./miri many-seeds: support MIRI_SEED_END to control the end of the tr…

…ied seed range

miri-script/src/commands.rs

@@ -356,11 +356,17 @@ impl Command {
             .unwrap_or_else(|_| "0".into())
             .parse()
             .context("failed to parse MIRI_SEED_START")?;
-        let seed_count: u64 = env::var("MIRI_SEEDS")
-            .unwrap_or_else(|_| "256".into())
-            .parse()
-            .context("failed to parse MIRI_SEEDS")?;
-        let seed_end = seed_start + seed_count;
+        let seed_end: u64 = match (env::var("MIRI_SEEDS"), env::var("MIRI_SEED_END")) {
+            (Ok(_), Ok(_)) => bail!("Only one of MIRI_SEEDS and MIRI_SEED_END may be set"),
+            (Ok(seeds), Err(_)) =>
+                seed_start + seeds.parse::<u64>().context("failed to parse MIRI_SEEDS")?,
+            (Err(_), Ok(seed_end)) => seed_end.parse().context("failed to parse MIRI_SEED_END")?,
+            (Err(_), Err(_)) => seed_start + 256,
+        };
+        if seed_end <= seed_start {
+            bail!("the end of the seed range must be larger than the start.");
+        }
+
         let Some((command_name, trailing_args)) = command.split_first() else {
             bail!("expected many-seeds command to be non-empty");
         };

miri-script/src/main.rs

@@ -113,8 +113,9 @@ sysroot, to prevent conflicts with other toolchains.
 ./miri many-seeds <command>:
 Runs <command> over and over again with different seeds for Miri. The MIRIFLAGS
 variable is set to its original value appended with ` -Zmiri-seed=$SEED` for
-many different seeds. The MIRI_SEEDS variable controls how many seeds are being
-tried; MIRI_SEED_START controls the first seed to try.
+many different seeds. MIRI_SEED_START controls the first seed to try (default: 0).
+MIRI_SEEDS controls how many seeds are being tried (default: 256);
+alternatively, MIRI_SEED_END controls the end of the (exclusive) seed range to try.
 
 ./miri bench <benches>:
 Runs the benchmarks from bench-cargo-miri in hyperfine. hyperfine needs to be installed.