordType {
-                fields: vec![InterfaceType::String]
+                fields: vec1![InterfaceType::String]
             },
             &[
                 0x01, // 1 field
@@ -464,7 +464,7 @@ mod tests {
         );
         assert_to_bytes!(
             RecordType {
-                fields: vec![InterfaceType::String, InterfaceType::I32]
+                fields: vec1![InterfaceType::String, InterfaceType::I32]
             },
             &[
                 0x02, // 2 fields
@@ -474,10 +474,10 @@ mod tests {
         );
         assert_to_bytes!(
             RecordType {
-                fields: vec![
+                fields: vec1![
                     InterfaceType::String,
                     InterfaceType::Record(RecordType {
-                        fields: vec![InterfaceType::I32, InterfaceType::I32],
+                        fields: vec1![InterfaceType::I32, InterfaceType::I32],
                     }),
                     InterfaceType::F64,
                 ],
@@ -542,7 +542,7 @@ mod tests {
     fn test_type_record() {
         assert_to_bytes!(
             Type::Record(RecordType {
-                fields: vec![InterfaceType::I32, InterfaceType::I64],
+                fields: vec1![InterfaceType::I32, InterfaceType::I64],
             }),
             &[
                 0x01, // record type

src/encoders/wat.rs

@@ -368,7 +368,7 @@ mod tests {
             (&InterfaceType::I32).to_string(),
             (&InterfaceType::I64).to_string(),
             (&InterfaceType::Record(RecordType {
-                fields: vec![InterfaceType::String],
+                fields: vec1![InterfaceType::String],
             }))
                 .to_string(),
         ];
@@ -397,18 +397,18 @@ mod tests {
     fn test_record_type() {
         let inputs = vec![
             (&RecordType {
-                fields: vec![InterfaceType::String],
+                fields: vec1![InterfaceType::String],
             })
                 .to_string(),
             (&RecordType {
-                fields: vec![InterfaceType::String, InterfaceType::I32],
+                fields: vec1![InterfaceType::String, InterfaceType::I32],
             })
                 .to_string(),
             (&RecordType {
-                fields: vec![
+                fields: vec1![
                     InterfaceType::String,
                     InterfaceType::Record(RecordType {
-                        fields: vec![InterfaceType::I32, InterfaceType::I32],
+                        fields: vec1![InterfaceType::I32, InterfaceType::I32],
                     }),
                     InterfaceType::F64,
                 ],
@@ -539,7 +539,7 @@ mod tests {
             })
                 .to_string(),
             (&Type::Record(RecordType {
-                fields: vec![InterfaceType::String, InterfaceType::I32],
+                fields: vec1![InterfaceType::String, InterfaceType::I32],
             }))
                 .to_string(),
         ];

src/interpreter/instructions/mod.rs

@@ -326,10 +326,10 @@ pub(crate) mod tests {
                 },
                 memory: Memory::new(vec![Cell::new(0); 128]),
                 wit_types: vec![ast::Type::Record(ast::RecordType {
-                    fields: vec![
+                    fields: vec1![
                         InterfaceType::I32,
                         InterfaceType::Record(ast::RecordType {
-                            fields: vec![InterfaceType::String, InterfaceType::F32],
+                            fields: vec1![InterfaceType::String, InterfaceType::F32],
                         }),
                         InterfaceType::I64,
                     ],

src/interpreter/instructions/records.rs

@@ -293,7 +293,7 @@ mod tests {
                 let mut instance = Instance::new();
                 instance.wit_types.push(
                     Type::Record(RecordType {
-                        fields: vec![InterfaceType::I32, InterfaceType::I32],
+                        fields: vec1![InterfaceType::I32, InterfaceType::I32],
                     })
                 );
 

src/interpreter/wasm/values.rs

@@ -1,7 +1,7 @@
 //! Defines WIT values and associated operations.
 
 pub use crate::ast::{InterfaceType, RecordType};
-use crate::errors::WasmValueNativeCastError;
+use crate::{errors::WasmValueNativeCastError, vec1::Vec1};
 use std::{convert::TryFrom, slice::Iter};
 
 #[cfg(feature = "serde")]
@@ -85,7 +85,8 @@ impl Default for InterfaceValue {
 impl From<&Vec<InterfaceValue>> for RecordType {
     fn from(values: &Vec<InterfaceValue>) -> Self {
         RecordType {
-            fields: values.iter().map(Into::into).collect(),
+            fields: Vec1::new(values.iter().map(Into::into).collect())
+                .expect("Record must have at least one field, zero given."),
         }
     }
 }
@@ -225,7 +226,7 @@ mod tests {
                 InterfaceValue::S8(2)
             ])),
             InterfaceType::Record(RecordType {
-                fields: vec![InterfaceType::I32, InterfaceType::S8]
+                fields: vec1![InterfaceType::I32, InterfaceType::S8]
             })
         );
 
