Support struct implicit conversions in type-checking (#870)

I should emphasize that I am **completely cheating** here. This PR does not add support for actually _performing_  implicit conversions at run time, because the AST doesn't yet contain the necessary type information. At run time, code like `var p: Point = {.x = 1, .y = 2};` directly initializes the name `p` with the _struct_ value `{.x = 1, .y = 2}`; no object of type `Point` is actually created. I'm only getting away with this because we don't yet have any tests that can tell the difference.

Co-authored-by: Jon Meow <46229924+jonmeow@users.noreply.github.com>

executable_semantics/testdata/class/assign.carbon

@@ -0,0 +1,24 @@
+// Part of the Carbon Language project, under the Apache License v2.0 with LLVM
+// Exceptions. See /LICENSE for license information.
+// SPDX-License-Identifier: Apache-2.0 WITH LLVM-exception
+//
+// RUN: executable_semantics %s 2>&1 | \
+// RUN:   FileCheck --match-full-lines --allow-unused-prefixes=false %s
+// RUN: executable_semantics --trace %s 2>&1 | \
+// RUN:   FileCheck --match-full-lines --allow-unused-prefixes %s
+// AUTOUPDATE: executable_semantics %s
+// CHECK: result: 0
+
+package ExecutableSemanticsTest api;
+
+class Point {
+  var x: i32;
+  var y: i32;
+}
+
+fn main() -> i32 {
+  var p1: Point = {.x = 1, .y = 2};
+  var p2: auto = p1;
+  p2 = {.x = 3, .y = 2};
+  return p1.x - 1;
+}

executable_semantics/testdata/class/assign_member.carbon

@@ -17,7 +17,7 @@ class Point {
 }
 
 fn main() -> i32 {
-  var p1: auto = Point(.x = 1, .y = 2);
+  var p1: Point = {.x = 1, .y = 2};
   var p2: auto = p1;
   p2.x = 3;
   return p1.x - 1;

executable_semantics/testdata/class/fail_field_access_mismatch.carbon

@@ -7,7 +7,7 @@
 // RUN: not executable_semantics --trace %s 2>&1 | \
 // RUN:   FileCheck --match-full-lines --allow-unused-prefixes %s
 // AUTOUPDATE: executable_semantics %s
-// CHECK: COMPILATION ERROR: {{.*}}/executable_semantics/testdata/class/fail_field_access_mismatch.carbon:20: class Point does not have a field named z
+// CHECK: COMPILATION ERROR: {{.*}}/executable_semantics/testdata/class/fail_field_access_mismatch.carbon:21: class Point does not have a field named z
 
 package ExecutableSemanticsTest api;
 
@@ -17,5 +17,6 @@ class Point {
 }
 
 fn main() -> i32 {
-  return Point(.x = 1, .y = 2).z - 1;
+  var p: Point = {.x = 1, .y = 2};
+  return p.z - 1;
 }

executable_semantics/testdata/class/fail_field_mismatch.carbon

@@ -7,9 +7,7 @@
 // RUN: not executable_semantics --trace %s 2>&1 | \
 // RUN:   FileCheck --match-full-lines --allow-unused-prefixes %s
 // AUTOUPDATE: executable_semantics %s
-// CHECK: COMPILATION ERROR: {{.*}}/executable_semantics/testdata/class/fail_field_mismatch.carbon:22: type error in call
-// CHECK: expected: (x = i32, y = i32)
-// CHECK: actual: (x = i32, z = i32)
+// CHECK: COMPILATION ERROR: {{.*}}/executable_semantics/testdata/class/fail_field_mismatch.carbon:20: type error in name binding: '{.x: i32, .z: i32}' is not implicitly convertible to 'class Point'
 
 package ExecutableSemanticsTest api;
 
@@ -19,5 +17,6 @@ class Point {
 }
 
 fn main() -> i32 {
-  return Point(.x = 1, .z = 2).x - 1;
+  var p: Point = {.x = 1, .z = 2};
+  return p.x - 1;
 }

executable_semantics/testdata/class/fail_field_missing.carbon

@@ -7,9 +7,7 @@
 // RUN: not executable_semantics --trace %s 2>&1 | \
 // RUN:   FileCheck --match-full-lines --allow-unused-prefixes %s
 // AUTOUPDATE: executable_semantics %s
-// CHECK: COMPILATION ERROR: {{.*}}/executable_semantics/testdata/class/fail_field_missing.carbon:22: type error in call
-// CHECK: expected: (x = i32, y = i32)
-// CHECK: actual: (x = i32)
+// CHECK: COMPILATION ERROR: {{.*}}/executable_semantics/testdata/class/fail_field_missing.carbon:20: type error in name binding: '{.x: i32}' is not implicitly convertible to 'class Point'
 
 package ExecutableSemanticsTest api;
 
@@ -19,5 +17,6 @@ class Point {
 }
 
 fn main() -> i32 {
-  return Point(.x = 1).x - 1;
+  var p: Point = {.x = 1};
+  return p.x - 1;
 }

executable_semantics/testdata/class/function_param.carbon

@@ -0,0 +1,25 @@
+// Part of the Carbon Language project, under the Apache License v2.0 with LLVM
+// Exceptions. See /LICENSE for license information.
+// SPDX-License-Identifier: Apache-2.0 WITH LLVM-exception
+//
+// RUN: executable_semantics %s 2>&1 | \
+// RUN:   FileCheck --match-full-lines --allow-unused-prefixes=false %s
+// RUN: executable_semantics --trace %s 2>&1 | \
+// RUN:   FileCheck --match-full-lines --allow-unused-prefixes %s
+// AUTOUPDATE: executable_semantics %s
+// CHECK: result: 0
+
+package ExecutableSemanticsTest api;
+
+class Point {
+  var x: i32;
+  var y: i32;
+}
+
+fn GetX(p: Point) -> i32 {
+  return p.x;
+}
+
+fn main() -> i32 {
+  return GetX({.x = 1, .y = 2}) - 1;
+}

