Denote ControlFlow as #[must_use]

[compiler-errors]

I've repeatedly hit bugs in the compiler due to ControlFlow not being marked #[must_use]. There seems to be an accepted ACP to make the type #[must_use] (rust-lang/libs-team#444), so this PR implements that part of it.

Most of the usages in the compiler that trigger this new warning are "root" usages (calling into an API that uses control-flow internally, but for which the callee doesn't really care) and have been suppressed by let _ = ..., but I did legitimately find one instance of a missing ? and one for a never-used ControlFlow value in #137448.

Presumably this needs an FCP too, so I'm opening this and nominating it for T-libs-api.

This PR also touches the tools (incl. rust-analyzer), but if this went into FCP, I'd split those out into separate PRs which can land before this one does.

r? libs-api
@rustbot label: T-libs-api I-libs-api-nominated

[rust-log-analyzer]

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#22 exporting to docker image format
#22 sending tarball 29.3s done
#22 DONE 35.7s
##[endgroup]
Setting extra environment values for docker:  --env ENABLE_GCC_CODEGEN=1 --env GCC_EXEC_PREFIX=/usr/lib/gcc/
[CI_JOB_NAME=x86_64-gnu-llvm-18]
debug: `DISABLE_CI_RUSTC_IF_INCOMPATIBLE` configured.
---
sccache: Listening on address 127.0.0.1:4226
##[group]Configure the build
configure: processing command line
configure: 
configure: build.configure-args := ['--build=x86_64-unknown-linux-gnu', '--llvm-root=/usr/lib/llvm-18', '--enable-llvm-link-shared', '--set', 'rust.randomize-layout=true', '--set', 'rust.thin-lto-import-instr-limit=10', '--enable-verbose-configure', '--enable-sccache', '--disable-manage-submodules', '--enable-locked-deps', '--enable-cargo-native-static', '--set', 'rust.codegen-units-std=1', '--set', 'dist.compression-profile=balanced', '--dist-compression-formats=xz', '--set', 'rust.lld=false', '--disable-dist-src', '--release-channel=nightly', '--enable-debug-assertions', '--enable-overflow-checks', '--enable-llvm-assertions', '--set', 'rust.verify-llvm-ir', '--set', 'rust.codegen-backends=llvm,cranelift,gcc', '--set', 'llvm.static-libstdcpp', '--enable-new-symbol-mangling']
configure: target.x86_64-unknown-linux-gnu.llvm-config := /usr/lib/llvm-18/bin/llvm-config
configure: llvm.link-shared     := True
configure: rust.randomize-layout := True
configure: rust.thin-lto-import-instr-limit := 10
---
  Downloaded boml v0.3.1
   Compiling boml v0.3.1
   Compiling y v0.1.0 (/checkout/compiler/rustc_codegen_gcc/build_system)
    Finished `release` profile [optimized] target(s) in 4.25s
     Running `/checkout/obj/build/x86_64-unknown-linux-gnu/stage1-codegen/x86_64-unknown-linux-gnu/release/y test --use-system-gcc --use-backend gcc --out-dir /checkout/obj/build/x86_64-unknown-linux-gnu/stage1-tools/cg_gcc --release --mini-tests --std-tests`
Using system GCC
warning: target feature `x87` must be enabled to ensure that the ABI of the current target can be implemented correctly
  |
  = note: this was previously accepted by the compiler but is being phased out; it will become a hard error in a future release!
  = note: for more information, see issue #116344 <https://github.com/rust-lang/rust/issues/116344>
---
   Compiling rustdoc v0.0.0 (/checkout/src/librustdoc)
error: unused `std::ops::ControlFlow` that must be used
  --> src/librustdoc/html/length_limit/tests.rs:12:5
   |
12 |     buf.push("Hello ");
   |
   = note: `-D unused-must-use` implied by `-D warnings`
   = help: to override `-D warnings` add `#[allow(unused_must_use)]`
help: use `let _ = ...` to ignore the resulting value
help: use `let _ = ...` to ignore the resulting value
   |
12 |     let _ = buf.push("Hello ");

error: unused `std::ops::ControlFlow` that must be used
  --> src/librustdoc/html/length_limit/tests.rs:14:5
   |
   |
14 |     buf.push("world");
   |
help: use `let _ = ...` to ignore the resulting value
   |
   |
14 |     let _ = buf.push("world");

error: unused `std::ops::ControlFlow` that must be used
  --> src/librustdoc/html/length_limit/tests.rs:16:5
   |
   |
16 |     buf.push("!");
   |
help: use `let _ = ...` to ignore the resulting value
   |
   |
16 |     let _ = buf.push("!");

error: unused `std::ops::ControlFlow` that must be used
  --> src/librustdoc/html/length_limit/tests.rs:23:5
   |
   |
23 |     buf.push("Hello");
   |
help: use `let _ = ...` to ignore the resulting value
   |
   |
23 |     let _ = buf.push("Hello");

error: unused `std::ops::ControlFlow` that must be used
  --> src/librustdoc/html/length_limit/tests.rs:24:5
   |
   |
24 |     buf.push(" world!");
   |
help: use `let _ = ...` to ignore the resulting value
   |
   |
24 |     let _ = buf.push(" world!");

