Auto merge of #13913 - Urgau:check-cfg-lints-sub-config, r=epage

Add special `check-cfg` lint config for the `unexpected_cfgs` lint

### What does this PR try to resolve?

This PR adds a special `check-cfg` lint config for the `unexpected_cfgs` lint, as it was decided by T-cargo (in today's meeting).

The goal of this lint config is to provide a simple and cost-less alternative to the build-script `cargo::rustc-check-cfg` instruction.

```toml
[lints.rust]
unexpected_cfgs = { level = "warn", check-cfg = ['cfg(foo, values("bar"))'] }
```

### How should we test and review this PR?

I recommand reviewing commit by commit; and looking at all the new tests added in `check_cfg.rs`, I tried making them as exhaustive as I could, many of them are very similar to their non-config counterpart.

### Additional information

I didn't add *(actually removed from the 1st version of this PR)* the possibility to omit the `level` field if `check-cfg` is specified, #13913 (comment).

Regarding the implementation, I tried making it is as straight forward as possible, nothing over-engineered or complex.

r? `@epage` (or `@weihanglo` maybe)

crates/cargo-util-schemas/src/manifest/mod.rs

@@ -1503,6 +1503,13 @@ impl TomlLint {
             Self::Config(config) => config.priority,
         }
     }
+
+    pub fn config(&self) -> Option<&toml::Table> {
+        match self {
+            Self::Level(_) => None,
+            Self::Config(config) => Some(&config.config),
+        }
+    }
 }
 
 #[derive(Serialize, Deserialize, Debug, Clone)]
@@ -1511,6 +1518,8 @@ pub struct TomlLintConfig {
     pub level: TomlLintLevel,
     #[serde(default)]
     pub priority: i8,
+    #[serde(flatten)]
+    pub config: toml::Table,
 }
 
 #[derive(Serialize, Deserialize, Debug, Copy, Clone, Eq, PartialEq)]

src/cargo/core/compiler/build_runner/mod.rs

@@ -246,7 +246,7 @@ impl<'a, 'gctx> BuildRunner<'a, 'gctx> {
                 let mut args = compiler::extern_args(&self, unit, &mut unstable_opts)?;
                 args.extend(compiler::lto_args(&self, unit));
                 args.extend(compiler::features_args(unit));
-                args.extend(compiler::check_cfg_args(&self, unit));
+                args.extend(compiler::check_cfg_args(&self, unit)?);
 
                 let script_meta = self.find_build_script_metadata(unit);
                 if let Some(meta) = script_meta {

src/cargo/core/compiler/mod.rs

@@ -63,7 +63,7 @@ use std::io::{BufRead, Write};
 use std::path::{Path, PathBuf};
 use std::sync::Arc;
 
-use anyhow::{Context as _, Error};
+use anyhow::{bail, Context as _, Error};
 use lazycell::LazyCell;
 use tracing::{debug, trace};
 
@@ -732,7 +732,7 @@ fn prepare_rustdoc(build_runner: &BuildRunner<'_, '_>, unit: &Unit) -> CargoResu
     let doc_dir = build_runner.files().out_dir(unit);
     rustdoc.arg("-o").arg(&doc_dir);
     rustdoc.args(&features_args(unit));
-    rustdoc.args(&check_cfg_args(build_runner, unit));
+    rustdoc.args(&check_cfg_args(build_runner, unit)?);
 
     add_error_format_and_color(build_runner, &mut rustdoc);
     add_allow_features(build_runner, &mut rustdoc);
@@ -1125,7 +1125,7 @@ fn build_base_args(
     }
 
     cmd.args(&features_args(unit));
-    cmd.args(&check_cfg_args(build_runner, unit));
+    cmd.args(&check_cfg_args(build_runner, unit)?);
 
     let meta = build_runner.files().metadata(unit);
     cmd.arg("-C").arg(&format!("metadata={}", meta));
@@ -1310,7 +1310,7 @@ fn trim_paths_args(
 }
 
 /// Generates the `--check-cfg` arguments for the `unit`.
-fn check_cfg_args(build_runner: &BuildRunner<'_, '_>, unit: &Unit) -> Vec<OsString> {
+fn check_cfg_args(build_runner: &BuildRunner<'_, '_>, unit: &Unit) -> CargoResult<Vec<OsString>> {
     if build_runner
         .bcx
         .target_data
@@ -1353,14 +1353,39 @@ fn check_cfg_args(build_runner: &BuildRunner<'_, '_>, unit: &Unit) -> Vec<OsStri
         // Cargo and docs.rs than rustc and docs.rs. In particular, all users of docs.rs use
         // Cargo, but not all users of rustc (like Rust-for-Linux) use docs.rs.
 
