Enable Kani to work with a stable toolchain (#3964)

As discussed in #3960, applying changes needed for the stable branch in
main to minimize the differences between the two branches.

By submitting this pull request, I confirm that my contribution is made
under the terms of the Apache 2.0 and MIT licenses.

---------

Co-authored-by: Michael Tautschnig <mt@debian.org>
Co-authored-by: Carolyn Zech <cmzech@amazon.com>

Author: 3 people

.cargo/config.toml

@@ -20,6 +20,8 @@ KANI_SYSROOT ={value = "target/kani", relative = true}
 KANI_BUILD_LIBS ={value = "target/build-libs", relative = true}
 # Build Kani library without `build-std`.
 KANI_LEGACY_LIBS ={value = "target/legacy-libs", relative = true}
+# This is only required for stable but is a no-op for nightly channels
+RUSTC_BOOTSTRAP = "1"
 
 [target.'cfg(all())']
 rustflags = [ # Global lints/warnings. Need to use underscore instead of -.

kani-compiler/src/main.rs

@@ -16,6 +16,9 @@
 #![feature(f128)]
 #![feature(f16)]
 #![feature(non_exhaustive_omitted_patterns_lint)]
+#![feature(cfg_version)]
+// Once the `stable` branch is at 1.86 or later, remove this line, since float_next_up_down is stabilized
+#![cfg_attr(not(version("1.86")), feature(float_next_up_down))]
 #![feature(try_blocks)]
 extern crate rustc_abi;
 extern crate rustc_ast;

kani-driver/src/call_cargo.rs

@@ -206,6 +206,8 @@ crate-type = ["lib"]
                     // Use CARGO_ENCODED_RUSTFLAGS instead of RUSTFLAGS is preferred. See
                     // https://doc.rust-lang.org/cargo/reference/environment-variables.html
                     .env("CARGO_ENCODED_RUSTFLAGS", rustc_args.join(OsStr::new("\x1f")))
+                    // This is only required for stable but is a no-op for nightly channels
+                    .env("RUSTC_BOOTSTRAP", "1")
                     .env("CARGO_TERM_PROGRESS_WHEN", "never");
 
                 match self.run_build_target(cmd, verification_target.target()) {

kani-driver/src/call_single_file.rs

@@ -84,6 +84,8 @@ impl KaniSession {
         let mut cmd = Command::new(&self.kani_compiler);
         let kani_compiler_args = to_rustc_arg(kani_args);
         cmd.arg(kani_compiler_args).args(rustc_args);
+        // This is only required for stable but is a no-op for nightly channels
+        cmd.env("RUSTC_BOOTSTRAP", "1");
 
         if self.args.common_args.quiet {
             self.run_suppress(cmd)?;

tests/cargo-ui/verbose-cmds/expected

@@ -1,5 +1,5 @@
 CARGO_ENCODED_RUSTFLAGS=
-cargo +nightly
+cargo +
 Running: `goto-cc
 Running: `goto-instrument
 Checking harness dummy_harness...