chore: basic tests exercising rw

tests/lean/rewrite.lean

@@ -1,3 +1,9 @@
+/-!
+# Tests exercising basic behaviour of `rw`.
+
+See also `tests/lean/run/rewrite.lean`.
+-/
+
 axiom appendNil {α} (as : List α) : as ++ [] = as
 axiom appendAssoc {α} (as bs cs : List α) : (as ++ bs) ++ cs = as ++ (bs ++ cs)
 axiom reverseEq {α} (as : List α) : as.reverse.reverse = as

tests/lean/rewrite.lean.expected.out

@@ -1,7 +1,7 @@
 α : Type u_1
 as bs : List α
 ⊢ as ++ bs ++ bs = as ++ (bs ++ bs)
-rewrite.lean:12:20-12:29: error: tactic 'rewrite' failed, did not find instance of the pattern in the target expression
+rewrite.lean:18:20-18:29: error: tactic 'rewrite' failed, did not find instance of the pattern in the target expression
   List.reverse (List.reverse ?as)
 α : Type u_1
 as bs : List α
@@ -10,7 +10,7 @@ x y z : Nat
 h₁ : x = y
 h₂ : y = z
 ⊢ x = z
-rewrite.lean:31:9-31:20: error: tactic 'rewrite' failed, did not find instance of the pattern in the current goal
+rewrite.lean:37:9-37:20: error: tactic 'rewrite' failed, did not find instance of the pattern in the current goal
 x y z : Nat
 h₁ : 0 + x = y
 h₂ : 0 + y = z
@@ -19,7 +19,7 @@ m n k : Nat
 h✝ : n = m
 h : k = m
 ⊢ k = n
-rewrite.lean:49:69-50:10: error: unsolved goals
+rewrite.lean:55:69-56:10: error: unsolved goals
 α : Type
 p : Prop
 a b c : α

tests/lean/run/rewrite.lean

@@ -0,0 +1,113 @@
+/-!
+# Tests exercising basic behaviour of `rw`.
+
+See also `tests/lean/rewrite.lean`.
+-/
+
+-- Rewriting by `iff`
+example (h : P ↔ Q) (q : Q) : P := by
+  rw [h]
+  exact q
+example (h : P ↔ Q) (q : Q) : P := by
+  rw [← h] at q
+  exact q
+
+def f (_ : Nat) : Nat := 0
+def g (n : Nat) : Nat := f n
+
+theorem t (n : Nat) : f n = f 0 := rfl
+theorem s (n m : Nat) : f n = f m := rfl
+
+example : f 1 = f 2 := by
+  rw [s 1 2] -- closes the goal `f 2 = f 2` via `rfl`
+example : f 1 = f 2 := by
+  rw [t] -- does not close the goal, because `rw` calls `rfl` under `withReducible`
+  rfl
+
+example (h : f (f 1) = 0) : f (g 1) = 0 := by
+  fail_if_success rw [h] -- Can not see through definition of `g`
+  erw [h] -- "Expensive" rewrite can
+
+-- Check `(config := ...)` works:
+example (h : f (f 1) = 0) : f (g 1) = 0 := by
+  rw (config := {transparency := .default}) [h] -- This is just `erw`.
+
+attribute [reducible] f in
+example : f 1 = f 2 := by
+  rw [] -- Empty rewrite closes the goal via `rfl`
+example : f 1 = f 2 := by
+  rw [t] -- Closes the goal via `rfl`
+example : f 1 = f 2 := by
+  rw [s 1 3, s 3 4] -- Closes the goal via `rfl`
+
+-- For the remaining tests we prevent `rfl` from closing the goal.
+attribute [irreducible] f
+
+-- Multiple rewrites
+example : f 3 = f 7 := by
+  rw [s 3 5, s 5 7]
+
+-- Rewrite only modifies the main goal.
+example (h : f 2 = f 3) : f 1 = f 2 := by
+  rw [s 2 3, s 1 2]
+  assumption
+
+-- Rewrite `at`
+example (h : f 2 = f 3) : f 1 = f 2 := by
+  rw [s 2 1] at h
+  rw [s 2 3]
+  assumption
+
+-- Rewriting only affects the first occurrence.
+-- Providing an argument allows rewriting other occurrences.
+example : [f 1, f 2] = [f 0, f 0] := by
+  rw [t]
+  rw [t]
+  rw [t 2]
+
+-- We can rewrite backwards.
+example : f 1 = f 0 := by
+  rw [← t 2]
+  rw [t]
+  rw [t 2]
+
+-- Rewriting simultaneously replaces all occurrences of the first match.
+example : [f 1, f 2, f 1] = [f 0, f 0, f 0] := by
+  rw [t]
+  rw [t 2]
+
+-- Rewriting by lemmas with undetermined arguments creates additional goals.
+example : f 2 = f 3 := by
+  rewrite [s]
+  rfl
+
+-- These are often solved by the implicit `rfl` at the end of `rw`.
+example : f 2 = f 3 := by
+  rw [s]
+
+-- We can rewrite at multiple locations,
+-- specializing the arguments differently at each location.
+-- This behaviour was accidentally broken by
+-- https://github.com/leanprover/lean4/pull/2317
+-- and required a revert.
+example (w₁ : f 1 = 0) (w₂ : f 2 = 0) : f 3 = 0 ∧ f 0 = 0 := by
+  rw [t] at w₁ w₂ ⊢
+  exact ⟨w₁, w₂⟩
+
+class P (n : Nat)
+
+theorem t' (n : Nat) [P n] : f n = f 0 := t n
+
+-- Rewriting uses typeclass synthesis to fill arguments.
+example [∀ n, P n] : f 2 = f 0 := by
+  rw [t']
+
+example : f 2 = f 0 := by
+  fail_if_success rw [t']      -- failed to synthesize P 2
+  fail_if_success rw [@t' 2 _] -- failed to synthesize P 2
+  -- By putting placeholders in a typeclass argument
+  -- we can return the problem to the user.
+  rw [@t' 2 ?_]
+  constructor
+
+