also accept odd number of hex digits; add README section on randomized alignment testing

Author: RalfJung

Cargo.lock

@@ -22,7 +22,6 @@ getrandom = { version = "0.2", features = ["std"] }
 env_logger = "0.9"
 log = "0.4"
 shell-escape = "0.1.4"
-hex = "0.4.0"
 rand = "0.8"
 smallvec = "1.7"
 

Cargo.toml

@@ -175,12 +175,28 @@ Here is an example job for GitHub Actions:
 The explicit `cargo miri setup` helps to keep the output of the actual test step
 clean.
 
+### Testing for alignment issues
+
+Miri can sometimes miss misaligned accesses since allocations can "happen to be"
+aligned just right. You can use `-Zmiri-symbolic-alignment-check` to definitely
+catch all such issues, but that flag will also cause false positives when code
+does manual pointer arithmetic to account for alignment. Another alternative is
+to call Miri with various values for `-Zmiri-seed`; that will alter the
+randomness that is used to determine allocation base addresses. The following
+snippet calls Miri in a loop with different values for the seed:
+
+```
+for seed in $({ echo obase=16; seq 255; } | bc); do
+  MIRIFLAGS=-Zmiri-seed=$seed cargo miri test || { echo "Last seed: $seed"; break; };
+done
+```
+
 ### Common Problems
 
 When using the above instructions, you may encounter a number of confusing compiler
 errors.
 
-### "note: run with `RUST_BACKTRACE=1` environment variable to display a backtrace"
+#### "note: run with `RUST_BACKTRACE=1` environment variable to display a backtrace"
 
 You may see this when trying to get Miri to display a backtrace. By default, Miri
 doesn't expose any environment to the program, so running

README.md

@@ -15,7 +15,6 @@ use std::num::NonZeroU64;
 use std::path::PathBuf;
 use std::str::FromStr;
 
-use hex::FromHexError;
 use log::debug;
 
 use rustc_data_structures::sync::Lrc;
@@ -377,22 +376,11 @@ fn main() {
                     if miri_config.seed.is_some() {
                         panic!("Cannot specify -Zmiri-seed multiple times!");
                     }
-                    let seed_raw = hex::decode(arg.strip_prefix("-Zmiri-seed=").unwrap())
-                        .unwrap_or_else(|err| match err {
-                            FromHexError::InvalidHexCharacter { .. } => panic!(
-                                "-Zmiri-seed should only contain valid hex digits [0-9a-fA-F]"
-                            ),
-                            FromHexError::OddLength =>
-                                panic!("-Zmiri-seed should have an even number of digits"),
-                            err => panic!("unknown error decoding -Zmiri-seed as hex: {:?}", err),
-                        });
-                    if seed_raw.len() > 8 {
-                        panic!("-Zmiri-seed must be at most 8 bytes, was {}", seed_raw.len());
-                    }
-
-                    let mut bytes = [0; 8];
-                    bytes[..seed_raw.len()].copy_from_slice(&seed_raw);
-                    miri_config.seed = Some(u64::from_be_bytes(bytes));
+                    let seed = u64::from_str_radix(arg.strip_prefix("-Zmiri-seed=").unwrap(), 16)
+                        .unwrap_or_else(|_| panic!(
+                            "-Zmiri-seed should only contain valid hex digits [0-9a-fA-F] and fit into a u64 (max 16 characters)"
+                        ));
+                    miri_config.seed = Some(seed);
                 }
                 arg if arg.starts_with("-Zmiri-env-exclude=") => {
                     miri_config