Implements the Never ! types as a TypeInfo bottom type. (FuelLabs#5602

) ## Description We now parse the `!` as a TypeInfo::Never, and remove the usage of empty enums as Never type in our code. This commit removes completely the DeterministicallyAborts and TypeCheckUnificationContext. The DeterministicallyAborts can be removed because the Never TypeInfo is now propagated using the type checker. Code blocks that return, break, continue, or call an expression that returns Never, are marked as Never. Partially fixes FuelLabs#5562. ## Checklist - [x] I have linked to any relevant issues. - [x] I have commented my code, particularly in hard-to-understand areas. - [x] I have updated the documentation where relevant (API docs, the reference, and the Sway book). - [x] I have added tests that prove my fix is effective or that my feature works. - [x] I have added (or requested a maintainer to add) the necessary `Breaking*` or `New Feature` labels where relevant. - [x] I have done my best to ensure that my PR adheres to [the Fuel Labs Code Review Standards](https://github.com/FuelLabs/rfcs/blob/master/text/code-standards/external-contributors.md). - [x] I have requested a review from the relevant team or maintainers.
sathvikvital · Feb 20, 2024 · b32d0e0 · b32d0e0
1 parent 08b102d
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diff --git a/docs/book/src/SUMMARY.md b/docs/book/src/SUMMARY.md
@@ -42,6 +42,7 @@
   - [Associated Types](./advanced/associated_types.md)
   - [Generics and Trait Constraints](./advanced/generics_and_trait_constraints.md)
   - [Assembly](./advanced/assembly.md)
+  - [Never Type](./advanced/never_type.md)
 - [Common Collections](./common-collections/index.md)
   - [Vectors on the Heap](./common-collections/vec.md)
   - [Storage Vectors](./common-collections/storage_vec.md)

