Fix how much is trimmed from an interp string part (#18123)

* Fix how much is trimmed from an interp string part

Only trim last 2 characters if they are "%s" and the '%' is not escaped

* Add release note

---------

Co-authored-by: Adam Boniecki <abonie@users.noreply.github.com>

docs/release-notes/.FSharp.Compiler.Service/9.0.200.md

@@ -16,7 +16,7 @@
 * Fix lowering of computed array expressions when the expression consists of a simple mapping from a `uint64` or `unativeint` array. [PR #18081](https://github.com/dotnet/fsharp/pull/18081)
 * Add missing nullable-metadata for C# consumers of records,exceptions and DU subtypes generated from F# code. [PR #18079](https://github.com/dotnet/fsharp/pull/18079)
 * Fix a race condition in file book keeping in the compiler service ([#18008](https://github.com/dotnet/fsharp/pull/18008))
-
+* Fix trimming '%' characters when lowering interpolated string to a concat call [PR #18123](https://github.com/dotnet/fsharp/pull/18123)
 
 ### Added
 

src/Compiler/Checking/Expressions/CheckExpressions.fs

@@ -174,7 +174,7 @@ let (|WithTrailingStringSpecifierRemoved|) (s: string) =
         let i = s.AsSpan(0, s.Length - 2).LastIndexOfAnyExcept '%'
         let diff = s.Length - 2 - i
         if diff &&& 1 <> 0 then
-            s[..i]
+            s[..s.Length - 3]
         else
             s
     else

tests/FSharp.Compiler.ComponentTests/Language/InterpolatedStringsTests.fs

@@ -130,6 +130,17 @@ type Foo () =
         |> compile
         |> shouldSucceed
 
+    [<Fact>]
+    let ``Percent signs and format specifiers with string expression`` () =
+        Fsx """
+let x = "abc"
+let s = $"%%%s{x}%%"
+printfn "%s" s
+        """
+        |> compileExeAndRun
+        |> shouldSucceed
+        |> withStdOutContains "%abc%"
+
     [<Theory>]
     // Test different number of interpolated string parts
     [<InlineData("$\"\"\"abc{\"d\"}e\"\"\"")>]