fix: strip trailing whitespace on codegenned new lines

.changelog/fix-generated-trailing-whitespace.md

@@ -0,0 +1,13 @@
+---
+applies_to:
+- server
+authors:
+- jlizen
+references:
+- smithy-rs#4634
+breaking: false
+new_feature: false
+bug_fix: true
+---
+
+Strip trailing whitespace from generated Rust code. Smithy's `AbstractCodeWriter` adds indentation to blank lines, producing whitespace-only lines that cause `cargo fmt` to fail with `error[internal]: left behind trailing whitespace`.

codegen-core/src/main/kotlin/software/amazon/smithy/rust/codegen/core/rustlang/RustWriter.kt

@@ -598,6 +598,8 @@ class RustWriter private constructor(
 ) :
     SymbolWriter<RustWriter, UseDeclarations>(UseDeclarations(namespace)) {
         companion object {
+            private val TRAILING_WHITESPACE_RE = Regex("[ \\t]+\\n")
+
             fun root() = forModule(null)
 
             fun forModule(module: String?): RustWriter =
@@ -925,6 +927,7 @@ class RustWriter private constructor(
                     null
                 }
             return listOfNotNull(preheader, header, useDecls, contents).joinToString(separator = "\n", postfix = "\n")
+                .replace(TRAILING_WHITESPACE_RE, "\n")
         }
 
         fun format(r: Any) = formatter.apply(r, "")

codegen-core/src/test/kotlin/software/amazon/smithy/rust/codegen/core/rustlang/RustWriterTest.kt

@@ -228,4 +228,37 @@ class RustWriterTest {
         val sut = RustWriter.forModule("src/module_name")
         sut.module().shouldBe("module_name")
     }
+
+    // Regression test for https://github.com/smithy-lang/smithy-rs/issues/4634
+    @Test
+    fun `output does not contain trailing whitespace`() {
+        val sut = RustWriter.root()
+        // Reproduce the two patterns from the issue:
+        sut.rustBlock("fn example()") {
+            // join("\n") separator creates a blank line that gets indented
+            val writables =
+                listOf(
+                    writable { rust("let a = 1;") },
+                    writable { rust("let b = 2;") },
+                )
+            writables.join("\n")(this)
+
+            // Empty string interpolation leaves a blank line that gets indented
+            val empty = ""
+            rust(
+                """
+                something
+                    .method()
+                    $empty
+                    .chain()
+                """,
+            )
+        }
+        val output = sut.toString()
+        val trailingWhitespaceLines =
+            output.lines().filter { line ->
+                line != line.trimEnd()
+            }
+        trailingWhitespaceLines shouldBe emptyList()
+    }
 }