diff --git a/docs/book/src/advanced/index.md b/docs/book/src/advanced/index.md
@@ -8,3 +8,4 @@ Advanced concepts.
 - [Associated Types](./associated_types.md)
 - [Generics and Trait Constraints](./generics_and_trait_constraints.md)
 - [Assembly](./assembly.md)
+- [Never Type](./never_type.md)
diff --git a/docs/book/src/advanced/never_type.md b/docs/book/src/advanced/never_type.md
@@ -0,0 +1,53 @@
+# Never Type
+
+The Never type `!` represents the type of computations which never resolve to any value at all.
+
+## Additional Information
+
+`break`, `continue` and `return` expressions also have type `!`. For example we are allowed to
+write:
+
+```sway
+let x: ! = {
+    return 123
+};
+```
+
+Although the `let` is pointless here, it illustrates the meaning of `!`. Since `x` is never
+assigned a value (because `return` returns from the entire function), `x` can be given type
+`Never`. We could also replace `return 123` with a `revert()` or a never-ending `loop` and this code
+would still be valid.
+
+A more realistic usage of `Never` is in this code:
+
+```sway
+let num: u32 = match get_a_number() {
+    Some(num) => num,
+    None => break,
+};
+```
+
+Both match arms must produce values of type [`u32`], but since `break` never produces a value
+at all we know it can never produce a value which isn't a [`u32`]. This illustrates another
+behaviour of the `!` type - expressions with type `!` will coerce into any other type.
+
+Note that `!` type coerces into any other type, another example of this would be:
+
+```sway
+let x: u32 = {
+    return 123
+};
+```
+
+Regardless of the type of `x`, the return block of type `Never` will always coerce into `x` type.
+
+## Examples
+
+```sway
+fn foo() {
+    let num: u64 = match Option::None::<u64> {
+        Some(num) => num,
+        None => return,
+    };
+}
+```
diff --git a/forc-plugins/forc-doc/src/tests/expects/impl_trait/mod.rs b/forc-plugins/forc-doc/src/tests/expects/impl_trait/mod.rs
@@ -37,7 +37,7 @@ fn test_impl_traits_default() {
     assert_search_js(
         &doc_path,
         &expect![[r#"
-            var SEARCH_INDEX={"core":[{"html_filename":"trait.AsRawSlice.html","module_info":["core","raw_slice"],"name":"AsRawSlice","preview":"Trait to return a type as a <code>raw_slice</code>.\n","type_name":"trait"},{"html_filename":"fn.from_str_array.html","module_info":["core","str"],"name":"from_str_array","preview":"","type_name":"function"},{"html_filename":"trait.Add.html","module_info":["core","ops"],"name":"Add","preview":"Trait for the addition of two values.\n","type_name":"trait"},{"html_filename":"trait.Subtract.html","module_info":["core","ops"],"name":"Subtract","preview":"Trait for the subtraction of two values.\n","type_name":"trait"},{"html_filename":"trait.Multiply.html","module_info":["core","ops"],"name":"Multiply","preview":"Trait for the multiplication of two values.\n","type_name":"trait"},{"html_filename":"trait.Divide.html","module_info":["core","ops"],"name":"Divide","preview":"Trait for the division of two values.\n","type_name":"trait"},{"html_filename":"trait.Mod.html","module_info":["core","ops"],"name":"Mod","preview":"Trait for the modulo of two values.\n","type_name":"trait"},{"html_filename":"trait.Not.html","module_info":["core","ops"],"name":"Not","preview":"Trait to invert a type.\n","type_name":"trait"},{"html_filename":"trait.Eq.html","module_info":["core","ops"],"name":"Eq","preview":"Trait to evaluate if two types are equal.\n","type_name":"trait"},{"html_filename":"trait.Ord.html","module_info":["core","ops"],"name":"Ord","preview":"Trait to evaluate if one value is greater or less than another of the same type.\n","type_name":"trait"},{"html_filename":"trait.BitwiseAnd.html","module_info":["core","ops"],"name":"BitwiseAnd","preview":"Trait to bitwise AND two values of the same type.\n","type_name":"trait"},{"html_filename":"trait.BitwiseOr.html","module_info":["core","ops"],"name":"BitwiseOr","preview":"Trait to bitwise OR two values of the same type.\n","type_name":"trait"},{"html_filename":"trait.BitwiseXor.html","module_info":["core","ops"],"name":"BitwiseXor","preview":"Trait to bitwise XOR two values of the same type.\n","type_name":"trait"},{"html_filename":"trait.Shift.html","module_info":["core","ops"],"name":"Shift","preview":"Trait to bit shift a value.\n","type_name":"trait"},{"html_filename":"enum.Never.html","module_info":["core","never"],"name":"Never","preview":"<code>Never</code> represents the type of computations which never resolve to any value at all.","type_name":"enum"},{"html_filename":"struct.StorageKey.html","module_info":["core","storage"],"name":"StorageKey","preview":"Describes a location in storage.\n","type_name":"struct"},{"html_filename":"struct.Buffer.html","module_info":["core","codec"],"name":"Buffer","preview":"","type_name":"struct"},{"html_filename":"trait.AbiEncode.html","module_info":["core","codec"],"name":"AbiEncode","preview":"","type_name":"trait"},{"html_filename":"fn.encode.html","module_info":["core","codec"],"name":"encode","preview":"","type_name":"function"}],"impl_traits":[{"html_filename":"trait.Foo.html","module_info":["impl_traits","foo"],"name":"Foo","preview":"","type_name":"trait"},{"html_filename":"trait.Baz.html","module_info":["impl_traits","foo"],"name":"Baz","preview":"","type_name":"trait"},{"html_filename":"struct.Bar.html","module_info":["impl_traits","bar"],"name":"Bar","preview":"","type_name":"struct"}]};
+            var SEARCH_INDEX={"core":[{"html_filename":"trait.AsRawSlice.html","module_info":["core","raw_slice"],"name":"AsRawSlice","preview":"Trait to return a type as a <code>raw_slice</code>.\n","type_name":"trait"},{"html_filename":"fn.from_str_array.html","module_info":["core","str"],"name":"from_str_array","preview":"","type_name":"function"},{"html_filename":"trait.Add.html","module_info":["core","ops"],"name":"Add","preview":"Trait for the addition of two values.\n","type_name":"trait"},{"html_filename":"trait.Subtract.html","module_info":["core","ops"],"name":"Subtract","preview":"Trait for the subtraction of two values.\n","type_name":"trait"},{"html_filename":"trait.Multiply.html","module_info":["core","ops"],"name":"Multiply","preview":"Trait for the multiplication of two values.\n","type_name":"trait"},{"html_filename":"trait.Divide.html","module_info":["core","ops"],"name":"Divide","preview":"Trait for the division of two values.\n","type_name":"trait"},{"html_filename":"trait.Mod.html","module_info":["core","ops"],"name":"Mod","preview":"Trait for the modulo of two values.\n","type_name":"trait"},{"html_filename":"trait.Not.html","module_info":["core","ops"],"name":"Not","preview":"Trait to invert a type.\n","type_name":"trait"},{"html_filename":"trait.Eq.html","module_info":["core","ops"],"name":"Eq","preview":"Trait to evaluate if two types are equal.\n","type_name":"trait"},{"html_filename":"trait.Ord.html","module_info":["core","ops"],"name":"Ord","preview":"Trait to evaluate if one value is greater or less than another of the same type.\n","type_name":"trait"},{"html_filename":"trait.BitwiseAnd.html","module_info":["core","ops"],"name":"BitwiseAnd","preview":"Trait to bitwise AND two values of the same type.\n","type_name":"trait"},{"html_filename":"trait.BitwiseOr.html","module_info":["core","ops"],"name":"BitwiseOr","preview":"Trait to bitwise OR two values of the same type.\n","type_name":"trait"},{"html_filename":"trait.BitwiseXor.html","module_info":["core","ops"],"name":"BitwiseXor","preview":"Trait to bitwise XOR two values of the same type.\n","type_name":"trait"},{"html_filename":"trait.Shift.html","module_info":["core","ops"],"name":"Shift","preview":"Trait to bit shift a value.\n","type_name":"trait"},{"html_filename":"struct.StorageKey.html","module_info":["core","storage"],"name":"StorageKey","preview":"Describes a location in storage.\n","type_name":"struct"},{"html_filename":"struct.Buffer.html","module_info":["core","codec"],"name":"Buffer","preview":"","type_name":"struct"},{"html_filename":"trait.AbiEncode.html","module_info":["core","codec"],"name":"AbiEncode","preview":"","type_name":"trait"},{"html_filename":"fn.encode.html","module_info":["core","codec"],"name":"encode","preview":"","type_name":"function"}],"impl_traits":[{"html_filename":"trait.Foo.html","module_info":["impl_traits","foo"],"name":"Foo","preview":"","type_name":"trait"},{"html_filename":"trait.Baz.html","module_info":["impl_traits","foo"],"name":"Baz","preview":"","type_name":"trait"},{"html_filename":"struct.Bar.html","module_info":["impl_traits","bar"],"name":"Bar","preview":"","type_name":"struct"}]};
             "object"==typeof exports&&"undefined"!=typeof module&&(module.exports=SEARCH_INDEX);"#]],
     );
     assert_file_tree(
@@ -50,8 +50,6 @@ fn test_impl_traits_default() {
             "core/codec/struct.Buffer.html",
             "core/codec/trait.AbiEncode.html",
             "core/index.html",
-            "core/never/enum.Never.html",
-            "core/never/index.html",
             "core/ops/index.html",
             "core/ops/trait.Add.html",
             "core/ops/trait.BitwiseAnd.html",

