better anonymous struct naming

this makes anonymous structs inherit the name of the function they are in
only when they are the return expression.

also document the behavior and provide examples.

closes #1243

Author: andrewrk

doc/langref.html.in

@@ -1918,6 +1918,32 @@ test "linked list" {
     assert(list2.first.?.data == 1234);
 }
       {#code_end#}
+      {#header_open|struct naming#}
+      <p>Since all structs are anonymous, Zig infers the type name based on a few rules.</p>
+      <ul>
+          <li>If the struct is in the initialization expression of a variable, it gets named after
+          that variable.</li>
+          <li>If the struct is in the <code>return</code> expression, it gets named after
+          the function it is returning from, with the parameter values serialized.</li>
+          <li>Otherwise, the struct gets a same such as <code>(anonymous struct at file.zig:7:38)</code>.</li>
+      </ul>
+      {#code_begin|exe|struct_name#}
+const std = @import("std");
+
+pub fn main() void {
+    const Foo = struct {};
+    std.debug.warn("variable: {}\n", @typeName(Foo));
+    std.debug.warn("anonymous: {}\n", @typeName(struct {}));
+    std.debug.warn("function: {}\n", @typeName(List(i32)));
+}
+
+fn List(comptime T: type) type {
+    return struct {
+        x: T,
+    };
+}
+      {#code_end#}
+      {#header_close#}
       {#see_also|comptime|@fieldParentPtr#}
       {#header_close#}
       {#header_open|enum#}

src/all_types.hpp

@@ -43,6 +43,7 @@ struct IrAnalyze;
 struct IrExecutable {
     ZigList<IrBasicBlock *> basic_block_list;
     Buf *name;
+    FnTableEntry *name_fn;
     size_t mem_slot_count;
     size_t next_debug_id;
     size_t *backward_branch_count;

src/ir.cpp

@@ -3186,7 +3186,11 @@ static IrInstruction *ir_gen_return(IrBuilder *irb, Scope *scope, AstNode *node,
             {
                 IrInstruction *return_value;
                 if (expr_node) {
+                    // Temporarily set this so that if we return a type it gets the name of the function
+                    FnTableEntry *prev_name_fn = irb->exec->name_fn;
+                    irb->exec->name_fn = exec_fn_entry(irb->exec);
                     return_value = ir_gen_node(irb, expr_node, scope);
+                    irb->exec->name_fn = prev_name_fn;
                     if (return_value == irb->codegen->invalid_instruction)
                         return irb->codegen->invalid_instruction;
                 } else {
@@ -6481,20 +6485,17 @@ static bool render_instance_name_recursive(CodeGen *codegen, Buf *name, Scope *o
 static Buf *get_anon_type_name(CodeGen *codegen, IrExecutable *exec, const char *kind_name, AstNode *source_node) {
     if (exec->name) {
         return exec->name;
+    } else if (exec->name_fn != nullptr) {
+        Buf *name = buf_alloc();
+        buf_append_buf(name, &exec->name_fn->symbol_name);
+        buf_appendf(name, "(");
+        render_instance_name_recursive(codegen, name, &exec->name_fn->fndef_scope->base, exec->begin_scope);
+        buf_appendf(name, ")");
+        return name;
     } else {
-        FnTableEntry *fn_entry = exec_fn_entry(exec);
-        if (fn_entry) {
-            Buf *name = buf_alloc();
-            buf_append_buf(name, &fn_entry->symbol_name);
-            buf_appendf(name, "(");
-            render_instance_name_recursive(codegen, name, &fn_entry->fndef_scope->base, exec->begin_scope);
-            buf_appendf(name, ")");
-            return name;
-        } else {
-            //Note: C-imports do not have valid location information
-            return buf_sprintf("(anonymous %s at %s:%" ZIG_PRI_usize ":%" ZIG_PRI_usize ")", kind_name,
-                (source_node->owner->path != nullptr) ? buf_ptr(source_node->owner->path) : "(null)", source_node->line + 1, source_node->column + 1);
-        }
+        //Note: C-imports do not have valid location information
+        return buf_sprintf("(anonymous %s at %s:%" ZIG_PRI_usize ":%" ZIG_PRI_usize ")", kind_name,
+            (source_node->owner->path != nullptr) ? buf_ptr(source_node->owner->path) : "(null)", source_node->line + 1, source_node->column + 1);
     }
 }