breaking changes to std.mem.Allocator interface API

Before, allocator implementations had to provide `allocFn`,
`reallocFn`, and `freeFn`.

Now, they must provide only `reallocFn` and `shrinkFn`.
Reallocating from a zero length slice is allocation, and
shrinking to a zero length slice is freeing.

When the new memory size is less than or equal to the
previous allocation size, `reallocFn` now has the option
to return `error.OutOfMemory` to indicate that the allocator
would not be able to take advantage of the new size.

For more details see #1306. This commit closes #1306.

This commit paves the way to solving #2009.

This commit also introduces a memory leak to all coroutines.
There is an issue where a coroutine calls the function and it
frees its own stack frame, but then the return value of `shrinkFn`
is a slice, which is implemented as an sret struct. Writing to
the return pointer causes invalid memory write. We could work
around it by having a global helper function which has a void
return type and calling that instead. But instead this hack will
suffice until I rework coroutines to be non-allocating. Basically
coroutines are not supported right now until they are reworked as
in #1194.

1 files changed, 5 insertions, 3 deletions

diff --git a/src/analyze.cpp b/src/analyze.cpp
index 79a723b2e5..0a04a14e3f 100644
--- a/src/analyze.cpp
+++ b/src/analyze.cpp
@@ -3707,9 +3707,11 @@ ZigVar *add_variable(CodeGen *g, AstNode *source_node, Scope *parent_scope, Buf
 
         ZigVar *existing_var = find_variable(g, parent_scope, name, nullptr);
         if (existing_var && !existing_var->shadowable) {
-            ErrorMsg *msg = add_node_error(g, source_node,
-                    buf_sprintf("redeclaration of variable '%s'", buf_ptr(name)));
-            add_error_note(g, msg, existing_var->decl_node, buf_sprintf("previous declaration is here"));
+            if (existing_var->var_type == nullptr || !type_is_invalid(existing_var->var_type)) {
+                ErrorMsg *msg = add_node_error(g, source_node,
+                        buf_sprintf("redeclaration of variable '%s'", buf_ptr(name)));
+                add_error_note(g, msg, existing_var->decl_node, buf_sprintf("previous declaration is here"));
+            }
             variable_entry->var_type = g->builtin_types.entry_invalid;
         } else {
             ZigType *type;