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bootstrap: Use cargo's build.warnings=deny rather than -Dwarnings
This has two major advantages. First, it makes us less dependent on the
rustc shim, which is nice but not super important. More importantly, it
gives us much nicer caching properties.
Previously, `x build --warnings warn && x build --warnings deny` would
rebuild all of bootstrap, and if there were any warnings emitted on the
last build of the compiler, they would *not* fail the build, because
cargo would cache the output rather than rerunning the shim.
After this change, bootstrap rebuilds instantly, and cargo knows that it
should fail the build but *without* invalidating the cache.
Here is an example of rebuilding bootstrap after a switch from
warn->deny:
```
INFO: Downloading and building bootstrap before processing --help command.
See src/bootstrap/README.md for help with common commands.
Building bootstrap
Compiling bootstrap v0.0.0 (/Users/jyn/src/ferrocene3/src/bootstrap)
warning: unused variable: `x`
--> src/bootstrap/src/core/builder/mod.rs:1792:13
|
1792 | let x: ();
| ^
|
= note: `#[warn(unused_variables)]` (part of `#[warn(unused)]`) on by default
help: if this is intentional, prefix it with an underscore
|
1792 | let _x: ();
| +
help: you might have meant to pattern match on the similarly named constant `_`
|
1792 - let x: ();
1792 + let utils::render_tests::_: ();
|
warning: `bootstrap` (lib) generated 1 warning (run `cargo fix --lib -p bootstrap` to apply 1 suggestion)
Finished `dev` profile [unoptimized] target(s) in 5.14s
error: warnings are denied by `build.warnings` configuration
failed to run: /Users/jyn/src/ferrocene3/build/aarch64-apple-darwin/stage0/bin/cargo build --jobs=default --manifest-path /Users/jyn/src/ferrocene3/src/bootstrap/Cargo.toml -Zroot-dir=/Users/jyn/src/ferrocene3 -Zwarnings
```
building the compiler from scratch with `deny`: https://gist.github.com/jyn514/493ed26c2aa6f61bf47c21e75efa2175
and rebuilding the compiler after switching from deny->warn (note that it reuses the whole cache, there are no invalidations):
```
$ x c compiler
Building bootstrap
Finished `dev` profile [unoptimized] target(s) in 0.15s
Checking stage1 compiler artifacts{rustc-main, rustc_abi, rustc_arena, rustc_ast, rustc_ast_ir, rustc_ast_lowering, rustc_ast_passes, rustc_ast_pretty, rustc_attr_parsing, rustc_baked_icu_data, rustc_borrowck, rustc_builtin_macros, rustc_codegen_llvm, rustc_codegen_ssa, rustc_const_eval, rustc_data_structures, rustc_driver, rustc_driver_impl, rustc_error_codes, rustc_error_messages, rustc_errors, rustc_expand, rustc_feature, rustc_fluent_macro, rustc_fs_util, rustc_graphviz, rustc_hashes, rustc_hir, rustc_hir_analysis, rustc_hir_id, rustc_hir_pretty, rustc_hir_typeck, rustc_incremental, rustc_index, rustc_index_macros, rustc_infer, rustc_interface, rustc_lexer, rustc_lint, rustc_lint_defs, rustc_llvm, rustc_log, rustc_macros, rustc_metadata, rustc_middle, rustc_mir_build, rustc_mir_dataflow, rustc_mir_transform, rustc_monomorphize, rustc_next_trait_solver, rustc_parse, rustc_parse_format, rustc_passes, rustc_pattern_analysis, rustc_privacy, rustc_proc_macro, rustc_public, rustc_public_bridge, rustc_query_impl, rustc_query_system, rustc_resolve, rustc_sanitizers, rustc_serialize, rustc_session, rustc_span, rustc_symbol_mangling, rustc_target, rustc_thread_pool, rustc_trait_selection, rustc_traits, rustc_transmute, rustc_ty_utils, rustc_type_ir, rustc_type_ir_macros, rustc_windows_rc} (stage0 -> stage1, aarch64-apple-darwin)
warning: function `foo` is never used
--> compiler/rustc_hashes/src/lib.rs:16:4
|
16 | fn foo() {}
| ^^^
|
= note: `#[warn(dead_code)]` (part of `#[warn(unused)]`) on by default
warning: `rustc_hashes` (lib) generated 1 warning
Finished `release` profile [optimized + debuginfo] target(s) in 0.53s
Build completed successfully in 0:00:08
```
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