diff --git a/sway-ast/src/ty/mod.rs b/sway-ast/src/ty/mod.rs
@@ -26,6 +26,9 @@ pub enum Ty {
         ampersand_token: AmpersandToken,
         ty: Box<Ty>,
     },
+    Never {
+        bang_token: BangToken,
+    },
 }
 
 impl Spanned for Ty {
@@ -43,6 +46,7 @@ impl Spanned for Ty {
                 ampersand_token,
                 ty,
             } => Span::join(ampersand_token.span(), ty.span()),
+            Ty::Never { bang_token } => bang_token.span(),
         }
     }
 }

diff --git a/sway-core/src/abi_generation/evm_abi.rs b/sway-core/src/abi_generation/evm_abi.rs
@@ -83,6 +83,7 @@ pub fn abi_str(type_info: &TypeInfo, type_engine: &TypeEngine, decl_engine: &Dec
     use TypeInfo::*;
     match type_info {
         Unknown => "unknown".into(),
+        Never => "never".into(),
         UnknownGeneric { name, .. } => name.to_string(),
         Placeholder(_) => "_".to_string(),
         TypeParam(n) => format!("typeparam({n})"),

diff --git a/sway-core/src/abi_generation/fuel_abi.rs b/sway-core/src/abi_generation/fuel_abi.rs
@@ -807,6 +807,7 @@ impl TypeInfo {
         use TypeInfo::*;
         match self {
             Unknown => "unknown".into(),
+            Never => "never".into(),
             UnknownGeneric { name, .. } => name.to_string(),
             Placeholder(_) => "_".to_string(),
             TypeParam(n) => format!("typeparam({n})"),

diff --git a/sway-core/src/asm_generation/fuel/fuel_asm_builder.rs b/sway-core/src/asm_generation/fuel/fuel_asm_builder.rs
@@ -1728,6 +1728,7 @@ impl<'ir, 'eng> FuelAsmBuilder<'ir, 'eng> {
     // XXX reassess all the places we use this
     pub(crate) fn is_copy_type(&self, ty: &Type) -> bool {
         ty.is_unit(self.context)
+            || ty.is_never(self.context)
             || ty.is_bool(self.context)
             || ty
                 .get_uint_width(self.context)

diff --git a/sway-core/src/ir_generation/convert.rs b/sway-core/src/ir_generation/convert.rs
@@ -159,6 +159,7 @@ fn convert_resolved_type(
             convert_resolved_typeid(type_engine, decl_engine, context, &ty.type_id, span)?
         }
         TypeInfo::Ref(_) => Type::get_uint64(context),
+        TypeInfo::Never => Type::get_never(context),
 
