remove -Zmiri-panic-on-unsupported flag

Author: RalfJung

src/tools/miri/README.md

@@ -392,11 +392,6 @@ to Miri failing to detect cases of undefined behavior in a program.
   but reports to the program that it did actually write. This is useful when you
   are not interested in the actual program's output, but only want to see Miri's
   errors and warnings.
-* `-Zmiri-panic-on-unsupported` will make some forms of unsupported functionality,
-  such as FFI and unsupported syscalls, panic within the context of the emulated
-  application instead of raising an error within the context of Miri (and halting
-  execution). Note that code might not expect these operations to ever panic, so
-  this flag can lead to strange (mis)behavior.
 * `-Zmiri-recursive-validation` is a *highly experimental* flag that makes validity checking
   recurse below references.
 * `-Zmiri-retag-fields[=<all|none|scalar>]` controls when Stacked Borrows retagging recurses into

src/tools/miri/src/bin/miri.rs

@@ -530,8 +530,6 @@ fn main() {
         } else if arg == "-Zmiri-ignore-leaks" {
             miri_config.ignore_leaks = true;
             miri_config.collect_leak_backtraces = false;
-        } else if arg == "-Zmiri-panic-on-unsupported" {
-            miri_config.panic_on_unsupported = true;
         } else if arg == "-Zmiri-strict-provenance" {
             miri_config.provenance_mode = ProvenanceMode::Strict;
         } else if arg == "-Zmiri-permissive-provenance" {

src/tools/miri/src/eval.rs

@@ -129,8 +129,6 @@ pub struct MiriConfig {
     /// If `Some`, enable the `measureme` profiler, writing results to a file
     /// with the specified prefix.
     pub measureme_out: Option<String>,
-    /// Panic when unsupported functionality is encountered.
-    pub panic_on_unsupported: bool,
     /// Which style to use for printing backtraces.
     pub backtrace_style: BacktraceStyle,
     /// Which provenance to use for int2ptr casts
@@ -183,7 +181,6 @@ impl Default for MiriConfig {
             track_outdated_loads: false,
             cmpxchg_weak_failure_rate: 0.8, // 80%
             measureme_out: None,
-            panic_on_unsupported: false,
             backtrace_style: BacktraceStyle::Short,
             provenance_mode: ProvenanceMode::Default,
             mute_stdout_stderr: false,

src/tools/miri/src/machine.rs

@@ -496,11 +496,6 @@ pub struct MiriMachine<'tcx> {
     /// `None` means no `Instance` exported under the given name is found.
     pub(crate) exported_symbols_cache: FxHashMap<Symbol, Option<Instance<'tcx>>>,
 
-    /// Whether to raise a panic in the context of the evaluated process when unsupported
-    /// functionality is encountered. If `false`, an error is propagated in the Miri application context
-    /// instead (default behavior)
-    pub(crate) panic_on_unsupported: bool,
-
     /// Equivalent setting as RUST_BACKTRACE on encountering an error.
     pub(crate) backtrace_style: BacktraceStyle,
 
@@ -667,7 +662,6 @@ impl<'tcx> MiriMachine<'tcx> {
             profiler,
             string_cache: Default::default(),
             exported_symbols_cache: FxHashMap::default(),
-            panic_on_unsupported: config.panic_on_unsupported,
             backtrace_style: config.backtrace_style,
             local_crates,
             extern_statics: FxHashMap::default(),
@@ -807,7 +801,6 @@ impl VisitProvenance for MiriMachine<'_> {
             profiler: _,
             string_cache: _,
             exported_symbols_cache: _,
-            panic_on_unsupported: _,
             backtrace_style: _,
             local_crates: _,
             rng: _,

src/tools/miri/src/shims/foreign_items.rs

@@ -83,17 +83,10 @@ pub trait EvalContextExt<'tcx>: crate::MiriInterpCxExt<'tcx> {
                     return interp_ok(Some(body));
                 }
 
-                let error_msg = format!(
+                throw_machine_stop!(TerminationInfo::UnsupportedForeignItem(format!(
                     "can't call foreign function `{link_name}` on OS `{os}`",
                     os = this.tcx.sess.target.os,
-                );
-                if this.machine.panic_on_unsupported {
-                    // message is slightly different here to make automated analysis easier
-                    let error_msg = format!("unsupported Miri functionality: {error_msg}");
-                    this.start_panic(error_msg.as_ref(), mir::UnwindAction::Continue)?;
-                } else {
-                    throw_machine_stop!(TerminationInfo::UnsupportedForeignItem(error_msg));
-                }
+                )));
             }
         }