feat: remove star projection and use-site variance (#597)

### Summary of Changes

There is currently no use-case for star projections and use-site
variance, so they are not worth the effort to implement. Moreover, these
concepts are difficult to explain.

Author: lars-reimann

docs/language/common/types.md

@@ -67,10 +67,7 @@ When we use this [class][classes] as a named type, we need to specify the value
 
 In the case of positional type arguments, they are mapped to [type parameters][type-parameters] by position, i.e. the first type argument is assigned to the first [type parameter][type-parameters], the second type argument is assigned to the second [type parameter][type-parameters] and so forth.
 
-If a positional type argument is used, we just write down its value. The value is either
-
-- a [type projection](#type-projection), or
-- a [star projection](#star-projection).
+If a positional type argument is used, we just write down its value, which is a [type projection](#type-projection).
 
 For example, if we expect a list of integers, we could use the following type:
 
@@ -98,7 +95,7 @@ These are the syntactic elements:
 - A named type argument (here `T = Int`). This in turn consists of
   - The name of the [type parameter][type-parameters] (here `T`)
   - An equals sign.
-  - The value of the type argument, which is still either a [type projection](#type-projection), or a [star projection](#star-projection).
+  - The value of the type argument, which is still a [type projection](#type-projection).
 - A closing angle bracket.
 
 Within a list of type arguments both positional and named type arguments can be used. However, after the first named type arguments all type arguments must be named.
@@ -136,43 +133,6 @@ SomeSpecialList<Int>
 
 The value of the type argument is just another named type (here `Int`).
 
-###### Use-Site Variance
-
-It is also possible to set the [variance][variance] of a [type parameter][type-parameters] at the use-site, i.e. where we use the containing declaration as a named type. This is only possible, however, if we did not [specify the variance at the declaration-site][declaration-site-variance].
-
-Covariance is denoted by the keyword `out`. If the variance of a [type parameter][type-parameters] is set to `out`, we can only access methods of the class that only use the [type parameter][type-parameters] in the out-position, i.e. as [results][results]. Methods that use the [type parameter][type-parameters] in the in-position, i.e. as [parameters][parameters] are hidden. Here is an example for the syntax:
-
-```txt
-SomeSpecialList<out Int>
-```
-
-The key element here is the keyword `out` that is added to the type argument. This essentially creates a list of integers that can be read from but that cannot be written to.
-
-Contravariance is denoted by the keyword `in`. If the variance of a [type parameter][type-parameters] is set to `in`, we can only access methods of the class that only use the [type parameter][type-parameters] in the in-position, i.e. as [parameters][parameters]. Methods that use the [type parameter][type-parameters] in the out-position, i.e. as [results][results] are hidden. Here is an example of the syntax:
-
-```txt
-SomeSpecialList<in Int>
-```
-
-The key element here is the keyword `in` that is added to the type argument. Here we essentially create a list of integers that can be written to but not read from.
-
-##### Star Projection
-
-If we do not want to specify a value for a [type parameter][type-parameters] and just accept everything, we can use a _star projection_. Here is the syntax:
-
-```txt
-SomeSpecialList<*>
-```
-
-It consists only of the `*`, which we use as the value of the type argument.
-
-The star projection is usually equivalent to the type projections
-
-- `out Any?` (`Any?` is the supertype of everything), or
-- `in Nothing` (`Nothing` is the subtype of everything).
-
-If the [type parameter][type-parameters] has [bounds][type-parameter-bounds], however, the star projection denotes that any type within the [bounds][type-parameter-bounds] is acceptable.
-
 ### Member Types
 
 A member type is essentially the same as a [named type](#named-types) with the difference that the declaration we refer to is nested inside [classes][classes] or [enums][enums].