         // Unsupported types which shouldn't exist in the AST after type checking and
         // monomorphisation.

diff --git a/sway-core/src/language/ty/ast_node.rs b/sway-core/src/language/ty/ast_node.rs
@@ -135,18 +135,6 @@ impl CollectTypesMetadata for TyAstNode {
     }
 }
 
-impl DeterministicallyAborts for TyAstNode {
-    fn deterministically_aborts(&self, decl_engine: &DeclEngine, check_call_body: bool) -> bool {
-        use TyAstNodeContent::*;
-        match &self.content {
-            Declaration(_) => false,
-            Expression(exp) => exp.deterministically_aborts(decl_engine, check_call_body),
-            SideEffect(_) => false,
-            Error(_, _) => false,
-        }
-    }
-}
-
 impl GetDeclIdent for TyAstNode {
     fn get_decl_ident(&self) -> Option<Ident> {
         self.content.get_decl_ident()

diff --git a/sway-core/src/language/ty/code_block.rs b/sway-core/src/language/ty/code_block.rs
@@ -5,7 +5,7 @@ use sway_types::Span;
 
 use crate::{
     decl_engine::*, engine_threading::*, language::ty::*, semantic_analysis::TypeCheckContext,
-    type_system::*, types::DeterministicallyAborts,
+    type_system::*,
 };
 
 #[derive(Clone, Debug)]
@@ -74,11 +74,3 @@ impl UpdateConstantExpression for TyCodeBlock {
             .for_each(|x| x.update_constant_expression(engines, implementing_type));
     }
 }
-
-impl DeterministicallyAborts for TyCodeBlock {
-    fn deterministically_aborts(&self, decl_engine: &DeclEngine, check_call_body: bool) -> bool {
-        self.contents
-            .iter()
-            .any(|x| x.deterministically_aborts(decl_engine, check_call_body))
-    }
-}
diff --git a/sway-core/src/language/ty/expression/expression.rs b/sway-core/src/language/ty/expression/expression.rs
@@ -317,109 +317,6 @@ impl CollectTypesMetadata for TyExpression {
     }
 }
 
