[WIP] Fix: Cargo fails to detect environment variable

src/cargo/core/compiler/fingerprint/dirty_reason.rs

@@ -29,6 +29,7 @@ pub enum DirtyReason {
     MetadataChanged,
     ConfigSettingsChanged,
     CompileKindChanged,
+    EnvConfigChanged,
     LocalLengthsChanged,
     PrecalculatedComponentsChanged {
         old: String,
@@ -172,6 +173,9 @@ impl DirtyReason {
             DirtyReason::CompileKindChanged => {
                 s.dirty_because(unit, "the rustc compile kind changed")
             }
+            DirtyReason::EnvConfigChanged => {
+                s.dirty_because(unit, "the environment variable changed")
+            }
             DirtyReason::LocalLengthsChanged => {
                 s.dirty_because(unit, "the local lengths changed")?;
                 s.note(

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

@@ -420,8 +420,14 @@ pub fn prepare_target(
     let mtime_on_use = build_runner.bcx.gctx.cli_unstable().mtime_on_use;
     let dirty_reason = compare_old_fingerprint(unit, &loc, &*fingerprint, mtime_on_use, force);
 
-    let Some(dirty_reason) = dirty_reason else {
-        return Ok(Job::new_fresh());
+    let dirty_reason = match dirty_reason {
+        Some(dr) => dr,
+        None => {
+            let Some(dr) = env_config_modified(bcx.gctx) else {
+                return Ok(Job::new_fresh());
+            };
+            dr
+        }
     };
 
     // We're going to rebuild, so ensure the source of the crate passes all
@@ -2231,3 +2237,17 @@ pub fn parse_rustc_dep_info(rustc_dep_info: &Path) -> CargoResult<RustcDepInfo>
         Ok(ret)
     }
 }
+
+/// Detects if environment variables from config `[env]` is newly seted.
+fn env_config_modified(gctx: &crate::GlobalContext) -> Option<DirtyReason> {
+    for (key, value) in gctx.env_config().unwrap().iter() {
+        if !gctx.env().any(|(k, _)| k == key) {
+            continue;
+        }
+
+        if !value.is_force() && gctx.env().find(|(k, _)| k == key).is_some() {
+            return Some(DirtyReason::EnvConfigChanged);
+        }
+    }
+    None
+}

tests/testsuite/cargo_env_config.rs

@@ -239,3 +239,32 @@ fn env_applied_to_target_info_discovery_rustc() {
         .with_stderr_contains("MAIN ENV_TEST:from-env")
         .run();
 }
+
+#[cargo_test]
+fn env_reset() {
+    let p = project()
+        .file("Cargo.toml", &basic_bin_manifest("foo"))
+        .file(
+            "src/main.rs",
+            r#"
+        use std::env;
+        fn main() {
+            println!( "{}", env!("ENV_TEST") );
+        }
+        "#,
+        )
+        .file(
+            ".cargo/config.toml",
+            r#"
+                [env]
+                ENV_TEST = "from-config"
+            "#,
+        )
+        .build();
+
+    p.cargo("run").with_stdout_contains("from-config").run();
+    p.cargo("run")
+        .env("ENV_TEST", "from-env")
+        .with_stdout_contains("from-env")
+        .run();
+}