Add future-incompatibility warning against some keyword-as-ident

crates/cargo-platform/src/cfg.rs

@@ -31,6 +31,14 @@ enum Token<'a> {
     String(&'a str),
 }
 
+/// The list of keywords.
+///
+/// We should consider all the keywords, but some are conditional on
+/// the edition so for now we just consider true/false.
+///
+/// <https://doc.rust-lang.org/reference/keywords.html>
+pub(crate) const KEYWORDS: &[&str; 2] = &["true", "false"];
+
 #[derive(Clone)]
 struct Tokenizer<'a> {
     s: iter::Peekable<str::CharIndices<'a>>,

crates/cargo-platform/src/lib.rs

@@ -11,12 +11,13 @@
 //!
 //! [`Platform`]: enum.Platform.html
 
-use std::fmt;
 use std::str::FromStr;
+use std::{fmt, path::Path};
 
 mod cfg;
 mod error;
 
+use cfg::KEYWORDS;
 pub use cfg::{Cfg, CfgExpr};
 pub use error::{ParseError, ParseErrorKind};
 
@@ -104,6 +105,46 @@ impl Platform {
             check_cfg_expr(cfg, warnings);
         }
     }
+
+    pub fn check_cfg_keywords(&self, warnings: &mut Vec<String>, path: &Path) {
+        fn check_cfg_expr(expr: &CfgExpr, warnings: &mut Vec<String>, path: &Path) {
+            match *expr {
+                CfgExpr::Not(ref e) => check_cfg_expr(e, warnings, path),
+                CfgExpr::All(ref e) | CfgExpr::Any(ref e) => {
+                    for e in e {
+                        check_cfg_expr(e, warnings, path);
+                    }
+                }
+                CfgExpr::Value(ref e) => match e {
+                    Cfg::Name(name) | Cfg::KeyPair(name, _) => {
+                        if KEYWORDS.contains(&name.as_str()) {
+                            if name.as_str() == "true" || name.as_str() == "false" {
+                                warnings.push(format!(
+                                    "[{}] future-incompatibility: the meaning of `cfg({e})` will change in the future\n \
+                                     | Cargo is erroneously allowing `cfg(true)` and `cfg(false)`, but both forms are interpreted as false unless manually overridden with `--cfg`.\n \
+                                     | In the future these will be built-in defines that will have the corresponding true/false value.\n \
+                                     | It is recommended to avoid using these configs until they are properly supported.\n \
+                                     | See <https://github.com/rust-lang/rust/issues/131204> for more information.",
+                                    path.display()
+                                ));
+                            } else {
+                                warnings.push(format!(
+                                    "[{}] future-incompatibility: `cfg({e})` is deprecated as `{name}` is a keyword \
+                                     and not an identifier and should not have have been accepted in this position.\n \
+                                     | this was previously accepted by Cargo but is being phased out; it will become a hard error in a future release!",
+                                     path.display()
+                                ));
+                            }
+                        }
+                    }
+                },
+            }
+        }
+
+        if let Platform::Cfg(cfg) = self {
+            check_cfg_expr(cfg, warnings, path);
+        }
+    }
 }
 
 impl serde::Serialize for Platform {

src/cargo/util/context/target.rs

@@ -3,7 +3,7 @@ use crate::core::compiler::{BuildOutput, LinkArgTarget};
 use crate::util::CargoResult;
 use serde::Deserialize;
 use std::collections::{BTreeMap, HashMap};
-use std::path::PathBuf;
+use std::path::{Path, PathBuf};
 use std::rc::Rc;
 
 /// Config definition of a `[target.'cfg(…)']` table.
@@ -53,6 +53,13 @@ pub(super) fn load_target_cfgs(
     let target: BTreeMap<String, TargetCfgConfig> = gctx.get("target")?;
     tracing::debug!("Got all targets {:#?}", target);
     for (key, cfg) in target {
+        if let Ok(platform) = key.parse::<cargo_platform::Platform>() {
+            let mut warnings = Vec::new();
+            platform.check_cfg_keywords(&mut warnings, &Path::new(".cargo/config.toml"));
+            for w in warnings {
+                gctx.shell().warn(w)?;
+            }
+        }
         if key.starts_with("cfg(") {
             // Unfortunately this is not able to display the location of the
             // unused key. Using config::Value<toml::Value> doesn't work. One

src/cargo/util/toml/mod.rs

@@ -1326,6 +1326,7 @@ pub fn to_real_manifest(
     for (name, platform) in original_toml.target.iter().flatten() {
         let platform_kind: Platform = name.parse()?;
         platform_kind.check_cfg_attributes(warnings);
+        platform_kind.check_cfg_keywords(warnings, manifest_file);
         let platform_kind = Some(platform_kind);
         validate_dependencies(
             platform.dependencies.as_ref(),

tests/testsuite/cfg.rs

@@ -550,9 +550,20 @@ fn cfg_keywords() {
 
     p.cargo("check")
         .with_stderr_data(str![[r#"
+[WARNING] [[ROOT]/foo/Cargo.toml] future-incompatibility: the meaning of `cfg(true)` will change in the future
+ | Cargo is erroneously allowing `cfg(true)` and `cfg(false)`, but both forms are interpreted as false unless manually overridden with `--cfg`.
+ | In the future these will be built-in defines that will have the corresponding true/false value.
+ | It is recommended to avoid using these configs until they are properly supported.
+ | See <https://github.com/rust-lang/rust/issues/131204> for more information.
+[WARNING] [.cargo/config.toml] future-incompatibility: the meaning of `cfg(false)` will change in the future
+ | Cargo is erroneously allowing `cfg(true)` and `cfg(false)`, but both forms are interpreted as false unless manually overridden with `--cfg`.
+ | In the future these will be built-in defines that will have the corresponding true/false value.
+ | It is recommended to avoid using these configs until they are properly supported.
+ | See <https://github.com/rust-lang/rust/issues/131204> for more information.
 [LOCKING] 1 package to latest compatible version
 [CHECKING] foo v0.1.0 ([ROOT]/foo)
-...
+[FINISHED] `dev` profile [unoptimized + debuginfo] target(s) in [ELAPSED]s
+
 "#]])
         .run();
 }