Consider boxing `Error` in wgsl parser.

[jrmuizel]

It looks like Error is 80 bytes on x86-64 which will make the Result<> returned from a lot of the lexing functions pretty big. A prototype of this reduced the size of naga::front::wgsl::Parser::parse from 24.2KiB to 21.9KiB. Unfortunately, because of rust-lang/rust#46607 the savings is not as much as it could be because every call to Box::new() ends up with an inlined test for a null return from _rust_alloc.

[jrmuizel]

Here are size profiles of parse_statement that show the impact of my prototype.

before: https://share.firefox.dev/3p6Jzol
after: https://share.firefox.dev/3J5De4s

[jrmuizel]

A fun fact revealed by this is that around 5% of parse_statements code size is spent in StatementContext::as_expression. I don't think many people would've predicted that.

[kvark]

Interesting! Any explanation there? I'd expect as_expression to be mostly a user-level construct that disappears when compiling down to the actual code.
So are you getting convinced that boxing Error is the way to go?

[jrmuizel]

as_expression is compiled down to a bunch of moves. It's a fairly big structure and is called a bunch so it adds up.

0x1001513d9: mov rcx, qword ptr [r15]    naga::front::wgsl::StatementContext::as_expression::h6d151787295027fb naga::front::wgsl::Parser::parse_statement::h9263f31b2d3df89c
0x1001513dc: mov rdx, qword ptr [r15 + 8] naga::front::wgsl::StatementContext::as_expression::h6d151787295027fb naga::front::wgsl::Parser::parse_statement::h9263f31b2d3df89c
0x1001513e0: mov r10, qword ptr [r15 + 0x28] naga::front::wgsl::StatementContext::as_expression::h6d151787295027fb naga::front::wgsl::Parser::parse_statement::h9263f31b2d3df89c
0x1001513e4: mov r11, qword ptr [r15 + 0x30] naga::front::wgsl::StatementContext::as_expression::h6d151787295027fb naga::front::wgsl::Parser::parse_statement::h9263f31b2d3df89c
0x1001513e8: mov rbx, qword ptr [r15 + 0x38] naga::front::wgsl::StatementContext::as_expression::h6d151787295027fb naga::front::wgsl::Parser::parse_statement::h9263f31b2d3df89c
0x1001513ec: mov r8, qword ptr [r15 + 0x10] naga::front::wgsl::StatementContext::as_expression::h6d151787295027fb naga::front::wgsl::Parser::parse_statement::h9263f31b2d3df89c
0x1001513f0: mov r9, qword ptr [r15 + 0x40] naga::front::wgsl::StatementContext::as_expression::h6d151787295027fb naga::front::wgsl::Parser::parse_statement::h9263f31b2d3df89c
0x1001513f4: mov rsi, qword ptr [r15 + 0x48] naga::front::wgsl::StatementContext::as_expression::h6d151787295027fb naga::front::wgsl::Parser::parse_statement::h9263f31b2d3df89c
0x1001513f8: mov rdi, qword ptr [r15 + 0x50] naga::front::wgsl::StatementContext::as_expression::h6d151787295027fb naga::front::wgsl::Parser::parse_statement::h9263f31b2d3df89c
0x1001513fc: mov qword ptr [rbp - 0x90], rcx naga::front::wgsl::StatementContext::as_expression::h6d151787295027fb naga::front::wgsl::Parser::parse_statement::h9263f31b2d3df89c
0x100151403: mov qword ptr [rbp - 0x88], rdx naga::front::wgsl::StatementContext::as_expression::h6d151787295027fb naga::front::wgsl::Parser::parse_statement::h9263f31b2d3df89c
0x10015140a: mov qword ptr [rbp - 0x80], rax naga::front::wgsl::StatementContext::as_expression::h6d151787295027fb naga::front::wgsl::Parser::parse_statement::h9263f31b2d3df89c
0x10015140e: mov qword ptr [rbp - 0x78], r10 naga::front::wgsl::StatementContext::as_expression::h6d151787295027fb naga::front::wgsl::Parser::parse_statement::h9263f31b2d3df89c
0x100151412: mov qword ptr [rbp - 0x70], r11 naga::front::wgsl::StatementContext::as_expression::h6d151787295027fb naga::front::wgsl::Parser::parse_statement::h9263f31b2d3df89c
0x100151416: mov qword ptr [rbp - 0x68], rbx naga::front::wgsl::StatementContext::as_expression::h6d151787295027fb naga::front::wgsl::Parser::parse_statement::h9263f31b2d3df89c
0x10015141a: mov qword ptr [rbp - 0x60], r8 naga::front::wgsl::StatementContext::as_expression::h6d151787295027fb naga::front::wgsl::Parser::parse_statement::h9263f31b2d3df89c
0x10015141e: mov qword ptr [rbp - 0x58], rsi naga::front::wgsl::StatementContext::as_expression::h6d151787295027fb naga::front::wgsl::Parser::parse_statement::h9263f31b2d3df89c
0x100151422: mov qword ptr [rbp - 0x50], rdi naga::front::wgsl::StatementContext::as_expression::h6d151787295027fb naga::front::wgsl::Parser::parse_statement::h9263f31b2d3df89c
0x100151426: mov qword ptr [rbp - 0x48], r9 naga::front::wgsl::StatementContext::as_expression::h6d151787295027fb naga::front::wgsl::Parser::parse_statement::h9263f31b2d3df89c
0x10015142a: mov qword ptr [rbp - 0x40], r12 naga::front::wgsl::StatementContext::as_expression::h6d151787295027fb naga::front::wgsl::Parser::parse_statement::h9263f31b2d3df89c
0x10015142e: lea rax, [rbp - 0x170]      naga::front::wgsl::StatementContext::as_expression::h6d151787295027fb naga::front::wgsl::Parser::parse_statement::h9263f31b2d3df89c
0x100151435: mov qword ptr [rbp - 0x38], rax naga::front::wgsl::StatementContext::as_expression::h6d151787295027fb naga::front::wgsl::Parser::parse_statement::h9263f31b2d3df89c
0x100151439: mov rbx, r12                naga::front::wgsl::StatementContext::as_expression::h6d151787295027fb naga::front::wgsl::Parser::parse_statement::h9263f31b2d3df89c
0x10015143c: lea r12, [rbp - 0x90]       naga::front::wgsl::StatementContext::as_expression::h6d151787295027fb naga::front::wgsl::Parser::parse_statement::h9263f31b2d3df89c

I haven't tried to see what's preventing the moves from being elided, but it doesn't look like it'd necessarily be easy for the compiler to do because it's not just plain memcpy.

[kvark]

so much for trying to limit the amount of data we pass in... we could try just passing StatementContext everywhere instead and see