Enforce starting newlines (#563)

This is an example of what I've been suggesting at
#506 (+ some linked issues):
- ~Stacked on #528 so that would
need to merge first~
- Enforces the correct number of newlines at the start of snapshots. The
start is easier because we don't need to worry about editor issues in
trailing newlines (and don't need to worry about how `Lines` deals
ambiguously with trailing new lines)
- Works for both inline & file snapshots
- Adds a `matches_legacy` method on snapshot contents — this
encapsulates the older formats we still support and warns about them

~It's not ready to merge yet, because we don't seem to actually support
`--force-update-snapshots` on inline values, which is somewhat required.
But I'm publishing for feedback & to put some weight on #528 in case we
don't want to pursue.~

---------

Co-authored-by: Claude <no-reply@anthropic.com>

Author: max-sixty

CHANGELOG.md

@@ -8,6 +8,14 @@ All notable changes to insta and cargo-insta are documented here.
   nextest. Shows a deprecation warning when nextest is used with doctests without this flag, to prepare `cargo insta` to no longer run
   a separate doctest process when using nextest in the future. #803
 
+- We no longer trim starting newlines during assertions, which allows asserting
+  the number of leading newlines match. Existing assertions with different
+  leading newlines will pass and print a warning suggesting running with
+  `--force-update-snapshots`.  They may fail in the future.  (Note that we still
+  currently allow differing _trailing_ newlines, though may adjust this in the
+  future).  #563
+
+
 ## 1.43.2
 
 - Fix panics when `cargo metadata` fails to execute or parse (e.g., when cargo is not in PATH or returns invalid output). Now falls back to using the manifest directory as the workspace root. #798 (@adriangb)

cargo-insta/tests/functional/binary.rs

@@ -26,8 +26,7 @@ fn test_binary_snapshot() {
     assert_snapshot!(test_project.file_tree_diff(), @r"
     --- Original file tree
     +++ Updated file tree
-    @@ -1,4 +1,8 @@
-     
+    @@ -1,3 +1,7 @@
     +  Cargo.lock
        Cargo.toml
        src
@@ -66,8 +65,7 @@ fn test_binary_snapshot() {
     assert_snapshot!(test_project.file_tree_diff(), @r"
     --- Original file tree
     +++ Updated file tree
-    @@ -1,4 +1,8 @@
-     
+    @@ -1,3 +1,7 @@
     +  Cargo.lock
        Cargo.toml
        src
@@ -123,8 +121,7 @@ fn test_binary_snapshot() {
     assert_snapshot!(test_project.file_tree_diff(), @r"
     --- Original file tree
     +++ Updated file tree
-    @@ -1,4 +1,10 @@
-     
+    @@ -1,3 +1,9 @@
     +  Cargo.lock
        Cargo.toml
        src
@@ -147,8 +144,7 @@ fn test_binary_snapshot() {
     assert_snapshot!(test_project.file_tree_diff(), @r"
     --- Original file tree
     +++ Updated file tree
-    @@ -1,4 +1,8 @@
-     
+    @@ -1,3 +1,7 @@
     +  Cargo.lock
        Cargo.toml
        src
@@ -199,8 +195,7 @@ fn test_binary_snapshot() {
     assert_snapshot!(test_project.file_tree_diff(), @r"
     --- Original file tree
     +++ Updated file tree
-    @@ -1,4 +1,6 @@
-     
+    @@ -1,3 +1,5 @@
     +  Cargo.lock
        Cargo.toml
        src
@@ -237,8 +232,7 @@ fn test() {
     assert_snapshot!(test_project.file_tree_diff(), @r"
     --- Original file tree
     +++ Updated file tree
-    @@ -1,4 +1,7 @@
-     
+    @@ -1,3 +1,6 @@
     +  Cargo.lock
        Cargo.toml
        src
@@ -268,8 +262,7 @@ fn test() {
     assert_snapshot!(test_project.file_tree_diff(), @r"
     --- Original file tree
     +++ Updated file tree
-    @@ -1,4 +1,8 @@
-     
+    @@ -1,3 +1,7 @@
     +  Cargo.lock
        Cargo.toml
        src
@@ -308,8 +301,7 @@ fn test() {
     assert_snapshot!(test_project.file_tree_diff(), @r"
     --- Original file tree
     +++ Updated file tree
-    @@ -1,4 +1,8 @@
-     
+    @@ -1,3 +1,7 @@
     +  Cargo.lock
        Cargo.toml
        src
@@ -340,8 +332,7 @@ fn test() {
     assert_snapshot!(test_project.file_tree_diff(), @r"
     --- Original file tree
     +++ Updated file tree
-    @@ -1,4 +1,7 @@
-     
+    @@ -1,3 +1,6 @@
     +  Cargo.lock
        Cargo.toml
        src
@@ -381,8 +372,7 @@ fn test_snapshot() {
     assert_snapshot!(test_project.file_tree_diff(), @r"
     --- Original file tree
     +++ Updated file tree
-    @@ -1,4 +1,8 @@
-     
+    @@ -1,3 +1,7 @@
     +  Cargo.lock
        Cargo.toml
        src
@@ -411,8 +401,7 @@ fn test_snapshot() {
     assert_snapshot!(test_project.file_tree_diff(), @r"
     --- Original file tree
     +++ Updated file tree
-    @@ -1,4 +1,6 @@
-     
+    @@ -1,3 +1,5 @@
     +  Cargo.lock
        Cargo.toml
        src

