Distinguish invocation and pattern substitutions on SILFunctionType.

In order to allow this, I've had to rework the syntax of substituted function types; what was previously spelled `<T> in () -> T for <X>` is now spelled `@substituted <T> () -> T for <X>`.  I think this is a nice improvement for readability, but it did require me to churn a lot of test cases.

Distinguishing the substitutions has two chief advantages over the existing representation.  First, the semantics seem quite a bit clearer at use points; the `implicit` bit was very subtle and not always obvious how to use.  More importantly, it allows the expression of generic function types that must satisfy a particular generic abstraction pattern, which was otherwise impossible to express.

As an example of the latter, consider the following protocol conformance:

```
protocol P { func foo() }
struct A<T> : P { func foo() {} }
```

The lowered signature of `P.foo` is `<Self: P> (@in_guaranteed Self) -> ()`.  Without this change, the lowered signature of `A.foo`'s witness would be `<T> (@in_guaranteed A<T>) -> ()`, which does not preserve information about the conformance substitution in any useful way.  With this change, the lowered signature of this witness could be `<T> @Substituted <Self: P> (@in_guaranteed Self) -> () for <A<T>>`, which nicely preserves the exact substitutions which relate the witness to the requirement.

When we adopt this, it will both obviate the need for the special witness-table conformance field in SILFunctionType and make it far simpler for the SILOptimizer to devirtualize witness methods.  This patch does not actually take that step, however; it merely makes it possible to do so.

As another piece of unfinished business, while `SILFunctionType::substGenericArgs()` conceptually ought to simply set the given substitutions as the invocation substitutions, that would disturb a number of places that expect that method to produce an unsubstituted type.  This patch only set invocation arguments when the generic type is a substituted type, which we currently never produce in type-lowering.

My plan is to start by producing substituted function types for accessors.  Accessors are an important case because the coroutine continuation function is essentially an implicit component of the function type which the current substitution rules simply erase the intended abstraction of.  They're also used in narrower ways that should exercise less of the optimizer.

Author: rjmccall

docs/ABI/Mangling.rst

@@ -583,10 +583,12 @@ mangled in to disambiguate.
 ::
 
   impl-function-type ::= type* 'I' FUNC-ATTRIBUTES '_'
-  impl-function-type ::= type* generic-signature 'I' PSEUDO-GENERIC? FUNC-ATTRIBUTES '_'
+  impl-function-type ::= type* generic-signature 'I' FUNC-ATTRIBUTES '_'
 
-  FUNC-ATTRIBUTES ::= CALLEE-ESCAPE? CALLEE-CONVENTION FUNC-REPRESENTATION? COROUTINE-KIND? PARAM-CONVENTION* RESULT-CONVENTION* ('Y' PARAM-CONVENTION)* ('z' RESULT-CONVENTION)?
+  FUNC-ATTRIBUTES ::= PATTERN-SUBS? INVOCATION-SUBS? PSEUDO-GENERIC? CALLEE-ESCAPE? CALLEE-CONVENTION FUNC-REPRESENTATION? COROUTINE-KIND? PARAM-CONVENTION* RESULT-CONVENTION* ('Y' PARAM-CONVENTION)* ('z' RESULT-CONVENTION)?
 
+  PATTERN-SUBS ::= 's'                       // has pattern substitutions
+  INVOCATION-SUB ::= 'I'                     // has invocation substitutions
   PSEUDO-GENERIC ::= 'P'
 
   CALLEE-ESCAPE ::= 'e'                      // @escaping (inverse of SIL @noescape)

docs/SIL.rst

@@ -592,6 +592,46 @@ autorelease in the callee.
     importer only imports non-native methods and types as ``throws``
     when it is possible to do this automatically.
 
