Use warn(unused_crate_dependencies) in compiler crates.

The commit also removes fifteen unnecessary dependencies from
`Cargo.toml` files that this finds. And tweaks some conditional
dependencies.

There are three crates with false positives, relating to tests and
doc comments. For these three crates `allow(unused_crate_dependencies)`
is used along with an explanatory comment.

Author: nnethercote

Cargo.lock

@@ -3216,7 +3216,6 @@ dependencies = [
  "rustc_index",
  "rustc_macros",
  "rustc_middle",
- "rustc_parse",
  "rustc_session",
  "rustc_span",
  "rustc_target",
@@ -3251,7 +3250,6 @@ version = "0.0.0"
 dependencies = [
  "itertools",
  "rustc_ast",
- "rustc_data_structures",
  "rustc_lexer",
  "rustc_span",
  "thin-vec",
@@ -3279,14 +3277,12 @@ dependencies = [
  "rustc_ast",
  "rustc_ast_pretty",
  "rustc_attr_data_structures",
- "rustc_data_structures",
  "rustc_errors",
  "rustc_feature",
  "rustc_fluent_macro",
  "rustc_hir",
  "rustc_lexer",
  "rustc_macros",
- "rustc_serialize",
  "rustc_session",
  "rustc_span",
  "thin-vec",
@@ -3413,15 +3409,13 @@ dependencies = [
  "rustc_abi",
  "rustc_arena",
  "rustc_ast",
- "rustc_ast_pretty",
  "rustc_attr_parsing",
  "rustc_data_structures",
  "rustc_errors",
  "rustc_fluent_macro",
  "rustc_fs_util",
  "rustc_hashes",
  "rustc_hir",
- "rustc_hir_pretty",
  "rustc_incremental",
  "rustc_index",
  "rustc_macros",
@@ -3536,7 +3530,6 @@ dependencies = [
  "rustc_hir_pretty",
  "rustc_hir_typeck",
  "rustc_incremental",
- "rustc_index",
  "rustc_infer",
  "rustc_interface",
  "rustc_lint",
@@ -3872,7 +3865,6 @@ dependencies = [
  "rustc_query_impl",
  "rustc_query_system",
  "rustc_resolve",
- "rustc_serialize",
  "rustc_session",
  "rustc_span",
  "rustc_symbol_mangling",
@@ -4037,13 +4029,11 @@ dependencies = [
 name = "rustc_mir_build"
 version = "0.0.0"
 dependencies = [
- "either",
  "itertools",
  "rustc_abi",
  "rustc_apfloat",
  "rustc_arena",
  "rustc_ast",
- "rustc_attr_parsing",
  "rustc_data_structures",
  "rustc_errors",
  "rustc_fluent_macro",
@@ -4250,7 +4240,6 @@ name = "rustc_query_impl"
 version = "0.0.0"
 dependencies = [
  "measureme",
- "rustc_attr_data_structures",
  "rustc_data_structures",
  "rustc_errors",
  "rustc_hashes",
@@ -4324,7 +4313,6 @@ version = "0.0.0"
 dependencies = [
  "bitflags",
  "rustc_abi",
- "rustc_ast",
  "rustc_data_structures",
  "rustc_hir",
  "rustc_middle",
@@ -4379,7 +4367,6 @@ name = "rustc_smir"
 version = "0.0.0"
 dependencies = [
  "rustc_abi",
- "rustc_ast",
  "rustc_data_structures",
  "rustc_hir",
  "rustc_hir_pretty",
@@ -4421,7 +4408,6 @@ dependencies = [
  "punycode",
  "rustc-demangle",
  "rustc_abi",
- "rustc_ast",
  "rustc_data_structures",
  "rustc_errors",
  "rustc_hashes",

compiler/rustc_abi/src/lib.rs

@@ -6,6 +6,7 @@
 #![cfg_attr(feature = "nightly", feature(rustdoc_internals))]
 #![cfg_attr(feature = "nightly", feature(step_trait))]
 #![warn(unreachable_pub)]
+#![warn(unused_crate_dependencies)]
 // tidy-alphabetical-end
 
 /*! ABI handling for rustc

compiler/rustc_arena/src/lib.rs

@@ -24,6 +24,7 @@
 #![feature(rustc_attrs)]
 #![feature(rustdoc_internals)]
 #![warn(unreachable_pub)]
+#![warn(unused_crate_dependencies)]
 // tidy-alphabetical-end
 
 use std::alloc::Layout;

compiler/rustc_ast/src/lib.rs

@@ -20,6 +20,7 @@
 #![feature(rustdoc_internals)]
 #![feature(stmt_expr_attributes)]
 #![warn(unreachable_pub)]
+#![warn(unused_crate_dependencies)]
 // tidy-alphabetical-end
 
 pub mod util {

compiler/rustc_ast_ir/src/lib.rs

@@ -10,6 +10,7 @@
 #![cfg_attr(feature = "nightly", feature(never_type))]
 #![cfg_attr(feature = "nightly", feature(rustc_attrs))]
 #![warn(unreachable_pub)]
+#![warn(unused_crate_dependencies)]
 // tidy-alphabetical-end
 
 #[cfg(feature = "nightly")]

compiler/rustc_ast_lowering/Cargo.toml

@@ -20,7 +20,6 @@ rustc_hir = { path = "../rustc_hir" }
 rustc_index = { path = "../rustc_index" }
 rustc_macros = { path = "../rustc_macros" }
 rustc_middle = { path = "../rustc_middle" }
-rustc_parse = { path = "../rustc_parse" }
 rustc_session = { path = "../rustc_session" }
 rustc_span = { path = "../rustc_span" }
 rustc_target = { path = "../rustc_target" }

compiler/rustc_ast_lowering/src/lib.rs

@@ -39,6 +39,7 @@
 #![feature(let_chains)]
 #![feature(rustdoc_internals)]
 #![warn(unreachable_pub)]
+#![warn(unused_crate_dependencies)]
 // tidy-alphabetical-end
 
 use std::sync::Arc;

compiler/rustc_ast_passes/src/lib.rs

@@ -11,6 +11,7 @@
 #![feature(let_chains)]
 #![feature(rustdoc_internals)]
 #![warn(unreachable_pub)]
+#![warn(unused_crate_dependencies)]
 // tidy-alphabetical-end
 
 pub mod ast_validation;

compiler/rustc_ast_pretty/Cargo.toml

@@ -7,7 +7,6 @@ edition = "2024"
 # tidy-alphabetical-start
 itertools = "0.12"
 rustc_ast = { path = "../rustc_ast" }
-rustc_data_structures = { path = "../rustc_data_structures" }
 rustc_lexer = { path = "../rustc_lexer" }
 rustc_span = { path = "../rustc_span" }
 thin-vec = "0.2.12"

compiler/rustc_ast_pretty/src/lib.rs

@@ -4,6 +4,7 @@
 #![feature(box_patterns)]
 #![feature(rustdoc_internals)]
 #![warn(unreachable_pub)]
+#![warn(unused_crate_dependencies)]
 // tidy-alphabetical-end
 
 mod helpers;