@@ -239,10 +240,10 @@ mod tests {
                 InterfaceValue::S8(2)
             ])),
             InterfaceType::Record(RecordType {
-                fields: vec![
+                fields: vec1![
                     InterfaceType::I32,
                     InterfaceType::Record(RecordType {
-                        fields: vec![InterfaceType::String, InterfaceType::F64]
+                        fields: vec1![InterfaceType::String, InterfaceType::F64]
                     }),
                     InterfaceType::S8
                 ]

src/lib.rs

@@ -57,3 +57,4 @@ pub mod decoders;
 pub mod encoders;
 pub mod errors;
 pub mod interpreter;
+pub mod vec1;

src/macros.rs

@@ -1,3 +1,28 @@
+/// This macro creates a `Vec1` by checking at compile-time that its
+/// invariant holds.
+#[allow(unused)]
+macro_rules! vec1 {
+    ($item:expr; 0) => {
+        compile_error!("Cannot create an empty `Vec1`, it violates its invariant.")
+    };
+
+    () => {
+        compile_error!("Cannot create an empty `Vec1`, it violates its invariant.")
+    };
+
+    ($item:expr; $length:expr) => {
+        {
+            crate::vec1::Vec1::new(vec![$item; $length]).unwrap()
+        }
+    };
+
+    ($($item:expr),+ $(,)?) => {
+        {
+            crate::vec1::Vec1::new(vec![$($item),*]).unwrap()
+        }
+    };
+}
+
 /// This macro runs a parser, extracts the next input and the parser
 /// output, and positions the next input on `$input`.
 macro_rules! consume {

src/vec1.rs

@@ -0,0 +1,62 @@
+//! `Vec1<T>` represents a non-empty `Vec<T>`.
+
+use std::{
+    error,
+    fmt::{self, Debug},
+    ops,
+};
+
+/// `Vec1<T>` represents a non-empty `Vec<T>`. It derefs to `Vec<T>`
+/// directly.
+#[derive(Clone, PartialEq)]
+pub struct Vec1<T>(Vec<T>)
+where
+    T: Debug;
+
+/// Represents the only error that can be emitted by `Vec1`, i.e. when
+/// the number of items is zero.
+#[derive(Debug)]
+pub struct EmptyVec;
+
+impl error::Error for EmptyVec {}
+
+impl fmt::Display for EmptyVec {
+    fn fmt(&self, formatter: &mut fmt::Formatter<'_>) -> fmt::Result {
+        write!(formatter, "Vec1 must as least contain one item, zero given")
+    }
+}
+
+impl<T> Vec1<T>
+where
+    T: Debug,
+{
+    /// Creates a new non-empty vector, based on an inner `Vec<T>`. If
+    /// the inner vector is empty, a `EmptyVec` error is returned.
+    pub fn new(items: Vec<T>) -> Result<Self, EmptyVec> {
+        if items.len() == 0 {
+            Err(EmptyVec)
+        } else {
+            Ok(Self(items))
+        }
+    }
+}
+
+impl<T> fmt::Debug for Vec1<T>
+where
+    T: Debug,
+{
+    fn fmt(&self, formatter: &mut fmt::Formatter<'_>) -> fmt::Result {
+        write!(formatter, "{:?}", self.0)
+    }
+}
+
+impl<T> ops::Deref for Vec1<T>
+where
+    T: Debug,
+{
+    type Target = Vec<T>;
+
+    fn deref(&self) -> &Self::Target {
+        &self.0
+    }
+}

tests/binary.rs

@@ -1,5 +1,6 @@
 use wasmer_interface_types::{
     ast::*, decoders::binary::parse, encoders::binary::ToBytes, interpreter::Instruction,
+    vec1::Vec1,
 };
 
 /// Tests an AST to binary, then binary to AST roundtrip.
@@ -16,7 +17,7 @@ fn test_binary_encoding_decoding_roundtrip() {
                 outputs: vec![InterfaceType::S32],
             },
             Type::Record(RecordType {
-                fields: vec![InterfaceType::String, InterfaceType::I32],
+                fields: Vec1::new(vec![InterfaceType::String, InterfaceType::I32]).unwrap(),
             }),
         ],
         imports: vec![Import {