diff --git a/src/librustc/middle/effect.rs b/src/librustc/middle/effect.rs
index 160b03132e06f..9c02f544fbaeb 100644
--- a/src/librustc/middle/effect.rs
+++ b/src/librustc/middle/effect.rs
@@ -102,8 +102,10 @@ impl Visitor<()> for EffectCheckVisitor {
     fn visit_block(&mut self, block:&Block, _:()) {
 
             let old_unsafe_context = self.context.unsafe_context;
-            if block.rules == ast::UnsafeBlock &&
-                    self.context.unsafe_context == SafeContext {
+            let is_unsafe = match block.rules {
+                ast::UnsafeBlock(*) => true, ast::DefaultBlock => false
+            };
+            if is_unsafe && self.context.unsafe_context == SafeContext {
                 self.context.unsafe_context = UnsafeBlock(block.id)
             }
 
diff --git a/src/librustc/middle/lint.rs b/src/librustc/middle/lint.rs
index 6f4d94e2a647a..da181ff2eb6b6 100644
--- a/src/librustc/middle/lint.rs
+++ b/src/librustc/middle/lint.rs
@@ -1131,8 +1131,11 @@ impl Visitor<@mut Context> for UnusedUnsafeLintVisitor {
     fn visit_expr(&mut self, e:@ast::Expr, cx:@mut Context) {
 
             match e.node {
-                ast::ExprBlock(ref blk) if blk.rules == ast::UnsafeBlock => {
-                    if !cx.tcx.used_unsafe.contains(&blk.id) {
+                // Don't warn about generated blocks, that'll just pollute the
+                // output.
+                ast::ExprBlock(ref blk) => {
+                    if blk.rules == ast::UnsafeBlock(ast::UserProvided) &&
+                       !cx.tcx.used_unsafe.contains(&blk.id) {
                         cx.span_lint(unused_unsafe, blk.span,
                                      "unnecessary `unsafe` block");
                     }
diff --git a/src/librustc/middle/typeck/check/mod.rs b/src/librustc/middle/typeck/check/mod.rs
index b6e0fd93fa9bb..0c0326e9317e7 100644
--- a/src/librustc/middle/typeck/check/mod.rs
+++ b/src/librustc/middle/typeck/check/mod.rs
@@ -200,7 +200,7 @@ impl PurityState {
 
             purity => {
                 let (purity, def) = match blk.rules {
-                    ast::UnsafeBlock => (ast::unsafe_fn, blk.id),
+                    ast::UnsafeBlock(*) => (ast::unsafe_fn, blk.id),
                     ast::DefaultBlock => (purity, self.def),
                 };
                 PurityState{ def: def,
diff --git a/src/libsyntax/ast.rs b/src/libsyntax/ast.rs
index 8d557125d3700..ef2e557b6ea6d 100644
--- a/src/libsyntax/ast.rs
+++ b/src/libsyntax/ast.rs
@@ -479,7 +479,13 @@ pub struct Field {
 #[deriving(Clone, Eq, Encodable, Decodable, IterBytes)]
 pub enum BlockCheckMode {
     DefaultBlock,
-    UnsafeBlock,
+    UnsafeBlock(UnsafeSource),
+}
+
+#[deriving(Clone, Eq, Encodable, Decodable, IterBytes)]
+pub enum UnsafeSource {
+    CompilerGenerated,
+    UserProvided,
 }
 
 #[deriving(Clone, Eq, Encodable, Decodable,IterBytes)]
diff --git a/src/libsyntax/ext/ifmt.rs b/src/libsyntax/ext/ifmt.rs
index b7722ffc29710..486069db4f0c4 100644
--- a/src/libsyntax/ext/ifmt.rs
+++ b/src/libsyntax/ext/ifmt.rs
@@ -632,7 +632,7 @@ impl Context {
            stmts: ~[],
            expr: Some(result),
            id: ast::DUMMY_NODE_ID,
-           rules: ast::UnsafeBlock,
+           rules: ast::UnsafeBlock(ast::CompilerGenerated),
            span: self.fmtsp,
         });
 
diff --git a/src/libsyntax/parse/parser.rs b/src/libsyntax/parse/parser.rs
index 4adc34d75a73e..b5772a9eede2d 100644
--- a/src/libsyntax/parse/parser.rs
+++ b/src/libsyntax/parse/parser.rs
@@ -1792,7 +1792,7 @@ impl Parser {
         } else if self.eat_keyword(keywords::Match) {
             return self.parse_match_expr();
         } else if self.eat_keyword(keywords::Unsafe) {
-            return self.parse_block_expr(lo, UnsafeBlock);
+            return self.parse_block_expr(lo, UnsafeBlock(ast::UserProvided));
         } else if *self.token == token::LBRACKET {
             self.bump();
             let mutbl = self.parse_mutability();
diff --git a/src/libsyntax/print/pprust.rs b/src/libsyntax/print/pprust.rs
index 55f052d769dce..9b9b157c9d85b 100644
--- a/src/libsyntax/print/pprust.rs
+++ b/src/libsyntax/print/pprust.rs
@@ -951,7 +951,7 @@ pub fn print_possibly_embedded_block_(s: @ps,
                                       attrs: &[ast::Attribute],
                                       close_box: bool) {
     match blk.rules {
-      ast::UnsafeBlock => word_space(s, "unsafe"),
+      ast::UnsafeBlock(*) => word_space(s, "unsafe"),
       ast::DefaultBlock => ()
     }
     maybe_print_comment(s, blk.span.lo);
diff --git a/src/test/run-pass/ifmt.rs b/src/test/run-pass/ifmt.rs
index 3f0c7e0704161..ab66bfc101110 100644
--- a/src/test/run-pass/ifmt.rs
+++ b/src/test/run-pass/ifmt.rs
@@ -10,6 +10,8 @@
 
 // xfail-fast: check-fast screws up repr paths
 
+#[deny(warnings)];
+
 use std::fmt;
 
 struct A;
@@ -226,6 +228,13 @@ pub fn main() {
     let a = ~3;
     format!("{:?}", a);
     format!("{:?}", a);
+
+    // make sure that format! doesn't cause spurious unused-unsafe warnings when
+    // it's inside of an outer unsafe block
+    unsafe {
+        let a: int = ::std::cast::transmute(3u);
+        format!("{}", a);
+    }
 }
 
 // Basic test to make sure that we can invoke the `write!` macro with an