-impl DeterministicallyAborts for TyExpression {
-    fn deterministically_aborts(&self, decl_engine: &DeclEngine, check_call_body: bool) -> bool {
-        use TyExpressionVariant::*;
-        match &self.expression {
-            FunctionApplication {
-                fn_ref, arguments, ..
-            } => {
-                if !check_call_body {
-                    return false;
-                }
-                let function_decl = decl_engine.get_function(fn_ref);
-                function_decl
-                    .body
-                    .deterministically_aborts(decl_engine, check_call_body)
-                    || arguments
-                        .iter()
-                        .any(|(_, x)| x.deterministically_aborts(decl_engine, check_call_body))
-            }
-            Tuple { fields, .. } => fields
-                .iter()
-                .any(|x| x.deterministically_aborts(decl_engine, check_call_body)),
-            Array { contents, .. } => contents
-                .iter()
-                .any(|x| x.deterministically_aborts(decl_engine, check_call_body)),
-            CodeBlock(contents) => contents.deterministically_aborts(decl_engine, check_call_body),
-            LazyOperator { lhs, .. } => lhs.deterministically_aborts(decl_engine, check_call_body),
-            StructExpression { fields, .. } => fields.iter().any(|x| {
-                x.value
-                    .deterministically_aborts(decl_engine, check_call_body)
-            }),
-            EnumInstantiation { contents, .. } => contents
-                .as_ref()
-                .map(|x| x.deterministically_aborts(decl_engine, check_call_body))
-                .unwrap_or(false),
-            AbiCast { address, .. } => {
-                address.deterministically_aborts(decl_engine, check_call_body)
-            }
-            StructFieldAccess { .. }
-            | Literal(_)
-            | StorageAccess { .. }
-            | VariableExpression { .. }
-            | ConstantExpression { .. }
-            | FunctionParameter
-            | TupleElemAccess { .. } => false,
-            IntrinsicFunction(kind) => kind.deterministically_aborts(decl_engine, check_call_body),
-            ArrayIndex { prefix, index } => {
-                prefix.deterministically_aborts(decl_engine, check_call_body)
-                    || index.deterministically_aborts(decl_engine, check_call_body)
-            }
-            AsmExpression { registers, .. } => registers.iter().any(|x| {
-                x.initializer
-                    .as_ref()
-                    .map(|x| x.deterministically_aborts(decl_engine, check_call_body))
-                    .unwrap_or(false)
-            }),
-            MatchExp { desugared, .. } => {
-                desugared.deterministically_aborts(decl_engine, check_call_body)
-            }
-            IfExp {
-                condition,
-                then,
-                r#else,
-                ..
-            } => {
-                condition.deterministically_aborts(decl_engine, check_call_body)
-                    || (then.deterministically_aborts(decl_engine, check_call_body)
-                        && r#else
-                            .as_ref()
-                            .map(|x| x.deterministically_aborts(decl_engine, check_call_body))
-                            .unwrap_or(false))
-            }
-            AbiName(_) => false,
-            EnumTag { exp } => exp.deterministically_aborts(decl_engine, check_call_body),
-            UnsafeDowncast { exp, .. } => {
-                exp.deterministically_aborts(decl_engine, check_call_body)
-            }
-            WhileLoop { condition, body } => {
-                condition.deterministically_aborts(decl_engine, check_call_body)
-                    || body.deterministically_aborts(decl_engine, check_call_body)
-            }
-            ForLoop { desugared } => {
-                desugared.deterministically_aborts(decl_engine, check_call_body)
-            }
-            Break => false,
-            Continue => false,
-            Reassignment(reassignment) => reassignment
-                .rhs
-                .deterministically_aborts(decl_engine, check_call_body),
-            ImplicitReturn(exp) => exp.deterministically_aborts(decl_engine, check_call_body),
-            // TODO: Is this correct?
-            // I'm not sure what this function is supposed to do exactly. It's called
-            // "deterministically_aborts" which I thought meant it checks for an abort/panic, but
-            // it's actually checking for returns.
-            //
-            // Also, is it necessary to check the expression to see if avoids the return? eg.
-            // someone could write `return break;` in a loop, which would mean the return never
-            // gets executed.
-            Return(_) => true,
-            Ref(exp) | Deref(exp) => exp.deterministically_aborts(decl_engine, check_call_body),
-        }
-    }
-}
-
 impl TyExpression {
     pub(crate) fn error(err: ErrorEmitted, span: Span, engines: &Engines) -> TyExpression {
         let type_engine = engines.te();

diff --git a/sway-core/src/language/ty/expression/intrinsic_function.rs b/sway-core/src/language/ty/expression/intrinsic_function.rs
@@ -3,9 +3,7 @@ use std::{
     hash::{Hash, Hasher},
 };
 
-use crate::{
-    decl_engine::DeclEngine, engine_threading::*, language::ty::*, type_system::*, types::*,
-};
+use crate::{engine_threading::*, language::ty::*, type_system::*, types::*};
 use itertools::Itertools;
 use sway_ast::Intrinsic;
 use sway_error::handler::{ErrorEmitted, Handler};
@@ -99,16 +97,6 @@ impl DebugWithEngines for TyIntrinsicFunctionKind {
     }
 }
 
-impl DeterministicallyAborts for TyIntrinsicFunctionKind {
-    fn deterministically_aborts(&self, decl_engine: &DeclEngine, check_call_body: bool) -> bool {
-        matches!(self.kind, Intrinsic::Revert)
-            || self
-                .arguments
-                .iter()
-                .any(|x| x.deterministically_aborts(decl_engine, check_call_body))
-    }
-}
-
 impl CollectTypesMetadata for TyIntrinsicFunctionKind {
     fn collect_types_metadata(
         &self,

diff --git a/sway-core/src/semantic_analysis.rs b/sway-core/src/semantic_analysis.rs
@@ -9,10 +9,8 @@ mod program;
 mod type_check_analysis;
 pub(crate) mod type_check_context;
 mod type_check_finalization;
-mod type_check_unification;
 pub use ast_node::*;
 pub use namespace::Namespace;
 pub(crate) use type_check_analysis::*;
 pub(crate) use type_check_context::TypeCheckContext;
 pub(crate) use type_check_finalization::*;
-pub(crate) use type_check_unification::*;