executable_semantics/testdata/class/global_var.carbon

@@ -0,0 +1,23 @@
+// Part of the Carbon Language project, under the Apache License v2.0 with LLVM
+// Exceptions. See /LICENSE for license information.
+// SPDX-License-Identifier: Apache-2.0 WITH LLVM-exception
+//
+// RUN: executable_semantics %s 2>&1 | \
+// RUN:   FileCheck --match-full-lines --allow-unused-prefixes=false %s
+// RUN: executable_semantics --trace %s 2>&1 | \
+// RUN:   FileCheck --match-full-lines --allow-unused-prefixes %s
+// AUTOUPDATE: executable_semantics %s
+// CHECK: result: 0
+
+package ExecutableSemanticsTest api;
+
+class Point {
+  var x: i32;
+  var y: i32;
+}
+
+var p: Point = {.x = 1, .y = 2};
+
+fn main() -> i32 {
+  return p.y - p.x - 1;
+}

executable_semantics/testdata/class/temp.carbon

@@ -16,6 +16,10 @@ class Point {
   var y: i32;
 }
 
+fn MakePoint() -> Point {
+  return {.x = 1, .y = 2};
+}
+
 fn main() -> i32 {
-  return Point(.x = 1, .y = 2).x - 1;
+  return MakePoint().x - 1;
 }

executable_semantics/testdata/class/var.carbon

@@ -17,6 +17,6 @@ class Point {
 }
 
 fn main() -> i32 {
-  var p: auto = Point(.x = 1, .y = 2);
+  var p: Point = {.x = 1, .y = 2};
   return p.y - p.x - 1;
 }

executable_semantics/testdata/function/fail_call_with_tuple.carbon

@@ -7,9 +7,7 @@
 // RUN: not executable_semantics --trace %s 2>&1 | \
 // RUN:   FileCheck --match-full-lines --allow-unused-prefixes %s
 // AUTOUPDATE: executable_semantics %s
-// CHECK: COMPILATION ERROR: {{.*}}/executable_semantics/testdata/function/fail_call_with_tuple.carbon:21: type error in call
-// CHECK: expected: (0 = i32, 1 = i32)
-// CHECK: actual: (0 = (0 = i32, 1 = i32))
+// CHECK: COMPILATION ERROR: {{.*}}/executable_semantics/testdata/function/fail_call_with_tuple.carbon:19: type error in call: '(0 = (0 = i32, 1 = i32))' is not implicitly convertible to '(0 = i32, 1 = i32)'
 
 package ExecutableSemanticsTest api;
 

executable_semantics/testdata/global_variable/fail_init_type_mismatch.carbon

@@ -7,9 +7,7 @@
 // RUN: not executable_semantics --trace %s 2>&1 | \
 // RUN:   FileCheck --match-full-lines --allow-unused-prefixes %s
 // AUTOUPDATE: executable_semantics %s
-// CHECK: COMPILATION ERROR: {{.*}}/executable_semantics/testdata/global_variable/fail_init_type_mismatch.carbon:18: type error in initializer of variable
-// CHECK: expected: i32
-// CHECK: actual: Bool
+// CHECK: COMPILATION ERROR: {{.*}}/executable_semantics/testdata/global_variable/fail_init_type_mismatch.carbon:16: type error in initializer of variable: 'Bool' is not implicitly convertible to 'i32'
 
 package ExecutableSemanticsTest api;
 

executable_semantics/testdata/struct/name_order.carbon

@@ -2,16 +2,16 @@
 // Exceptions. See /LICENSE for license information.
 // SPDX-License-Identifier: Apache-2.0 WITH LLVM-exception
 //
-// RUN: not executable_semantics %s 2>&1 | \
+// RUN: executable_semantics %s 2>&1 | \
 // RUN:   FileCheck --match-full-lines --allow-unused-prefixes=false %s
-// RUN: not executable_semantics --trace %s 2>&1 | \
+// RUN: executable_semantics --trace %s 2>&1 | \
 // RUN:   FileCheck --match-full-lines --allow-unused-prefixes %s
 // AUTOUPDATE: executable_semantics %s
-// CHECK: COMPILATION ERROR: {{.*}}/executable_semantics/testdata/struct/fail_name_order.carbon:17: Type pattern '{.x: i32, .y: i32}' does not match actual type '{.y: i32, .x: i32}'
+// CHECK: result: 0
 
 package ExecutableSemanticsTest api;
 
-// Test the that field order matters for structs.
+// Test the that field order doesn't matter for structs.
 
 fn main() -> i32 {
   var t: {.x: i32, .y: i32} = {.y = 2, .x = 3};

executable_semantics/testdata/tuple/fail_name_order.carbon

@@ -7,7 +7,7 @@
 // RUN: not executable_semantics --trace %s 2>&1 | \
 // RUN:   FileCheck --match-full-lines --allow-unused-prefixes %s
 // AUTOUPDATE: executable_semantics %s
-// CHECK: PROGRAM ERROR: {{.*}}/executable_semantics/testdata/tuple/fail_name_order.carbon:17: Tuple field name 'y' does not match pattern field name 'x'
+// CHECK: COMPILATION ERROR: {{.*}}/executable_semantics/testdata/tuple/fail_name_order.carbon:17: type error in name binding: '(y = i32, x = i32)' is not implicitly convertible to '(x = i32, y = i32)'
 
 package ExecutableSemanticsTest api;