Add Kani usage and verify-std section to verification book (#12)

Adds a section in the book assuming no knowledge from the reader about
Kani or the verification effort.

Co-authored-by: Felipe R. Monteiro <rms.felipe@gmail.com>

doc/book.toml

@@ -20,6 +20,5 @@ runnable = false
 
 [output.linkcheck]
 
-
 [rust]
 edition = "2021"

doc/src/tools/kani.md

@@ -1,7 +1,118 @@
 # Kani Rust Verifier
 
-The Kani Rust Verifier is a bit-precise model checker for Rust.
-This page will give more details on how to use Kani to verify the standard library.
-You can find more informations about how to install and use Kani in the
-[Kani book](https://model-checking.github.io/kani/).
+The [Kani Rust Verifier](https://github.com/model-checking/kani) is a bit-precise model checker for Rust.
+Kani is designed to prove safety properties in your code as well as
+the absence of some forms of undefined behavior. It uses model checking under the hood to ensure that
+Rust programs adhere to user specified properties.
+
+You can find more information about how to install in [this section of the Kani book](https://model-checking.github.io/kani/install-guide.html).
+
+## Usage
+
+Consider a Rust function that takes an integer and returns its absolute value and
+a Kani proof that invokes the function that you want to verify
+
+``` rust
+fn abs_diff(a: i32, b: i32) -> i32 {
+    if a > b {
+        a - b
+    } else {
+        b - a
+    }
+}
+
+#[cfg(kani)]
+#[kani::proof]
+fn harness() {
+    let a = kani::any::<i32>();
+    let b = kani::any::<i32>();
+    let result = abs_diff(a, b);
+    kani::assert(result >= 0, "Result should always be more than 0");}
+```
+
+Running the command `cargo kani` in your cargo crate will give the result
+
+```
+RESULTS:
+Check 1: abs_diff.assertion.1
+         - Status: FAILURE
+         - Description: "attempt to subtract with overflow"
+         - Location: src/main.rs:5:9 in function abs_diff
+
+... Other properties that might have failed or succeeded.
+
+Summary:
+Verification failed for - harness
+Complete - 0 successfully verified harnesses, 1 failures, 1 total.
+```
+
+
+## Using Kani to verify the Rust Standard Library
+
+To aid the Rust Standard Library verification effort, Kani provides a sub-command out of the box to help you get started.
+
+### Step 1
+
+Modify your local copy of the Rust Standard Library by writing proofs for the functions/methods that you want to verify.
+
+For example, insert this short blob into your copy of the library. This blob imports the building-block APIs such as
+`assert`, `assume`, `proof` and [function-contracts](https://github.com/model-checking/kani/blob/main/rfc/src/rfcs/0009-function-contracts.md) such as `proof_for_contract` and `fake_function`.
+
+``` rust
+#[cfg(kani)]
+kani_core::kani_lib!(core);
 
+#[cfg(kani)]
+#[unstable(feature = "kani", issue = "none")]
+pub mod verify {
+    use crate::kani;
+
+    #[kani::proof]
+    pub fn harness() {
+        kani::assert(true, "yay");
+    }
+
+    #[kani::proof_for_contract(trivial_function)]
+    fn dummy_proof() {
+        trivial_function(true);
+    }
+
+    #[kani::requires(x == true)]
+    fn trivial_function(x: bool) -> bool {
+        x
+    }
+}
+```
+
+### Step 2
+
+Run the following command in your local terminal:
+
+`kani verify-std -Z unstable-options "path/to/library" --target-dir "/path/to/target" -Z function-contracts -Z stubbing`.
+
+The command `kani verify-std` is a sub-command of the `kani`. This specific sub-command is used to verify the Rust Standard Library with the following arguments.
+
+- `"path/to/library"`: This argument specifies the path to the modified Rust Standard Library that was prepared earlier in the script.
+- `--target-dir "path/to/target"`: This optional argument sets the target directory where Kani will store its output and intermediate files.
+
+Apart from these, you can use your regular `kani-args` such as `-Z function-contracts` and `-Z stubbing` depending on your verification needs.
+For more details on Kani's features, refer to [the features section in the Kani Book](https://model-checking.github.io/kani/reference/attributes.html)
+
+### Step 3
+
+After running the command, you can expect an output that looks like this:
+
+```
+SUMMARY:
+ ** 0 of 1 failed
+
+VERIFICATION:- SUCCESSFUL
+Verification Time: 0.017101772s
+
+Complete - 2 successfully verified harnesses, 0 failures, 2 total.
+```
+
+## More details
+
+You can find more information about how to install and how you can customize your use of Kani in the
+[Kani book](https://model-checking.github.io/kani/).