Auto merge of #9026 - kassens:master, r=Eh2406

Clarify fingerprint log messages

I had to look up the source to figure out which side was the old value for a RUSTFLAGS fingerprint mismatch.

Making the logs a bit more explicit about which value is old and new would've helped me.

Author: bors

src/cargo/core/compiler/fingerprint.rs

@@ -817,9 +817,9 @@ impl Fingerprint {
         }
         if self.features != old.features {
             bail!(
-                "features have changed: {} != {}",
-                self.features,
-                old.features
+                "features have changed: previously {}, now {}",
+                old.features,
+                self.features
             )
         }
         if self.target != old.target {
@@ -833,9 +833,9 @@ impl Fingerprint {
         }
         if self.rustflags != old.rustflags {
             bail!(
-                "RUSTFLAGS has changed: {:?} != {:?}",
-                self.rustflags,
-                old.rustflags
+                "RUSTFLAGS has changed: previously {:?}, now {:?}",
+                old.rustflags,
+                self.rustflags
             )
         }
         if self.metadata != old.metadata {
@@ -853,15 +853,23 @@ impl Fingerprint {
             match (new, old) {
                 (LocalFingerprint::Precalculated(a), LocalFingerprint::Precalculated(b)) => {
                     if a != b {
-                        bail!("precalculated components have changed: {} != {}", a, b)
+                        bail!(
+                            "precalculated components have changed: previously {}, now {}",
+                            b,
+                            a
+                        )
                     }
                 }
                 (
                     LocalFingerprint::CheckDepInfo { dep_info: adep },
                     LocalFingerprint::CheckDepInfo { dep_info: bdep },
                 ) => {
                     if adep != bdep {
-                        bail!("dep info output changed: {:?} != {:?}", adep, bdep)
+                        bail!(
+                            "dep info output changed: previously {:?}, now {:?}",
+                            bdep,
+                            adep
+                        )
                     }
                 }
                 (
@@ -875,13 +883,17 @@ impl Fingerprint {
                     },
                 ) => {
                     if aout != bout {
-                        bail!("rerun-if-changed output changed: {:?} != {:?}", aout, bout)
+                        bail!(
+                            "rerun-if-changed output changed: previously {:?}, now {:?}",
+                            bout,
+                            aout
+                        )
                     }
                     if apaths != bpaths {
                         bail!(
-                            "rerun-if-changed output changed: {:?} != {:?}",
-                            apaths,
+                            "rerun-if-changed output changed: previously {:?}, now {:?}",
                             bpaths,
+                            apaths,
                         )
                     }
                 }
@@ -896,11 +908,11 @@ impl Fingerprint {
                     },
                 ) => {
                     if *akey != *bkey {
-                        bail!("env vars changed: {} != {}", akey, bkey);
+                        bail!("env vars changed: previously {}, now {}", bkey, akey);
                     }
                     if *avalue != *bvalue {
                         bail!(
-                            "env var `{}` changed: previously {:?} now {:?}",
+                            "env var `{}` changed: previously {:?}, now {:?}",
                             akey,
                             bvalue,
                             avalue