-        vec![
+        let mut args = vec![
             OsString::from("--check-cfg"),
             OsString::from("cfg(docsrs)"),
             OsString::from("--check-cfg"),
             arg_feature,
-        ]
+        ];
+
+        // Also include the custom arguments specified in `[lints.rust.unexpected_cfgs.check_cfg]`
+        if let Ok(Some(lints)) = unit.pkg.manifest().resolved_toml().resolved_lints() {
+            if let Some(rust_lints) = lints.get("rust") {
+                if let Some(unexpected_cfgs) = rust_lints.get("unexpected_cfgs") {
+                    if let Some(config) = unexpected_cfgs.config() {
+                        if let Some(check_cfg) = config.get("check-cfg") {
+                            if let Ok(check_cfgs) =
+                                toml::Value::try_into::<Vec<String>>(check_cfg.clone())
+                            {
+                                for check_cfg in check_cfgs {
+                                    args.push(OsString::from("--check-cfg"));
+                                    args.push(OsString::from(check_cfg));
+                                }
+                            // error about `check-cfg` not being a list-of-string
+                            } else {
+                                bail!("`lints.rust.unexpected_cfgs.check-cfg` must be a list of string");
+                            }
+                        }
+                    }
+                }
+            }
+        }
+
+        Ok(args)
     } else {
-        Vec::new()
+        Ok(Vec::new())
     }
 }
 

src/cargo/util/toml/mod.rs

@@ -2265,7 +2265,7 @@ supported tools: {}",
         if tool == "cargo" && !gctx.cli_unstable().cargo_lints {
             warn_for_cargo_lint_feature(gctx, warnings);
         }
-        for name in lints.keys() {
+        for (name, config) in lints {
             if let Some((prefix, suffix)) = name.split_once("::") {
                 if tool == prefix {
                     anyhow::bail!(
@@ -2278,6 +2278,19 @@ supported tools: {}",
                 } else {
                     anyhow::bail!("`lints.{tool}.{name}` is not a valid lint name")
                 }
+            } else if let Some(config) = config.config() {
+                for config_name in config.keys() {
+                    // manually report unused manifest key warning since we collect all the "extra"
+                    // keys and values inside the config table
+                    //
+                    // except for `rust.unexpected_cfgs.check-cfg` which is used by rustc/rustdoc
+                    if !(tool == "rust" && name == "unexpected_cfgs" && config_name == "check-cfg")
+                    {
+                        let message =
+                            format!("unused manifest key: `lints.{tool}.{name}.{config_name}`");
+                        warnings.push(message);
+                    }
+                }
             }
         }
     }

src/doc/src/reference/build-scripts.md

@@ -292,32 +292,6 @@ if foo_bar_condition {
 }
 ```
 
-##### Example of local-only `build.rs`/build script
-
-Build scripts can impose costs on downstream users, and crate authors who wish to avoid
-this can use "local-only" build scripts, which are active locally but not packaged in the
-distributed package. Completly removing the cost from downstream users. 
-
-The way to achieved this, is by excluding the `build.rs` in the `Cargo.toml` with the
-`exclude` key:
-
-```diff
-  [package]
-  name = "foo"
-  version = "0.1.0"
-+ exclude = ["build.rs"]
-```
-
-*`build.rs`:*
-```rust
-fn main() {
-    // Warning: build.rs is not published to crates.io.
-
-    println!("cargo::rerun-if-changed=build.rs");
-    println!("cargo::rustc-check-cfg=cfg(foo)");
-}
-```
-
 For a more complete example see in the [build script examples][build-script-examples] page
 the [conditional compilation][conditional-compilation-example] example.