error: unused `std::ops::ControlFlow` that must be used
  --> src/librustdoc/html/length_limit/tests.rs:31:5
   |
   |
31 |     buf.push("Hello ");
   |
help: use `let _ = ...` to ignore the resulting value
   |
   |
31 |     let _ = buf.push("Hello ");

error: unused `std::ops::ControlFlow` that must be used
  --> src/librustdoc/html/length_limit/tests.rs:33:5
   |
   |
33 |     buf.push("world");
   |
help: use `let _ = ...` to ignore the resulting value
   |
   |
33 |     let _ = buf.push("world");

error: unused `std::ops::ControlFlow` that must be used
  --> src/librustdoc/html/length_limit/tests.rs:35:5
   |
   |
35 |     buf.push("!");
   |
help: use `let _ = ...` to ignore the resulting value
   |
   |
35 |     let _ = buf.push("!");

error: unused `std::ops::ControlFlow` that must be used
  --> src/librustdoc/html/length_limit/tests.rs:42:5
   |
   |
42 |     buf.push("Hello ");
   |
help: use `let _ = ...` to ignore the resulting value
   |
   |
42 |     let _ = buf.push("Hello ");

error: unused `std::ops::ControlFlow` that must be used
  --> src/librustdoc/html/length_limit/tests.rs:44:5
   |
   |
44 |     buf.push("world");
   |
help: use `let _ = ...` to ignore the resulting value
   |
   |
44 |     let _ = buf.push("world");

error: unused `std::ops::ControlFlow` that must be used
  --> src/librustdoc/html/length_limit/tests.rs:46:5
   |
   |
46 |     buf.push("!");
   |
help: use `let _ = ...` to ignore the resulting value
   |
   |
46 |     let _ = buf.push("!");

error: unused `std::ops::ControlFlow` that must be used
  --> src/librustdoc/html/length_limit/tests.rs:54:5
   |
   |
54 |     buf.push("This is a ");
   |
help: use `let _ = ...` to ignore the resulting value
   |
   |
54 |     let _ = buf.push("This is a ");

error: unused `std::ops::ControlFlow` that must be used
  --> src/librustdoc/html/length_limit/tests.rs:56:5
   |
   |
56 |     buf.push("paragraph");
   |
help: use `let _ = ...` to ignore the resulting value
   |
   |
56 |     let _ = buf.push("paragraph");

error: unused `std::ops::ControlFlow` that must be used
  --> src/librustdoc/html/length_limit/tests.rs:58:5
   |
   |
58 |     buf.push("!");
   |
help: use `let _ = ...` to ignore the resulting value
   |
   |
58 |     let _ = buf.push("!");

error: unused `std::ops::ControlFlow` that must be used
  --> src/librustdoc/html/length_limit/tests.rs:69:5
   |
   |
69 |     buf.push("This is a ");
   |
help: use `let _ = ...` to ignore the resulting value
   |
   |
69 |     let _ = buf.push("This is a ");

error: unused `std::ops::ControlFlow` that must be used
  --> src/librustdoc/html/length_limit/tests.rs:71:5
   |
   |
71 |     buf.push("paragraph");
   |
help: use `let _ = ...` to ignore the resulting value
   |
   |
71 |     let _ = buf.push("paragraph");

error: unused `std::ops::ControlFlow` that must be used
  --> src/librustdoc/html/length_limit/tests.rs:73:5
   |
   |
73 |     buf.push("!");
   |
help: use `let _ = ...` to ignore the resulting value
   |
   |
73 |     let _ = buf.push("!");

error: unused `std::ops::ControlFlow` that must be used
  --> src/librustdoc/html/length_limit/tests.rs:81:5
   |
   |
81 | /     (0..10).try_for_each(|n| {
82 | |         buf.open_tag("strong");
83 | |         buf.push("word#")?;
84 | |         buf.push(&n.to_string())?;
85 | |         buf.close_tag();
86 | |         ControlFlow::Continue(())
   | |______^
   |
help: use `let _ = ...` to ignore the resulting value
   |
   |
81 |     let _ = (0..10).try_for_each(|n| {

error: unused `std::ops::ControlFlow` that must be used
   --> src/librustdoc/html/length_limit/tests.rs:103:5
    |
    |
103 |     buf.push("Hello");
    |
help: use `let _ = ...` to ignore the resulting value
    |
    |
103 |     let _ = buf.push("Hello");

error: unused `std::ops::ControlFlow` that must be used
   --> src/librustdoc/html/length_limit/tests.rs:104:5
    |
    |
104 |     buf.push(" World");
    |
help: use `let _ = ...` to ignore the resulting value
    |
    |
104 |     let _ = buf.push(" World");

error: unused `std::ops::ControlFlow` that must be used
   --> src/librustdoc/html/length_limit/tests.rs:113:5
    |
    |
113 |     buf.push("Hello");
    |
help: use `let _ = ...` to ignore the resulting value
    |
    |
113 |     let _ = buf.push("Hello");

error: could not compile `rustdoc` (lib test) due to 21 previous errors
Build completed unsuccessfully in 0:34:40
  local time: Sun Feb 23 00:42:26 UTC 2025