docs/language/common/variance.md

@@ -2,7 +2,7 @@
 
 **Note:** This is an advanced section. Feel free to skip it initially.
 
-Variance deals with the question which generic types are compatible with each other. We explain this concept using the following [class][classes]:
+Variance deals with the question, which generic types are compatible with each other. We explain this concept using the following [class][classes]:
 
 ```txt
 class Stack<T>(vararg initialElements: T) {
@@ -64,20 +64,17 @@ class Stack<in T> {
 
 ## Specifying Variance
 
-The variance of a [type parameter][type-parameters] can either be declared at its [declaration site][declaration-site-variance] or its [use site][use-site-variance]. If it is specified already at the [declaration site][declaration-site-variance], however, [use-site variance][use-site-variance] is no longer available.
+The variance of a [type parameter][type-parameters] can only be declared at its [declaration site][declaration-site-variance], using the syntax shown in the following table:
 
-The following table sums up the syntax of [declaration-site variance][declaration-site-variance], where the [class][classes] declaration is changed, and [use-site variance][use-site-variance], where the [type arguments][type-arguments] passed by the [named type][named-types] Refer to the linked documents for more details.
-
-| Desired Variance | Declaration Site     | Use Site       |
-|------------------|----------------------|----------------|
-| Invariant        | `class Stack<T>`     | `Stack<T>`     |
-| Covariant        | `class Stack<out T>` | `Stack<out T>` |
-| Contravariant    | `class Stack<in T>`  | `Stack<in T>`  |
+| Desired Variance | Declaration Site     |
+|------------------|----------------------|
+| Invariant        | `class Stack<T>`     |
+| Covariant        | `class Stack<out T>` |
+| Contravariant    | `class Stack<in T>`  |
 
 [types]: types.md
 [named-types]: types.md#named-types
 [type-arguments]: types.md#type-arguments
-[use-site-variance]: types.md#use-site-variance
 [parameters]: parameters.md
 [results]: results.md
 [classes]: ../stub-language/classes.md

src/language/formatting/safe-ds-formatter.ts

@@ -173,8 +173,6 @@ export class SafeDsFormatter extends AbstractFormatter {
             this.formatSdsTypeArgumentList(node);
         } else if (ast.isSdsTypeArgument(node)) {
             this.formatSdsTypeArgument(node);
-        } else if (ast.isSdsTypeProjection(node)) {
-            this.formatSdsTypeProjection(node);
         }
 
         // -----------------------------------------------------------------------------
@@ -858,12 +856,6 @@ export class SafeDsFormatter extends AbstractFormatter {
         formatter.keyword('=').surround(oneSpace());
     }
 
-    private formatSdsTypeProjection(node: ast.SdsTypeProjection) {
-        const formatter = this.getNodeFormatter(node);
-
-        formatter.property('variance').append(oneSpace());
-    }
-
     /**
      * Returns whether the type is considered complex and requires special formatting like placing the associated
      * parameter on its own line.

src/language/grammar/safe-ds.langium

@@ -957,16 +957,12 @@ SdsTypeArgument returns SdsTypeArgument:
 
 interface SdsTypeArgumentValue extends SdsObject {}
 
-interface SdsStarProjection extends SdsTypeArgumentValue {}
-
 interface SdsTypeProjection extends SdsTypeArgumentValue {
-	variance?: string
 	^type: SdsType
 }
 
 SdsTypeArgumentValue returns SdsTypeArgumentValue:
-    {SdsStarProjection} '*'
-  | {SdsTypeProjection} variance=SdsTypeParameterVariance? ^type=SdsType
+    {SdsTypeProjection} ^type=SdsType
 ;
 
 

tests/resources/formatting/types/named types/type argument and nullable.sdstest

@@ -1,9 +1,9 @@
 segment mySegment(
-    x: Int < * ,  in  Number ,  out  Number ,  T  =  Number > ?
+    x: Int < Number ,  T  =  Number > ?
 ) {}
 
 // -----------------------------------------------------------------------------
 
 segment mySegment(
-    x: Int<*, in Number, out Number, T = Number>?
+    x: Int<Number, T = Number>?
 ) {}

tests/resources/formatting/types/named types/type arguments and not nullable.sdstest

@@ -1,9 +1,9 @@
 segment mySegment(
-    x: Int < * ,  in  Number ,  out  Number ,  T  =  Number>
+    x: Int < Number ,  T  =  Number>
 ) {}
 
 // -----------------------------------------------------------------------------
 
 segment mySegment(
-    x: Int<*, in Number, out Number, T = Number>
+    x: Int<Number, T = Number>
 ) {}

tests/resources/grammar/types/named types/good-type argument and nullable.sdstest

@@ -1,5 +1,5 @@
 // $TEST$ no_syntax_error
 
 segment mySegment(
-    x: Int<*, in Number, out Number, T = Number>?
+    x: Int<Number, T = Number>?
 ) {}

tests/resources/grammar/types/named types/good-type arguments and not nullable.sdstest

@@ -1,5 +1,5 @@
 // $TEST$ no_syntax_error
 
 segment mySegment(
-    x: Int<*, in Number, out Number, T = Number>
+    x: Int<Number, T = Number>
 ) {}