+- SIL function types may provide a pattern signature and substitutions
+  to express that values of the type use a particular generic abstraction
+  pattern.  Both must be provided together.  If a pattern signature is
+  present, the component types (parameters, yields, and results) must be
+  expressed in terms of the generic parameters of that signature.
+  The pattern substitutions should be expressed in terms of the generic
+  parameters of the overall generic signature, if any, or else
+  the enclosing generic context, if any.
+
+  A pattern signature follows the ``@substituted`` attribute, which
+  must be the final attribute preceding the function type.  Pattern
+  substitutions follow the function type, preceded by the ``for``
+  keyword.  For example::
+
+    @substituted <T: Collection> (@in T) -> @out T.Element for Array<Int>
+
+  The low-level representation of a value of this type may not match
+  the representation of a value of the substituted-through version of it::
+
+    (@in Array<Int>) -> @out Int
+
+  Substitution differences at the outermost level of a function value
+  may be adjusted using the ``convert_function`` instruction.  Note that
+  this only works at the outermost level and not in nested positions.
+  For example, a function which takes a parameter of the first type above
+  cannot be converted by ``convert_function`` to a function which takes
+  a parameter of the second type; such a conversion must be done with a
+  thunk.
+
+  Type substitution on a function type with a pattern signature and
+  substitutions only substitutes into the substitutions; the component
+  types are preserved with their exact original structure.
+
+- In the implementation, a SIL function type may also carry substitutions
+  for its generic signature.  This is a convenience for working with
+  applied generic types and is not generally a formal part of the SIL
+  language; in particular, values should not have such types.  Such a
+  type behaves like a non-generic type, as if the substitutions were
+  actually applied to the underlying function type.
+
 Coroutine Types
 ```````````````
 

include/swift/AST/DiagnosticsParse.def

@@ -828,6 +828,14 @@ ERROR(sil_function_subst_expected_l_angle,none,
       "expected '<' to begin SIL function type substitution list after 'for'", ())
 ERROR(sil_function_subst_expected_r_angle,none,
       "expected '>' to end SIL function type substitution list after 'for <...'", ())
+ERROR(sil_function_subst_expected_generics,none,
+      "expected '<' to begin substituted parameter list after '@substituted'", ())
+ERROR(sil_function_subst_expected_function,none,
+      "expected function type after '@substituted'", ())
+ERROR(sil_function_subst_expected_subs,none,
+      "expected 'for' to begin substitutions after '@substituted' function type", ())
+ERROR(sil_function_subs_without_generics,none,
+      "unexpected 'for' to begin substitutions after non-generic function type", ())
 
 // Opaque types
 ERROR(opaque_mid_composition,none,

include/swift/AST/DiagnosticsSema.def

@@ -4019,6 +4019,8 @@ ERROR(sil_non_coro_yields,PointsToFirstBadToken,
 ERROR(sil_function_repeat_convention,PointsToFirstBadToken,
       "repeated %select{parameter|result|callee}0 convention attribute",
       (unsigned))
+ERROR(ast_subst_function_type,none,
+      "substitutions cannot be provided on a formal function type", ())
 ERROR(sil_function_multiple_error_results,PointsToFirstBadToken,
       "SIL function types cannot have multiple @error results", ())
 ERROR(unsupported_sil_convention,none,

include/swift/AST/TypeRepr.h

@@ -472,12 +472,15 @@ inline IdentTypeRepr::ComponentRange IdentTypeRepr::getComponentRange() {
 ///   (x: Foo, y: Bar) -> Baz
 /// \endcode
 class FunctionTypeRepr : public TypeRepr {
-  // These fields are only used in SIL mode, which is the only time
-  // we can have polymorphic and substituted function values.
+  // The generic params / environment / substitutions fields are only used
+  // in SIL mode, which is the only time we can have polymorphic and
+  // substituted function values.
   GenericParamList *GenericParams;
   GenericEnvironment *GenericEnv;
-  bool GenericParamsAreImplied;
-  ArrayRef<TypeRepr *> GenericSubs;
+  ArrayRef<TypeRepr *> InvocationSubs;
+  GenericParamList *PatternGenericParams;
+  GenericEnvironment *PatternGenericEnv;
+  ArrayRef<TypeRepr *> PatternSubs;
 
   TupleTypeRepr *ArgsTy;
   TypeRepr *RetTy;
@@ -487,21 +490,37 @@ class FunctionTypeRepr : public TypeRepr {
 public:
   FunctionTypeRepr(GenericParamList *genericParams, TupleTypeRepr *argsTy,
                    SourceLoc throwsLoc, SourceLoc arrowLoc, TypeRepr *retTy,
-                   bool GenericParamsAreImplied = false,
-                   ArrayRef<TypeRepr *> GenericSubs = {})
+                   GenericParamList *patternGenericParams = nullptr,
+                   ArrayRef<TypeRepr *> patternSubs = {},
+                   ArrayRef<TypeRepr *> invocationSubs = {})
     : TypeRepr(TypeReprKind::Function),
-      GenericParams(genericParams),
-      GenericEnv(nullptr),
-      GenericParamsAreImplied(GenericParamsAreImplied),
-      GenericSubs(GenericSubs),
+      GenericParams(genericParams), GenericEnv(nullptr),
+      InvocationSubs(invocationSubs),
+      PatternGenericParams(patternGenericParams), PatternGenericEnv(nullptr),
+      PatternSubs(patternSubs),
       ArgsTy(argsTy), RetTy(retTy),
       ArrowLoc(arrowLoc), ThrowsLoc(throwsLoc) {
   }
 
   GenericParamList *getGenericParams() const { return GenericParams; }
   GenericEnvironment *getGenericEnvironment() const { return GenericEnv; }
-  bool areGenericParamsImplied() const { return GenericParamsAreImplied; }
-  ArrayRef<TypeRepr*> getSubstitutions() const { return GenericSubs; }
+
+  GenericParamList *getPatternGenericParams() const {
+    return PatternGenericParams;
+  }
+  GenericEnvironment *getPatternGenericEnvironment() const {
+    return PatternGenericEnv;
+  }
+
+  ArrayRef<TypeRepr*> getPatternSubstitutions() const { return PatternSubs; }
+  ArrayRef<TypeRepr*> getInvocationSubstitutions() const {
+    return InvocationSubs;
+  }
+
+  void setPatternGenericEnvironment(GenericEnvironment *genericEnv) {
+    assert(PatternGenericEnv == nullptr);
+    PatternGenericEnv = genericEnv;
+  }
 
   void setGenericEnvironment(GenericEnvironment *genericEnv) {
     assert(GenericEnv == nullptr);