Add LoggingAllocator prototype

[daurnimator]

Concept for #2337

Open questions:

• how to allow custom logging function? trying

  pub fn init(parent_allocator: *Allocator, logFn: fn (comptime fmt: []const u8, args: ...) void) LoggingAllocator {

  fails with error: parameter of type 'fn([]const u8,var)var' must be declared comptime
• Log message formatting

[Hejsil]

Instead of a logFn, you could store an *std.io.OutStream.

[andrewrk]

Please feel free to open a new PR with something that is ready for review.

[daurnimator]

This was ready to review?

[andrewrk]

[daurnimator]

I guess I was looking for a 2nd opinion on the phrasing in the log messages.

[andrewrk]

This is related to a proposal I've been meaning to type up. I'll go ahead and do that now.

[andrewrk]

doc comments please. when should someone use this? what are the semantics?

[andrewrk]

How does one acquire a NoErrorOutStream? If I were to for example use (try std.io.getStdErr()).outStream() this outstream has errors.

[daurnimator]

I've used this before....:

diff --git a/std/debug.zig b/std/debug.zig
index c5741f390..6425a7526 100644
--- a/std/debug.zig
+++ b/std/debug.zig
@@ -2339,7 +2339,15 @@ fn readILeb128(in_stream: var) !i64 {
 }
 
 /// This should only be used in temporary test programs.
-pub const global_allocator = &global_fixed_allocator.allocator;
+var NoErrorStdErrStream = std.heap.NoErrorOutStream {
+    .writeFn = struct {
+        pub fn writeFn(out_stream: *std.heap.NoErrorOutStream, bytes: []const u8) error{}!void {
+            const stderr = io.getStdErr() catch return;
+            stderr.write(bytes) catch return;
+        }
+    }.writeFn,
+};
+pub const global_allocator = &std.heap.LoggingAllocator.init(&global_fixed_allocator.allocator, &NoErrorStdErrStream).allocator;

[andrewrk]

This proposal would remove the need for the OutStream component: #2586