cargo-insta/tests/functional/delete_pending.rs

@@ -45,8 +45,7 @@ fn test_snapshot_file() {
     assert_snapshot!(test_project.file_tree_diff(), @r"
     --- Original file tree
     +++ Updated file tree
-    @@ -1,4 +1,7 @@
-     
+    @@ -1,3 +1,6 @@
     +  Cargo.lock
        Cargo.toml
        src
@@ -70,8 +69,7 @@ fn test_snapshot_file() {
     assert_snapshot!(test_project.file_tree_diff(), @r"
     --- Original file tree
     +++ Updated file tree
-    @@ -1,4 +1,6 @@
-     
+    @@ -1,3 +1,5 @@
     +  Cargo.lock
        Cargo.toml
        src
@@ -116,8 +114,7 @@ fn test_file_snapshot() {
     assert_snapshot!(test_project.file_tree_diff(), @r"
     --- Original file tree
     +++ Updated file tree
-    @@ -1,4 +1,7 @@
-     
+    @@ -1,3 +1,6 @@
     +  Cargo.lock
        Cargo.toml
        src
@@ -158,8 +155,7 @@ fn test_file_snapshot() {
     assert_snapshot!(test_project.file_tree_diff(), @r"
     --- Original file tree
     +++ Updated file tree
-    @@ -1,4 +1,9 @@
-     
+    @@ -1,3 +1,8 @@
     +  Cargo.lock
        Cargo.toml
        src
@@ -183,8 +179,7 @@ fn test_file_snapshot() {
     assert_snapshot!(test_project.file_tree_diff(), @r"
     --- Original file tree
     +++ Updated file tree
-    @@ -1,4 +1,7 @@
-     
+    @@ -1,3 +1,6 @@
     +  Cargo.lock
        Cargo.toml
        src
@@ -206,8 +201,7 @@ fn test_file_snapshot() {
     assert_snapshot!(test_project.file_tree_diff(), @r"
     --- Original file tree
     +++ Updated file tree
-    @@ -1,4 +1,9 @@
-     
+    @@ -1,3 +1,8 @@
     +  Cargo.lock
        Cargo.toml
        src
@@ -250,8 +244,7 @@ fn test_file_snapshot() {
     assert_snapshot!(test_project.file_tree_diff(), @r"
     --- Original file tree
     +++ Updated file tree
-    @@ -1,4 +1,7 @@
-     
+    @@ -1,3 +1,6 @@
     +  Cargo.lock
        Cargo.toml
        src